Title: Calculation of the electronic structure of the U center and related color centers in alkali halides by using the multiple scattering method / by Hsi-Ling Yu
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Title: Calculation of the electronic structure of the U center and related color centers in alkali halides by using the multiple scattering method / by Hsi-Ling Yu
Physical Description: v, 125 leaves. : illus. ; 28 cm.
Language: English
Creator: Yu, Hsi-Ling, 1945-
Publication Date: 1975
Copyright Date: 1975
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Subject: Halides   ( lcsh )
Physics thesis Ph. D
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Thesis: Thesis -- University of Florida.
Bibliography: Bibliography: leaves 121-124.
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CALCULATION OF THE ELECTRONIC STRUCTURE OF THE U CENTER
AND RELATED COLOR CENTERS IN ALKALI HALIDES
BY USING THE MULTIPLE SCATTERING METHOD










By

HSI-LING YU


A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR TEE
DEGREE OF DOCTOR OF PHILOSOPHY




UNIVERSITY OF FLORIDA


1975















ACKNOWLEDGEMENT S

I would like to thank Dr. J. W. D. Connolly for his

suggestion of the present topic and for his advice and

encouragement in the course of this report.

Special thanks are extended to Dr. S. B. Trickey for

his constructive criticism and his patience in correcting

errors in the manuscript. I would like also to acknowledge

Dr. J. R. Sabin and other members of the Quantum Theory

Project at University of Florida for their helpful cosrents.

Thanks are also due to Dr. M. L. De Siqueira for his

helpful discussion concerning this work.

Finally, I would like to thank, especially, my wife,

Ning, for her help in preparation of the manuscript and

for her patience and continuous encouragement throughout

this research.

The use of the facilities of the North East Regional

Data Center and the financial support from the National

Science Foundation for part of this research are gratefully

acknowledged.















TABLE OF CONTENTS


Page

ACKNOWLEDGMENTS . . . . . . . . ii

ABSTRACT . . . . . . . . . . iv

CHAPTER I INTRODUCTION . . . . . .. 1

CHAPTER II COLOR CENTERS IN ALKALI HALIDES . 3

2.1 Optical Absorption. . . . . .. 3
2.2 Experimental Methods . . . . . 4
2.3 Model for Color Centers . . . . 7
2.4 Theoretical Approaches . . . . 9

CHAPTER III METHODS OF CALCULATION. . . ,. 16

3.1 The Multiple Scattering Method ..... 16
3.2 Choice of a in Vxa . . . . .. 24
3.3 Transition State . . . . . . 25
3.4 Radii for Ions . . . . . . 27
3.5 Ionic Correction Potential . . 31

CHAPTER IV RESULTS AND DISCUSSION . . . . 38

4.1 The U Centers in Alkali Halides . . 38
4.2 The F Centers in Alkali Halides . . 71
4.3 The U2 Center in Potassium Chloride .86
4.4 The U1 Center in Potassium Chloride . 92
4.5 Cluster Calculation of a Pure KCi Crystal 95

CHAPTER V CONCLUSIONS . . . . .. . 109

APPENDIX A THE EVJEN METHOD. . . . .. 114

APPENDIX B SYMMETRY ORBITALS . . . .. 117


BIBLIOGRAPHY . . . . . . . . . .

BIOGRAPHICAL SKETCH . . . . . . . .


121

125


iii














Abstract of Dissertation Presented to the Graduate Council
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy


CALCULATION OF THE ELECTRONIC STRUCTURE OF THE U CENTER
AND RELATED COLOR CENTERS IN ALKALI HALIDES
BY USING THE MULTIPLE SCATTERING METHOD

By

HSI-LING YU

March, 1975

Chairman: J. W. D. Connolly
Major Department: Physics and Astronomy


The spin-polarized multiple scattering Xa self-consistent

field method has been applied to the problem of color centers

in ionic crystal. Within the muffin-tin approximation, the

electronic structures of the U and F centers in several

alkali halides and the U1 and U2 centers in potassium chloride

have been calculated and studied using a cluster of ions

including the first nearest-neighbors. From the transition

state calculation, the U and F center optical absorption

energies in several alkali halides are found to be in good

agreement with experiment except in the case of a crystal

with a very small interionic distance. In the U2 center case,

two possible transitions are investigated and one transition

agrees well with experiment while the other, a charge

transfer process, gives an energy too small. The hyperfine

iv








interaction of the U2 center and the optical absorption of

the Ui center have also been studied.

Larger clusters including the second nearest-neighbors

and the third nearest-neighbors for the U and F centers

in KC1 have been calculated and the absorption energies

are found to oscillate with the size of the cluster. This

problem has been investigated and discussed by comparing

the cluster calculation of the pure KC1 crystal with the

energy band calculation and the experimental data. One of

the improvements for the large cluster calculation is a

more accurate crystalline potential. In the present calcu-

lation this potential is calculated from a point-ion

assumption.














CHAPTER I

INTRODUCTION

Color centers in alkali halide crystals have been

observed and studied for many years. But the calculation of

the electronic structure and the properties of the color

center or impurity in solid is difficult because of the

complexity of the system involved. Thus, most of the studies

are done by methods that are semi-empirical in nature.

Recently, the Xa method (Slater, 1972; Slater and

Johnson, 1972) which makes use of the so-called Xa statisti-

cal exchange, has been successfully applied to many problems.

For molecules, the multiple scattering Xa self-consistent

field (MS-Xa-SCF) method (Johnson, 1973) gives satisfactory

results in many cases and is also very efficient in computing

time. Thus, one of the purposes of the present dissertation

is to study the electronic structure of color centers by

direct calculation of the properties of a cluster of ions

using the MS-Xa-SCF method. From the calculation the optical

absorption process of the color center can be studied and the

transition energy can be calculated.

Several clusters with different sizes were used in the

calculation, but only one cluster (Figure 2.1) is used for

extensive study of the U center from LiF to KBr. Other color

centers (F, U1 and U2 centers) are studied mainly in the KC1
1




2



crystal. From these investigations, the capability and

limitation of the present approach can be checked.

Another purpose of this dissertation is to compare the

present cluster calculation of a pure crystal with energy

band calculations and the experimental data. From this

comparison, the characteristics of the present cluster method

and possible improvements on the method for future applica-

tions can be found.















CHAPTER II

COLOR CENTERS IN ALKALI HALIDES

2.1 Optical Absorption

The alkali halides are, in many respects, ideal crystals

for theoretical and experimental studies. They have cubic

structure and many of their properties are affected by the

strong Coulomb interactions which, in turn, are due to the

highly ionic nature of these crystals. For a pure alkali

halide crystal, the energy gap between the conduction band

and the valence band is of the order of 10 eV. This corres-

ponds to a light quantum in the far ultraviolet and, hence,

the crystal appears transparent to the eye. When imperfections

develop in these crystals extra absorptions may appear in the

visible region and the crystals will appear colored. One of

the more interesting problems in solid state physics for

many years has been the study of these imperfections in

ionic crystals both experimentally and theoretically.

The color centers occurring in alkali halides come

from many varieties of defects in the crystal. They occur

when a few alkali or halogen ions are replaced by other ions

or atoms, or when ions are removed, leaving ion vacancies

in the crystal. There are also cases in which extra ions or

atoms exist in the solids, forming interstitial impurities.




4



Distortions of the lattice from the perfect configuration of

the crystal also can create color centers. In all cases, the

effect of these color centers is to allow certain energy

levels to exist in the gap between the conduction band and

valence band of the energy band picture. Thus, energies

needed to excite electrons from these color center levels

to the allowed energy levels above are small compared

to the energies of the interband transition or the energies

from the valence band to the exciton levels. In some cases,

this will happen in the visible light region, causing the

crystal to appear colored, e.g., the F centers. In other

cases, the excitation energies are outside the visible

light region; thus, the crystal will remain transparent,

e.g., the U centers.


2.2 Experimental Methods

Color centers are produced by many different kinds

of techniques. They can be produced by exposing the crystals

to X-rays or y-rays, or by injecting electrons directly

into the crystals. Bombarding the sample with heavy particles

or electrons can also create imperfections in the solid.

Another method is the process of additive coloration in

which the crystal is heated in the presence of excess

alkali or halogen or other atoms. Several review articles

by Markham (1966) and Seitz (1946, 1954) have discussed

these and other methods for producing color centers.

The optical absorption spectrum of a color center








generally shows a broad band whose position and shape are

temperature dependent. For example, in the case of the F

center, it is found that the width of the absorption band

broadens with an increase of the temperature, while the

peak energy of the band decreases with the increase of the

temperature. It will be seen in section 2.4 that a very

simple model for the F center can give a qualitatively

good explanation for these characteristics of the absorption

band.

A study of the peaks of the absorption bands in various

alkali halides for a certain color center shows that there

is a relationship between these energies and the nearest-

neighbor distances of the solid. This kind of relation

was first found empirically by Mollwo (1931) and later

refined by Ivey (1947). In the cases of the U center and the

F center, the relations are


20.16 d-1.10 for U band
max = -1.84 (2.1)
17.64 d for F band,


where E is in Electron Volts and d (the nearest-neighbor
max
distance) is in Angstroms.

From the optical absorption data alone, very little

information about the color center and its surroundings

can be obtained. There are two other methods which can

provide more insight into the structure of imperfections

in the crystal. One of these methods is the Electron Spin

Resonance method (ESR) (Hutchison, 1949) and the other is









the Electron Nuclear Double Resonance method (ENDOR)

(Feher, 1957). These methods have been reviewed by Seidel

and Wolf (1968) and Markham (1966).

In ESR spectroscopy, magnetic dipole transitions

between the spin levels of an unpaired electron of the color

center can be measured in a static magnetic field. In

ENDOR spectroscopy, the nuclear spin resonances of the

neighboring nuclei that are coupled to the unpaired electron

of the color center are measured. Usually, these two reson-

ance methods are applied only to the ground states of color

centers. Measurements on excited states are difficult

due to the short lifetime of these states.

From the ESR measurements, one can determine the

nature of the center, that is, whether it is paramagnetic

or diamagnetic. Sometimes the ESR absorption line is broad-

ened due to the interactions between the magnetic moment

of the unpaired electron and those of the neighboring

nuclei, and information about the neighboring ions is

difficult to obtain from the structureless absorption line

shape. With the relatively new ENDOR measurements, more

precise determination about the surroundings of the imper-

fection can be obtained and this provides an accurate way

to check the ground state wave function of the paramagnetic

center from theoretical calculations.

Using the data from these optical and magnetic measure-

ments, models for these centers can be established and the








electronic structure of the color centers can be studied by

various theoretical approaches.

2.3 Models for Color Centers


In the present calculations, only U, F, U-, and U2

centers in alkali halides are considered. From various

experimental studies, especially ESR (Hutchison, 1949; Kip

et al., 1953) and ENDOR (Feher, 1957; Seidel, 1961; Doyle,

1962), the F center can be established to be an electron

trapped at an anion vacancy in the alkali halide crystal.

The U center is believed to be a negative hydrogen ion

substituted for an anion in the ionic crystal (Delbecq et al.,

1956). The Ul and U2 centers are interstitial defects which

occupy a position in-between the regular sites of the

crystal. The U1 center is believed to be a negative

hydrogen ion at an interstitial position in the crystal and

the U2 center is a hydrogen atom at the same site (Delbecq

et al., 1956; Spaeth, 1966).

The clusters of ions that are used in the present

calculations for these color centers are shown in Figures

2.1-2.3.

Figure 2.1 shows the eight-center cluster (including

the outer sphere) for the U center and the F center calcula-

tion with Oh symmetry. For the U center, it is a negative

hydrogen ion in the center surrounded by six alkali ions. For

the F center, the cluster is an electron trapped in the

central sphere with six nearest alkali ions surrounding it.






















II




















Figure 2.1. The eight-center cluster used in the
calculation. Regions I, II and III are
the atomic, intersphere and outer regions,
respectively.








Figure 2.2 shows the cluster with Td symmetry for the

U1 and U2 center. In the case of the U2 center, this cluster

is a hydrogen atom in the center with four anions and four

cations forming two interpenetrating tetrahedra. For the

U1 center, the central sphere is a negative hydrogen ion.

Figure 2.3 shows a large cluster with several shells

of ions that is used in the U and F center calculations.

This cluster has Oh symmetry, and the difference for the

U center and the F center is at the central sphere which is

occupied by a hydrogen ion in the case of the U center and

an electron in the case of the F center. Calculations with

the halogen ion in the central sphere of this cluster have

also been done in order to compare with energy band calcu-

lations.



2.4 Theoretical Approaches

The F center is the simplest defect that can occur in

alkali halides. As mentioned in the previous section, this

center is an electron at the vacant anion site trapped in

the potential well due to the near-by positive ions. An

extremely simple model (Stockmann, 1952; van Doom, 1962)

in which the F center is thought to be a particle in an

infinitely deep three-dimensional square well, i.e., a

particle in a box, can qualitatively explain several pro-

perties of the optical absorption spectra that werementioned

before. Using this model, the excitation energy from the

ground state to the first excited state is

























































Figure 2.2. The cluster used in the U1 and U2 center
calculations.





















































Figure 2.3. The large cluster with 01 symmetry used in
the calculation. C=central ion, l=first nearest-
neighbor, 2=second nearest-neighbor, 3=third
nearest-neighbor.













A (/ (2.2)



where d is the distance from center of the potential well

to the potential barrier. This relation shows the same form

as the empirical relation for the F center in Equation (2.1).

Thus, from this model the F band energy is found to be rela-

ted directly to the spacing of the lattice. The decrease

of the peak energy of the absorption band can be explained

as the expansion of the lattice due to the increase of the

temperature, and from the temperature dependence of lattice

vibrations, the width of the absorption band can be expected

to increase with the temperature.

Because of the simplicity of the electronic structure

and the availability of the magnetic properties of an F

center in an ionic crystal, this center served as a good

testing example for the theoretical methods developed for the

general impurity problem. Thus, most of the methods in the

beginning are concentrated on the F center. Before 1957, the

techniques used are mostly semi-empirical in nature. For

example, there is a continuum model (Simpson, 1949) in

which the polarizable crystalline medium is replaced by

a dielectric continuum and the influence of lattice periodi-

city on the electron is considered by using an effective

mass for the electron. Modified version, the semicontinuum

model (Krumhansl and Schwartz, 1953), employs more detailed








treatment of potential for the central part of a color

center in which the previous model fails. There are other

methods using the variation techniques, e.g., the point-ion

approximation (Gourary and Adrian, 1957) and molecular

orbital calculation (Inui and Uemura, 1950). A summary of

these methods up to 1960 is given by Gourary and Adrian

(1960). More extensive calculations using the extended point-

ion approximation (Wood and Joy, 1964) and pseudopotential

approximation (Kubler and Friauf, 1965) have also been done.

These methods are discussed in a review article by Fowler

(1968). Since the U center has the same structure as the F

center, calculations of U centers (Spector et al., 1967;

Wood and Opik, 1967) have employed the same methods that were

developed for the F center.

As to the U1 center, there have been no theoretical

calculations, partially because it has no interesting

magnetic properties (the ground state is a closed shell)

and partially because the peak of U1 band in the absorption

spectrum is generally not well defined (Delbecq et al.,

1956; Rolfe, 1958). Contrary to the case of the U1 center,

a great deal of work has been done on the U2 center. Kerkhoff,

Martienssenand Sander (1963) examined various possibilities

of the optical process for the U2 center in a semi-classical

way, and concluded that the optical U2 band is due to a

charge transfer from the nearest-neighbor halogen ions to

the hydrogen atom. The configuration of the excited state

of the U2 center is thought to be a negative hydrogen ion








plus a hole in the p orbitals of the nearest-neighbor

halogen ions. Using these configurations, Cho et al. (1966)

and Cho (1967) obtain wave functions to discuss the hyper-

fine interaction.

Other calculations involving the variation technique

have been done by Sammel (1969). Calculations of the transi-

tion energy of the U2 band have been done semi-empirically

by Hagston (1971) and Schechter (1969) and earlier by

Mimura and Uemura (1959).

In this dissertation, the optical absorption energies

of the U and F centers in alkali halides and the U1 and U2

centers in KC1 are calculated. Unlike other approaches, the

U centers, instead of the F centers, in various alkali

halides were investigated first by using the multiple

scattering method. This is because the starting potential

used in the present calculation usually is a superimposed

ion-potential and it is relatively simple to get an ion

potential for the H- ion by using the Herman-Skillman

atomic program (Herman and Skillman, 1963).

In all calculations, the effects of the lattice dis-

tortion and polarization and the temperature dependence

mentioned before are not included. Some of these effects,

though, will be discussed in later sections. One of the

purposes of the present calculation is to investigate the

capability of the multiple scattering cluster method of

handling the problem of impurities in solid; therefore, no

adjustment of parameters to match the experimental data





15


was made. However, the effects of some of the parameters

will be discussed in the results of the calculations given

in Chapter 4.














CHAPTER III

METHODS OF CALCULATION

3.1 The Multiple Scattering Method

One practical way to solve the problem of a many-

electron system is the one-electron self-consistent field

method in which the interactions between the electrons and

nuclei are replaced by the interactions of one electron

with an averaged electronic charge distributions and with

all nuclei plus some exchange-correlation effects. By

solving the Schroedinger Equation in such a field, a set of

eigenfuncticns can be found and from these eigenfunctions

the charge densities can be calculated. Thus, by repeating

the procedures a self-consistent field can be reached. This

method was first used by Hartree (1928) in his proposed

one-electron equation.

If one startsfrom an anti-symmetrized one-electron

product function or a Slater determinant function (Slater,

1929) to express the total energy of the system, a set of

one-electron equations can be obtained by the variation

technique. These are the Hartree-Fock equations (Fock, 1930)

which have been used in many problems. For a complicated

system, the exchange potential term in the Hartree-Fock

equation becomes extremely difficult to calculate.








An approximate method called the Xa. method (Slater,

1972; Slater and Johnson, 1972) can be used to avoid the

difficulty in the exchange-correlation potential. In this

method, the exchange term in the Hartree-Fock one-electron

equation is replaced by a local exchange potential (Slater

and Wood, 1971) (see Equation (3.2)) which gives an approxi-

mate exchange-correlation term and is easy to calculate.

A numerical technique to solve the X<- equation in a

large molecule or cluster is the multiple scattering (MS-XA )

method (Johnson, 1973). In its formulation, the cluster is

divided into three regions, The first is the atomic sphere

region which contains one atom or ion inside the sphere.

The second region is the intersphere region that is the

space outside all the atomic spheres but inside an outer

sphere which enclosesthe whole cluster. The third region is

the outer region which includesall space outside the outer

sphere. The problem is to solve the Schroedinger problem

of a single electron in each region and after the matching

of the wave functions and their first derivatives on all

boundaries of the cluster we get a relation in which the

eicenvalues can be determined. The one-electron wave func-

tion with spin up, u (1), satisfies the following Xo

equation (in Rydberg units).



-VS* VC !4 Vx1t ( V ] VI %Litj!)= i C ILtl)t (3.1)








where V (1) is the Coulomb potential at position 1 due to
c
all electronic and nuclear charges of the cluster, and

V x(1) is the statistical exchange-correlation potential

which is given by




L kiLi') (3.2)




where the summation is over all eigenfunctions with spin up

and n. is the occupation number of the spin-orbital u.. A

discussion of the choice of a in this local exchange potential

is given in the next section. The term V represents the

Coulomb potential due to all ions outside the cluster. The

determination of VI is treated in section 3.5. Similar

expressions of Equations (3.1) and (3.2) with spin down give

the one-electron equation satisfied by ui(l1).

In order to simplify the problem, a muffin-tin form

of potential has been used in the calculations. In this

approximation, the potential inside the atomic sphere

region is spherically averaged and the potential in the

intersphere region is a constant equal to the volume average

of potentials in that region. The potential in the outer

region is also spherically averaged. Because of the locally

spherically symmetric potential, we can express the solution

of the one-electron Equation (3.1) in the ath sphere of the

first region (for simplicity, the spin subscripts are








dropped for the following discussion) as


j = z C'R" ( 1, -,0 ,(3.3)
Jm
where r, is a vector measured from the center of the Eth

sphere and m (r )'s are the real spherical harmonics where

r, represents the angular part of r, and C m' are coeffi-

cients to be determined, and R(JIrl ,e) is the solution of

the one-electron radial Schroedinger equation with a certain

energy E, namely,


r. -- -- V ) R) ) o

where V is the spherically averaged potential in the cth

sphere.

Similarly, the solution in the outer region can be

written as

0 20 ( A
Cn R, c; r (3.4)

where r5 is a vector measured from the center of the outer

sphere. In the intersphere region, the volume average of Vc,

Vx and VI gives a constant potential VII, and the solutions

of the Schroedinger equation for the cases
expanded as


S. L( Am.
(3.5)
2L ('A Kr 2 Kr
j M J m Ot








where K=(VII- ) and i k (1) are modified spherical

Bessel and Hankel function, respectively. For the cases

where E > V II the solution in the intersphere region can

be expanded as


0 A r A
Sd"k'~ (3.6)

where K=( E-V I) and j is an ordinary spherical Bessel

function and r. is an ordinary spherical Neumann function.

In order to match the logarithmic derivative of the

wave functions on the boundaries of a particular atomic

sphere or the outer sphere, we have to transform the multi-

centered wave function in region II in terms of only the

one center that is under consideration. Several expansion

theorems which are useful for this purpose have been

discussed by Johnson (1973).

Suppose we consider the case of < (V and try to

match u and u (and their first derivatives) on the

boundary of the th sphere. After cancelling the C m

coefficients, we get a set of linear equations in terms

of the AIm coefficients, namely, for a particular j and m


4 4 -I P ( C3.7)
"A + ALM L L NA
3*cA LM
0o ,(3.7)
+1 ALM J, LM





21


where


-t ) r 4L E-,A K[ r ) W \N K (rI


evaluated at r= b (the radius of the oCth sphere). Here

we have used the Wronskian bracket notation


'J r- adr The G (U) coefficients
SHLM
depend only on the relative positions of centers in the

cluster and are defined as





(3.8)
S1^1) for


where R is a vector from the center of the oth sphere

to the center of the (i th sphere. The term I ,, (km;LM)

is a Gaunt coefficient and is defined as anintegral of the

product of three spherical harmonics over all solid angles,

namely,


I(; L M') I((r) (3.9)


Similarly,










^ (, 4^-0^2 iT (-)I) Z, &.)
ICt, LM JPv0
(3.10)





From above we know that in Equation (3.7) the dependence

on the nature of a particular atom or ion in the cluster

is only from the term t (e) through its dependence on

RI ( r ,c ), the solution of the radial Schroedinger equa-

tion, and the G (0) matrix element is only a structure

factor with dependence on eigenvalue parameter E through

its relation with K .

After matching the logarithmic derivatives of the wave

functions on the boundaries of all atomic spheres and the

outer sphere, we complete the set of equations in terms of

A 's. It is well known that for such a set of homogeneous

linear equations, the condition for having non-vanishing

solutions for all variables requires that the determinant

of the coefficients of these equations equals to zero. Thus,




et I L rM + -0( (3.11)



where the indices t and ^ start from the outer sphere and

run through all atomic centers. The highest I value used








in the calculation is dependent on the atoms and the

symmetry of the cluster.

In the case that the cluster possesses symmetry, we

can simplify the determinant by expanding the wave function

in each region in terms of some linear combinations of

the products of spherical harmonics and radial functions

centered on the equivalent sites of the cluster that form

bases for various irreducible representations of the

appropriate symmetry group. The size of the symmetrized

array of a particular representation that we need to solve

is equal to the number of the basis functions that we used

in that representation and is considerably smaller than that

of (3.11) especially in the cases where the cluster has a

high symmetry or many centers. In any case, the determinant

can not be solved directly, but the zeroes of the determinant

can be searched by evaluating the determinant in steps of

the energy parameter E .

For each eigenvalue EL found, the eigenfunction in

each region can be obtained. From these eigenfunctions, we

can calculate the charge densities. After all eigenvalues

F's are found, we can calculate the Coulomb potentials in

each region by using the muffin-tin form of the total

charge densities. Using the spherically averaged potential

in region I and region III and the volume averaged potential

in region II, the entire calculation can be started again.

In order to start the initial calculation, a super-








position of atomic potentials is used. For each new iteration,

a weighted mixture of the newly generated potential and the

previous one is used as the input potential. This procedure

is repeated until the difference between the new and the

old potentials is within a preset tolerance. Thus, self-

consistency is achieved.


3.2 Choice of a in V
xc

The statistical exchange-correlation potential (Slater

and Wood, 1971) given in Equation (3.2) was originally

derived by Slater (1951) with a = 1, and later with another

approach it was derived independently by Gaspar (1954) and by

Kohn and Sham (1965) with a = 2/3. In actual cases, values of

a in between the above two values are expected to give better

exchange-correlation effects. In the case of atoms, Schwarz

(1972) has determined two setsof a's by using two different

criteria. The first is to adjust the statistical total energy

to the Hartree-Fock total energy of the isolated atom and the

second is to adjust the parameter a so that the virial theorem

is satisfied when Xa orbitals are used in the RHF expressions

for the expectation value of the kinetic and potential energies

of the isolated atom.

The two sets of a's turn out to be very close to each

other and the variation of a from atom to ion is not

significant. Thus, it seems justified to use the same a for

atom or ion in the solid. All of Schwarz' determinations

were done without spin polarization. In the present

calculations, the a~HF values with the Hartree-Fock total









energy criterion were used for all atomic spheres except

for the hydrogen ion sphere in which the oL value is equal

to 0.77, which is appropriate for a spin-polarized calcula-

tion (Slater, 1973; Singh and Smith, 1971). For other ions,

the oHF values are approximately the same in a spin-

polarized calculation. For the case of KC1, the same value

of 0.72 was used both for the K ion and the Cl ion. A

weighted average of V is also used for the intersphere and

the outer sphere regions. All the oc values used for various

ions in the present calculations and the XHF and oVT
HF VI
values determined by Schwarz (1972) are given in Table 3.1.



3.3 Transition State

In order to find the absorption energies of color

centers in ionic crystals, the transition state concept

(Slater, 1972) has been used in the multiple scattering Xo(

method.

Instead of calculating the difference of the total

energies of the ground state and the excited state of the

whole system, the excitation energy can be approximated by

taking the difference of the one-electron eigenvalues of

the two eigenstates involved in the transition with the

occupation numbers set to be half-way between the initial

and the final state of the system. In other words, one can

remove one-half of the charge from the lower eigenstate

and add one-half of a charge to the higher eigenstate to














TABLE 3.1

The a values used in the present calculations and the
aHF and aV values determined by Schwarz.

a a
Ion a aHFa VT


H 0.77000b

Li+ 0.78147 0.78147 0.78087

F 0.73732 0.73732 0.73651
+
Na 0.73115 0.73115 0.73044

Cl 0.72000 0.72325 0.72277

K+ 0.72000 0.72117 0.72072


Schwarz (1972).
Singh and Smith (1971)









form a "transition state" of the system, and the excitation

energy can be easily obtained from the eigenvalues.

The justification behind the transition state calcula-

tion follows from the fact that the eigenvalue in the Xx

method can be expressed as a first derivative of the total

energy with respect to the occupation number of that state.

Using the power expansion of the total energy, it can be

shown that the error of the excitation energy for using

the transition state calculation is mainly from the third

order terms which can be shown to be relatively small

(Slater and Wood, 1971).



3.4 Radii for Ions

In section 3.1, the division of space into three

regions in the multiple scattering formulation has been

discussed and the atomic region for each atom or ion is

defined by a sphere. Thus, a set of radii acting as para-

meters for all atomic spheres is required in the calcula-

tion. Because of the muffin-tin approximation, it is

natural to choose the radii of atomic sphere and outer

sphere to touch one another so that the space in the inter-

sphere region will not be too large, since the use of the

constant potential in this region will probably contribute

some errors to the final results.

For ionic crystals, there are a few sets of empirical

ionic radii (Goldschmidt, 1926; Pauling, 1948) which are









widely used in many books. Thus, at the beginning of this

investigation, several calculations were carried out by

using the radii of Goldschmidt (1926) for the ion spheres.

However, the basic reason for having such a set of radii

is the fact that the differences between the values of the

nearest-neighbor distance in the corresponding halides of a

pair of alkali metals are approximately constant, and the

same holds for the alkali metal salts of pairs of halogens;

thus, by specifying any one radius, the whole set of ionic

radii can be determined with a small correction so that the

radii of anion and cation can add up exactly to the nearest-

neighbor distance of that particular crystal. Different

methods for choosing the radii generally will lead to a

different set of ionic radii and a review of various kinds

of ionic radii is given by Tosi (1965).

In order to make best use of the muffin-tin approxima-

tion, it is desirable to have a set of radii which are

close to the relative sizes of ions in crystals. There are

a few experiments employing the X-ray diffraction techniques

to study the electron charge distributions of ionic crystal

(Witte and Wolfel, 1955; Krug et al., 1955).

From the minimum of the charge densities in between a

pair of anion and cation the crystal radii for this pair of

ions can be defined. Using the above criterion, the crystal

radii of the Na+ and Cl- ions are 1.17 A and 1.64 A, respec-

tively. Starting from these two radii, Gourary and Adrian









(1960) were able to deduce a set of corrected radii for the

ionic crystals. This set of radii give a larger relative

size of cation to anion than that of the previous sets.

Values of different sets of ionic radii mentioned are given

in Table 3.2.

Calculations of various color centers by using Gourary

and Adrian X-ray corrected ionic radii have been done for

many alkali halides. The electronic charge in the intersphere

region was observed to be less than those of previous calcu-

lations and the potential has a smaller discontinuity when

we go from one atomic region to another. Therefore, in most of

thecalculations in this dissertation the corrected radii of

Gourary and Adrian were used.

There are other ways to find the crystal radii for a

particular ionic crystal, for example, by using the charge

densities of ions or the potentials of the ions. But in

most cases the radii determined are not far from the

X-ray corrected radii.

Above all, the radii of the atomic spheres are not

used as adjustable parameters in the present calculation.

However, for some color centers, calculations with different

setsof radii were available and effects of this variation

of radii will be discussed in the results of those parti-

cular color centers.















TABLE 3.2

The ionic radii of Goldschmidt and the X-ray
corrected radii of Gourary and Adrian.


Ion Goldschmidta Gourary and Adrian

(in A) (in a.u.) (in A) (in a.u.)


Li+ (LiF only) 0.78 1.47 0.92 1.74

Li+ 0.78 1.47 0.94 1.78

Na 0.98 1.85 1.17 2.21

K+ 1.33 2.51 1.49 2.82

F (LiF only) 1.33 2.51 1.09 2.06

F 1.33 2.51 1.16 2.19

Cl 1.81 3.42 1.64 3.10

Br 1.96 3.70 1.80 3.40


aGoldschmidt (1926).
bGourary and Adrian (1960).








3.5 Ionic Correction Potential

In all of the present calculations except the U2 center

calculations, the cluster being used is not neutral. Thus,

some form of the potential correction is needed to stabilize

it. The most common method is by adding the potential from

a charged sphere to the potential of the cluster. This

method was first used by Watson (1958) to stabilize the 0

ion. Using this technique with the charge on the Watson

sphere being the negative of the charge Q of the cluster

(see Table 3.3), the ionic correction potential VI for an

electron in Equation (3.1) is given by



R2 Ry for r R
V (r) = (3.12)
I (2Q Ry for r > R
r


where R is the radius of the Watson sphere, usually chosen

to be the same as the outer sphere, and r is measured from

the center of the cluster. With this charged sphere, the

potential for the cluster will approach zero outside the

cluster, and presumably the potential inside the cluster is

close to the actual potential of the cluster imbedded in the

solid. Several calculations were carried out with this

potential correction, and some characteristics of this

correction were found. That is, the correction inside the

cluster depends on the choice of the radii of the ion spheres

through the fact that the R in Equation (3.12) is always
















TABLE 3.3

Constants in Equations (3.12) and (3.13) for
various clusters used in the U center
and F center calculations.


Clustera A Q


Eight-center 3.96 5.00

Twenty-center -3.97 -7.00

Twenty-eight-center 0.50 1.00


See Figure 2.3. The eight-center cluster includes the
first nearest-neighbors, and the twenty-center cluster
includes the second nearest-neighbors, and the twenty-
eight-center cluster includes the third nearest-neigh-
bors.





33


equal to the sum of a constant (in the eight-center calcu-

lation, this constant is the nearest-neighbor distance) and

the radius of the most distant ion of the cluster. The

situation is more serious when the cluster has a large Q.

A better potential correction,which depends only on the

nearest-neighbor distance of the crystal, is needed. There

are several methods for calculating the Madelung potential

inside the ionic crystal (for a review, see Tosi (1965)),

and for the present purpose the Evjen method (Evjen, 1932)

is a simple method which can be easily programed to use in

a computer. A volume average of the potential due to the

surrounding ions is calculated inside the cluster and

details of this calculation are discussed in Appendix A.

Thus, the ionic correction potential for an electron can be

rewritten as



2A
d Ry for r < R
VI(r) = (3.13)
2Q
Ry for r > R
r


where d is the nearest-neighbor distance or the anion-cation

distance in an ionic crystal, A is a constant, which depends

on the size and structure of the cluster. The different

values of A for various clusters are given in Table 3.3.

In a large cluster, the volume averaged correction

potential is not sufficient to give a reasonable correction









for each ion, therefore, separate ionic correction poten-

tials for ions were sought. Since the potential for each

ion in the muffin-tin approximation is spherically averaged,

the correction potential for each ion is particularly

simple to find.

We note that in the jth ion sphere of the cluster the

ionic correction potential at an arbitrary point, rj, is

given by



V = r (3.14)
I i r Rjij


where r. is measured from the center of the jth sphere,

the summation of i is over all ions outside the cluster

and q. is the point charge of ith ion, and R.. is a vector

measured from the center of the jth sphere to the ith ion.

It can be shown very easily that the spherically averaged

V (r.) is equal to the ionic correction potential at the
I 3
center of the jth sphere:



(r,) 41... ^ -1.--- ^ (3.15)


Therefore, the ionic correction potential for an electron

inside any ionic sphere is a constant and can be calculated

by using the Madelung constant c (see Appendix A ), e.g.,




35





Z% L 2 v. 2z 2A (3.16)
V, ---- 1



where d is the nearest-neighbor distance of the crystal,

the summation of k is over all ions in the cluster

except the jth, and R. is the distance from the jth center
jk
to the kth center. The ionic correction potential in the

intersphere region of the cluster can also be found by

the following method. The spherically averaged or the

volume averaged potential inside the outer sphere of a

cluster due to all ions outside will give a constant poten-

tial that is the same as the potential at the origin of the

cluster, and since the potentials inside the ion spheres

are known, the ionic correction potential for the inter-

sphere region can be calculated from the following equation:




o) 5 I^ Sk 2,At ^(3.17)
V ---- Ry
1 So - s_ )




where V (0) is the potential at the center of the cluster,

S is the volume of the outer sphere, the summation of k
0
is over all the ion spheres in the cluster, and Sk is

the volume of the kth ion sphere. The value of Aint depends





36


on the relative size of the cation and anion, and values of

Aint for KC1 and LiF along with the Aj's for the large

cluster shown in Figure 2.3 are given in Table 3.4. The

correction potentials for the outer region in all cases are

the same as defined in Equation (3.12).



















TABLE 3.4

Constants in the separate ionic correction potentials
for the twenty-eight-center cluster.


Region Aj or Ainta


Central ion sphere 0.386

First nearest-neighbor 0.439

Second nearest-neighbor 0.235

Third nearest-neighbor 0.682

Intersphere (KC1) 0.381

Intersphere (LiF) 0.477


aThe twenty-eight-center cluster is shown in Figure 2.3.
Constants are defined in Equations (3.16) and (3.17).














CHAPTER IV

RESULTS AND DISCUSSION

4.1 The U Centers in Alkali Halides

The peak energies of the optical absorption band of

the U centers in several alkali halides have been calculated

with the use of the eight-center cluster (see Figure 2.1),

which has been discussed in section 2.3. The electronic

structure and the absorption energy of the U center in KC1

have also been calculated using the twenty-center cluster.

This has a negative hydrogen ion at its center and includes

up to the second nearest-neighbors. Calculations with the

twenty-eight-center cluster shown in Figure 2.3 including

up to the third nearest-neighbors have also been done.

All calculations have been carried out using the spin-

polarized multiple scattering Xa method in which a separate

local exchange potential for each spin is used in the one-

electron equation. The a values used for the various atomic

regions are given in Table 3.1. In the U center calculation,

the a value for the hydrogen sphere-is 0.77 which is

usually used in a spin-polarized calculation of a two-

electron ion (Slater, 1973; Singh and Smith, 1971).

The input data for the U center calculations using the

eight-center cluster are given in Table 4.1. The interionic

















TABLE 4.1

Radii used in the eight-center cluster calculations
of the U centers and the F centers in alkali halides.


Crystal da R+ R Rut


LiF 3.800 1.740 2.060 5.540

NaF 4.370 2.190 2.180 6.560

NaCl 5.310 2.210 3.100 7.520

NaBr 5.630 2.210 3.420 7.840

KF 5.046 2.834 2.212 7.880

KC1 5.934 2.834 3.100 8.768

KBr 6.220 2.820 3.400 9.040


All quantities are measured in atomic
aTosi (1965).


units.









distance (d), shown in the first columnis the same as the

equilibrium nearest-neighbor distances in a pure crystal.

The R 's are the radii for the alkali spheres and the R_'s
+
are the radii for the halogen spheres, which is occupied by

a negative hydrogen ion in the present case. All radii are

chosen according to Gourary and Adrian's corrected ionic

radii shown in Table 3.1), although small adjustments were

made so that the atomic spheres touch one another. Rout is

the radius of the outer sphere. The averaged Madelung

potential correction defined in Equation (3.13) was used for

all these calculations.

The electronic structure of the ground state of the U

center in KC1 calculated by using the eight-center cluster

is shown in Figure 4.1. In this diagram, one can see that

no spin-polarization effect appeared in the eigenvalues.

This is expected from a system consisting of closed shell

ions. The hydrogen Is level appears as two alg symmetry

orbitals, one with each spin. These two hydrogen alg levels

are well localized in the central part of the cluster as the

calculated result showed that 85% of the charge of these

levels is within the central sphere and the rest of the

charge is distributed mostly in the intersphere region.

Below the hydrogen levels, the next occupied levels

are a group of seven levels for each spin that arise from

the atomic 3p levels of the six K ions. These levels are

relatively deep compared to the hydrogen alg orbital and








SPIN DOWN


O.OOF0


tlu


-0.10-


tlu


















alg


-0.20-


-0.30 -


-0.40-


-1.24

-1.25

-1.26

-1 27 -

-1.28
-1.281


alg


(U)


tig
tlu
e
t2u
t2g
tlu
alg


tig
tlu
eg
t2u
t2g
tlu
alg


Figure 4.1. The ground state electronic structure of the
U center in KC1 calculated by using the
eight-center cluster.


SPIN UP








their charge distribution is concentrated in the six K+

spheres. Hence, only a small interaction is expected between

this group of levels and the hydrogen levels. The excitation

of the U center in this picture is expected to be an electron

excited from the hydrogen alg orbital to the next higher

allowed level, which is a tlu symmetry orbital with the same

spin. By using the transition state concept discussed before,

the excitation energy for the U center can be calculated.

The calculated transition energies of the U center for

several alkali halides are shown in Table 4.2 along with

the experimental absorption energies and other theoretical

results. The e(alg) and E(tlu) are the eigenvalues of the

two states involved in the excitation process in a "transi-

tion state" calculation, and the difference of these two

energies gives the present value, AE, for the U center

absorption. Comparing with the experimental results, one

can see that AE's are in good agreement with the observed

values, especially for the two potassium halides. Two

other sets of energies are also given in Table 4.2. One of

them is calculated by Gourary and Adrian (1957) and Spector

et al. (1967) using the point-ion model and their values

are given under AEpi, and the other set of values, AE vey,

is calculated by Ivey's relation given in Equation (2.1),

which is determined empirically from the experimental

data.

The absorption energies obtained both theoretically and






















































































A4 A U

2 z


H i 4
r u


U
u
>1




.0











4
x







09
I
Q-


Md
<3


!
H,
ft


09
r'r
.-,-I
r-I

1o1
H


r0
-H








CdJ
-r-
rHU







4)2
O U






t0
4-9


aC (
4J

00)


J c

4 -1

4-4

0,

LO 4-







r ro
1,-



H H
0







04
0






E-i


4-
*

C.,
*H





0

4-l)

-H 0










U) a I
oa o




CN








*o
tr>-


S r-4


I H


-H 4C -


U 0 CN
co u ;-'




0 C C O




io: U) C0 ()








experimentally are plotted against the nearest-neighbor

distances of alkali halides in Figure 4.2. As one can see

from this figure, Ivey's relation is the result of averaging

experimental data, and there is in fact considerable devia-

tion from it.

The absorption energies calculated by the point-ion

model show a smooth curve in parallel with the Ivey's

empirical curve, and their values can be fitted to a rela-

tion with the same form as the Ivey's. Unfortunately, this

simple model produces results which are approximately 15%

lower than the observed values. In the point-ion model

calculation of the U center, one uses a trial wave function

for a ground state, with variation parameters to be deter-

mined, in a field produced by all surrounding ions which

are treated as point charges. By minimizing the energy

expression of the Hamiltonian for this two-electron system

using the trial wave function, one can obtain the energy

for the ground state. A similar procedure using another

trial wave function gives the energy of the excited state,

thus, the excitation energy of the U center can be found.

In this point-ion model, only the nearest-neighbor distance

of the crystal is directly involved in the calculation, it

is not surprising that the calculated energy changes

smoothly with the interionic distance of the crystal. On

the contrary, the results calculated from the present

cluster method do show some deviations from the smooth


















IVEY'S RELATION
/"


O EXP
* MS-Xo
A PI


'N
'N


\:"


6\


\ /


-F 1 --


5.0


5.5


6.0


6.5


NEAREST-NEIGHBOR DISTANCE (A.U.)



Figure 4.2. The optical absorption energies of the U center
as a function of the nearest-neighbor distance
of the crystal.


0.60




0.55-


0.50




0.45




0.40 -


0.35-


4.0


4.5









variation of an Ivey type of relation. This can be seen,

especially for the case of the NaBr crystal with an inter-

ionic distance of 5.63 a.u., in Figure 4.2. In that case,

the U center energy is lower than the energy for a U center

in KC1 which has an interionic distance of 5.934 a.u. As

one can see, the experimental results also indicate the

same characteristics in that region. Therefore, the present

model, including the detailed structure of the first

nearest-neighbors, does show that although the interactions

between the hydrogen ion and the six alkali ions are expected

to be small as discussed before, the effects on the energy

levels and wave function of the U center due to the structure

of the surrounding alkali ions are, nevertheless, not

negligible. Wood and Opik (1967) have also calculated the

U center absorption energies in potassium halides by a semi-

empirical method. In their calculation, the electronic

structure of the first nearest-neighbor ions is considered,

and the calculated transition energies, 0.409 Rydbergs in

KC1 and 0.403 Rydbergs in KBr, showed an energy somewhat too

low for KC1 and a comparable energy for the KBr case. They

also tried another calculation by including polarization

effects and taking more surrounding ions into consideration

and obtained a transition energy of 0.437 Rydbergs for KC1

which is higher than the experimental value by approximately

the same amount as the difference of their first calculated

result. In any case, the ability of a model to predict a








single transition energy is not a very satisfactory criterion

for the validity of the model. It is necessary at least to

examine several cases to obtain a better understanding of the

model that is used. Thus, the calculation of Wood and Opik

and the calculation of Hayns (1972), in which only the U

center transition energy in LiF was calculated by a semi-

empirical CNDO method, will not be discussed.

Experimental data for the U center absorption energy are

available for several alkali halides with interionic distance

greater than and equal to that of the sodium chloride

crystal. Comparison of the present calculated energies with

the observed values shows that the average deviation is

about 3.5%, which is very good. For alkali halides, with

interionic distances less than that of the sodium chloride

crystal, one has to consider that the results from Ivey's

empirical relation will probably give a reasonable guess as

to the transition energy of the U center. From Figure 4.2,

the results from. the present cluster method can be seen to

deviate away from the predictions of the Ivey relation as

the nearest-neighbor distance gets smaller.

For the case of the LiF crystal, which has the smallest

interionic distance of 3.8 a.u., the transition energy

calculated is 0.577 Ry which is approximately 16% lower

than Ivey's result and is comparable to the calculated value

of the point-ion model. Discussion of the possible causes

for this drop in the calculated U center excitation energy

in the region of small interionic distance and of the








neglected factors, namely, the lattice distortion and

polarization effect, will be given later.

At first, it may seem that the resulting curve of the

cluster model in Figure 4.2 is quite complicated as to its

variation with the interionic distance despite the fact

that it has been shown to be consistent with the experi-

mental data. With a closer examination, the calculated

energies do show a characteristic variation with the inter-

ionic distance within the sodium halides and the potassium

halides (see Figure 4.3). This is reasonable, since in the

eight-center calculation only the nearest cations are

considered directly in the calculation and the variation

within a certain family of cation only changes the space

occupied by the H- ion. Therefore, the difference in energy

between the ground state and the excited state of the U

center decreases with the increase of the space occupied by

the H- ion, but for the same space occupied by H- ion with a

different set of cations, the lighter cations will give a

larger difference between the two levels of the U center. In

the point-ion calculation, the effect of the nearest cation

is only related to the distance measured from the center of

the cation, since only the Coulomb potential of point-charge

of that cation is considered. Thus, the interionic distance,

which is the sum of the size of H and that of the nearest

cation, will be related directly to the U center transition

energies. On the other hand, the effects of cations are

shown in the cluster calculation. This implies that the





49










0.5 -
POINT-ION MODEL




A
0.4 /
/7 /0 '/KBr










0.2 -
I 7
oC /7 KF

'0.3 /,5



LiF /
-------IVEY'S RELATION

0.2





0.6 0.7 0.8

LOG (d)




Figure 4.3. The logarithmic graph of the optical
absorption energies of the U center, AE
(in Ry), as a function of the nearest-
neighbor distance of the crystal, d (in
a.u.).







interactions between the hydrogen ion and different cations

have different degrees of deviation from the interactions

between a hydrogen ion and the approximated point-charges of

the cations. We can see in Figure 4.3 that the transition

energies calculated from the cluster for the sodium halides

and the potassium halides fall on different curves. The

reason for this is probably the different ionicities of the

sodium ion and the potassium ion. In other words, the

electronic charge distributions of different cations have

slightly different forms, a feature which cannot be included

in the point-ion model.

In order to investigate this possible difference, the

electronic charge distribution within each region in the

transition state cluster calculation for various alkali

halides is shown in Table 4.3. Now, the transition state

is, in the exact case, a mixed state corresponding to equal

weighting of single determinant ground and excited states

(Trickey, 1973). Comparing the charge distribution of the

ground state to that of the transition state in KC1, one

finds that the charge inside the K+ sphere, Q+, almost stays

the same (difference is less than 0.001 e), and a fraction

of an electronic charge (about 0.32 e) is transferred from

the hydrogen sphere to both the intersphere region and the

outer region as the cluster goes from the ground state to

the transition state. With this in mind, one can use the

general features of the charge distributions given in Table

4.3 to discuss the problem mentioned above.















TABLE 4.3

The distribution of electronic charges within each
in the U center transition state calculation.


Crystal Q Q- Qint out


KBr 17.842 1.450 1.274 0.224

KC1 17.846 1.410 1.242 0.271

KF 17.849 1.260 1.187 0.455

NaBr 9.954 1.455 0.681 0.143

NaCI 9.954 1.408 0.682 0.187

NaF 9.954 1.239 0.659 0.380

LiF 1.998 1.201 0.442 0.369


All calculations are
cluster.


carried out using the eight-center


region








From Table 4.3, one finds that the charges in the K+,

Na+, and Li ion spheres are approximately 17,85, 9.95, and

2.00 electron charges, respectively, thus, the net charges

inside the K Na+, and Li+ ion spheres are approximately

+1.15, +1.05, and +1.00, respectively. The electronic

charges of cations outside the ion sphere are mostly in the

intersphere region, and the Coulomb potential due to this

diffused charge of the cations in the hydrogen sphere is

smaller than the Coulomb potential with all these charges

in the cation spheres. Therefore, the net Coulomb effect of

a potassium ion gives an equivalent charge of more than +1.

The sodium ion also has an effective ionicity greater than

+1 but less than that of the potassium ion. The lithium ion

has an ionicity equal to +1. Since the alg orbital of the

U center is localized in the central part of the cluster

as one can see from the general features of the charge

distribution of this orbital given in Table 4.4, the energy

of this alg orbital is expected to be lower than that in a

point-ion field, and the amount of this difference for the

potassium halides is larger than that of the sodium halides.

On the other hand, the charge distribution of the excited

state (the tlu orbital) of the U center is mainly in the

intersphere region and the outer region (see Table 4.4).

Since in general there is more charge in the intersphere

region for the potassium halides than for the sodium halides,

the energy of this tlu orbital is expected to have a higher





53


TABLE 4.4

The charge distribution of the ground state and
the excited state orbitals of the U center
in a transition state calculation.


Orbital Crystal QH Q Qint Qo
H int out

KBr 0.891 0.004 0.085 0.001

KC1 0.872 0.005 0.096 0.001

KF 0.775 0.014 0.142 0.001

alg NaBr 0.883 0.003 0.099 0.000

NaC1 0.864 0.004 0.113 0.000

NaF 0.774 0.009 0.169 0.001

LiF 0.754 0.009 0.191 0.001


KBr 0.059 0.013 0.506 0.360

KC1 0.032 0.012 0.432 0.465

KF 0.002 0.006 0.163 0.801

tlu NaBr 0.119 0.013 0.541 0.263

NaCI 0.077 0.013 0.497 0.349

NaF 0.007 0.008 0.223 0.723

LiF 0.011 0.006 0.226 0.725









energy in the case of the potassium ions surrounding a H

ion than in the case with sodium ions surrounding a H- ion.

Therefore, the U center transition energies for potassium

halides will be generally higher than those of the sodium

halides. Precisely this sort of behavior is exhibited in

Figure 4.3. Of course, one should not compare the U center

transition energies between these two families of alkali

halides quite so directly. For example, in the cases of NaC1

and KC1, the small interionic distance of NaC1 is still the

dominating factor which makes the transition energy in NaCI

greater than that in KC1. On the other hand, the argument

given above does explain the cause of the calculated U

center energy in NaBr being smaller than that in KC1.

Another result is that there is a larger difference

between the calculated energy and Ivey's result in an alkali

fluoride than the corresponding differences in an alkali

chloride and an alkali bromide (see Figure 4.3). This

suggests that the relatively small size of the F ion has

a strong effect on the hydrogen ion. As shown in Table 4.4,

the charge distribution of the ground state of the U center

inside the central hydrogen sphere for an alkali fluoride

is less than that of the corresponding alkali chloride or

alkali bromide and the charge in the intersphere region is

increased in the case of an alkali fluoride. Thus, the energy

of this state is expected to have an additional shift upward

as compared to the other cases. For the U center excited









state in an alkali fluoride, the charge distribution is

mostly in the outer region, thus, this is a very extensive

orbital as compared to the other cases. A more accurate

representation of this state probably needs the consideration

of more neighbors into the calculation or other semi-empirical

methods, but the energy of this state is believed to be not

far from the present value. There are no experimental data

on the U center energies for the alkali fluorides. Thus, the

present calculated values cannot be evaluated directly.

Nevertheless, U center calculations in various alkali halides

which use the small cluster multiple scattering calculation

give a good description of the electronic structure and the

absorption energies. The absorption energies calculated for

KC1 and KBr are in excellent agreement with the observed

values, although this agreement is probably somewhat coin-

cidental since some effects were neglected in the calcula-

tions and the effects of the parameters and approximations

used in the calculation should be considered.

The most common effects discussed in the color center

calculation are lattice distortion and polarization. To

incorporate the lattice distortion into the present calcula-

tion is relatively simple. A preliminary result showed that

the U center transition energy is increased if the neighboring

ions are allowed to displace inward, and in fact, an estimate

can be obtained directly by using the graph shown in Figure

4.3. At present, the determination of the amount of the









lattice distortion near the U center by fitting data to the

experimental results is certainly unwarranted since there

are other effects to be considered. Thus, no attempt was made

in this regard, though a rough estimate showed that the U

center transition energy in KCl will increase about 0.004 Ry

if the first nearest-neighbor ions are allowed to displace

inward by 1% of the interionic distance of the crystal.

Furthermore, in the excited state the probability for finding

the electron outside the first nearest-neighbor distance

from the center will be appreciable. Thus, the lattice

distortion effect is expected to be more important in the

excited state than the ground state of the U center. Never-

theless, the lattice distortion caused by the U center is

certainly smaller than the distortion caused by the F center,

and one calculation (Wood and Joy, 1964) estimated the dis-

tortion of the ground state of the F center is of the order

of 1% or less.

When an electron moves out of the vicinity of the color

center, the surroundings of the electron will be polarized,

and this polarization will follow the movement of the elec-

tron. Gourary and Adrian (1960) have discussed the quasi-

adiabatic approximation to consider the instantaneous field

at the position of the electron. But there are many

difficulties in this approximation. For the one-electron

self-consistent field method, each electron is supposed to

respond only to the averaged positions of all other electrons.









Thus, the polarization effect cannot be considered in the

present calculation. For the ground state of the U center,

the electron will stay mainly in the central region and the

polarization effect will be negligible. For the excited

state, the polarization effect probably will lower the

energy of this state, but the effect is expected to be

small.

As to the effects of the approximations and parameters

used in the present calculation, the muffin-tin approximation

is expected to be a good approximation, since the X-ray

diffraction experiment (Witte and Wolfel, 1955) for NaCl

showed that the charge distribution is nearly spherical for

the ions and only a very small amount of charge is in-

between the ions. The gradient of the charge densities in

this interior region is small. For the case of LiF (Krug et

al., 1955), the charge distribution is less spherical near

the outer part of the ions as compared to the case of NaC1.

It is thought in this case the anions are relatively soft,

so that some overlapping of charge densities from different

ions can occur. One of the problems of using this approxima-

tion usually comes from the uncertainty in choosing the

radii for the atomic spheres. From the treatment of the

intersphere potential as a constant, one knows that an

appropriate choice of radius for each ion is certainly

essential. In section 3.4, the choice of the radii for the

ions has been discussed, and the X-ray corrected radii of








Gourary and Adrian (X.R.) are used for all the calculations.

In order to see the effects for using different sets of

radii, a U center calculation in KC1 with the ordinary ionic

radii of Goldschmidt (O.R.) was carried cut. The results of

these two transition state calculations in KC1 are shown in

Table 4.5. In the O.R. case, the size of the K+ ion is

decreased as compared to the size in the X.R. case, and this

decrease is the direct cause for the drop in electron charge

inside the K sphere. Thus, the net charge inside the K

sphere is about +1.32 which is higher than the net charge

(about +1.15) in the X.R. case. Similarly, the charge inside

the H sphere is increased due to the increase of the size

of the hydrogen sphere. The major effects of using the O.R.

as compared to the X.R. are the increase of the electronic

charge in the intersphere region and the decrease in energies

of the ground state and the excited state of the U center, and

also a decrease of the U center transition energy. The

increase of the charge in the intersphere region indicates

that the O.R. gives a worse representation for the size of

the ions in crystal than the X.R. does. The more positive

charge of the K+ sphere in the O.R. case gives a deeper

potential for the hydrogen ion, consequently, the energy

levels of the U center are lowered. The transition energy

of the U center is lowered by 0.01 Ry in the O.R. case which

is about 2.5% of the energy.

So far all the calculations have used the ionic













TABLE 4.5

Comparison of the U center transition state calculations
in KC1 with two different sets of ionic sphere radii.


X.R.a


VI(r Rout) c


Rout


E (alg)

(tlu)


Qint


1.335

8.768

3.100

2.834

0.427

-0.488

-0.061

0.270

1.240

1.410

17.846


O.R.b


1.335

8.448

3.420

2.514

0.417

-0.497

-0.080

0.255

2.157

1.509

17.680


All energies are in Rydbergs and distances are in a. u.
aRadii of Gourary and Adrian (1960).
bRadii of Goldschmidt (1926).
CCalculated from Equation (3.13).









correction potential defined in Equation (3.13). Several

calculations with the Watson sphere correction (see Equation

(3.12)) are shown in Table 4.6. In this table two sets of

radii are used in both the KCl and the LiF calculations.

Comparing the charge distribution of the cluster in KCl

with two different kinds of radii, one finds essentially the

same differences that are found in the calculations with the

averaged correction potential (see Table 4.5). With the

Watson sphere in the calculation, the potential inside the

cluster is lower than that with the averaged correction

potential in these calculations. This shifts the energy

levels of the U center down and a small fraction of the

charge redistributes from the outer region to the inside of

the cluster. The charges inside the cation spheres stay the

same. Comparing the transition energies found for different

ionic correction potentials, one finds that these energies

vary due to the relative insensitivity of the U center

excited state as compared to the ground state. Thus, the

correction potential for a cluster embedded in the solid

is essential in finding an accurate optical absorption

energy in the U center. Further, as can be seen in Table 4.6

the differences in the transition energies between calcula-

tions which use the X.R. and the O.R. are 0.02 Ry and 0.04 Ry

in the cases of KCl and LiF, respectively. The difference

for the case of KC1 is doubled as compared to that of the

previous calculations shown in Table 4.5. One important













TABLE 4.6

Comparison of the U center calculations
with different sets of radii using
the Watson sphere potential correction.


KC1 LiF

(O.R.) (X.R.) (O.R.) (X.R.)


VI(r Rout) 1.184 1.141 1.965 1.805

Rout 8.448 8.768 5.090 5.540

R 2.514 2.834 1.290 1.740

R_ 3.420 3.100 2.510 2.060

AE 0.438 0.458 0.624 0.662

E(alg) -0.628 -0.653 -0.766 -0.865

E(tlu) -0.190 -0.195 -0.142 -0.203

Qout 0.138 0.091 0.174 0.104

Qnt 2.200 1.350 0.860 0.652

QH- 1.540 1.439 1.380 1.217

Q+ 17.680 17.850 1.928 2.004


All energies are in Rydbergs and distances are in atomic
units. O.R.=ionic radii of Goldschmidt (1926). X.R.=
X-ray corrected radii of Gourary and Adrian (1960).








factor for causing these differences is the dependence of

the Watson sphere potential on Rout for the cluster (see

section 3.5). Therefore, the averaged correction potential

of Equation (3.13) is a better potential to use in the present

eight-center cluster calculations.

For most of the calculations in various alkali halides,

the X-ray corrected radii give fairly good results for the

transition energies, and the potential calculated has a

smaller discontinuity in crossing the boundary of the

neighboring ionic spheres than is the case when the ordinary

ionic radii are used. Exceptions are the cases of the alkali

fluorides, namely, KF and NaF. In these cases, preliminary

results show that the transition energies calculated by

using O.R. are somewhat higher than the previous results

and closer to Ivey's results. Above all, the variations of

the results due to different sizes of the ions are relatively

small, as one can see in the case of KC1, for which the

variation is only 2.5%.

Actually, any set of ionic radii is only an approximation

to the apparent sizes of the ions in crystals. Thus, in order

to get a more accurate result on the sizes of ions in each

crystal, one needs to find them separately by using the

criteria of charge densities or potentials. Eventually, the

removal of the muffin-tin restriction on the potential can

eliminate the necessity of choosing radii for the ion spheres

and the error introduced by this approximation.









In all calculations, the parameter a in the local

exchange potential for each region is not varied. The

detailed effects of choices of a have not been investigated,

but in general a higher a value will give the electron a

deeper potential. Thus, an a value greater than 0.77 in the

hydrogen sphere will probably lower the states of the U

center and will probably result in a slightly higher

transition energy for the U center.

Several effects associated with the present cluster

calculation have been discussed and most of these effects

are small. From Figure 4.2, the calculated energies can be

seen to deviate from Ivey's relation in the region of small

interionic distance. One possible cause is the assumption

of the point charges for the surrounding ions in calculating

the average correction potential. This may become improper

as the interionic distance gets too small. There are no

experimental results on a U center in the region of small

interionic distances. A similar problem will be discussed

later, in the section on F center calculations.

A calculation on the U center in KC1 using the twenty-

center cluster (including the outer sphere) has also been

carried out. In this cluster the central hydrogen ion is

surrounded by six K ions and twelve Cl ions. The radii of

the atomic spheres are the same as those used in the eight-

center cluster calculation. The a values used in various

regions are given in Table 3.1. At first, the correction









potential used in the calculation is chosen simply to be

an average of the ionic correction potentials at the centers

of all atomic spheres which can be calculated by using

Equation (3.16). This constant correction potential for an

electron inside the cluster equals to -8.32/d Ry, where d is

the nearest-neighbor distance of the crystal. From the tran-

sition state calculation the U center absorption energy is

found to be 0.34 Ry which is about 20% lower than the

observed value.

Another calculation has used the averaged correction

potential defined in Equation (3.13). This correction

potential for an electron inside the cluster equals

-7.94/d Ry which is higher than the previous potential by

0.067 Ry. The results of the transition state calculation

show no significant change from the previous calculation.

The calculated U center absorption energy is equal to 0.32

Ry. The charge in the intersphere region has 10.12

electronic charges which is very large compared to that of

an eight-center cluster calculation. This is' mainly from the

fact that only 17.19 electronic charges are within the Cl

sphere and the rest of the charge of the Cl ion is in the

intersphere region.

The possible cause for the low transition energy is

believed to be the inaccurate correction potential used in

the calculation. For a small cluster the averaged potential

defined by Equation (3.13) may be an appropriate approxima-








tion. But for a larger cluster, the deviation of this correc-

tion potential from the averaged value in each region may be

appreciable. This is especially important in the present

calculation of the U center absorption energy. Since the

ground state of the U center is in general concentrated in

the hydrogen sphere region and the excited state is mainly

distributed in the intersphere region, a difference of the

correction potentials in these two regions will affect the

resulting transition energy almost directly.

The ionic correction potential inside any atomic sphere

can be calculated from Equation (3.16), and this potential

inside the hydrogen sphere is -8.46/d Ry. The correction

potential in the intersphere region can be calculated by

Equation (3.17). Since the averaged correction potential

inside the cluster region has been calculated for this

twenty-center cluster (see Table 3.3), one can use this

potential to replace the Vi(o) term in Equation (3.17).

This will give a more accurate correction potential for the

intersphere region (because of the fact that the parts of

the surrounding ions inside the "outer sphere" of the

cluster are excluded in the calculation of the averaged

correction potential) and this correction potential calculated

in the present case is -7.7/d Ry.

Using the correction potential for each region discussed

above, one can see that the difference of these potentials

between the intersphere region and the hydrogen region is








about 0.13 Ry. Thus, this correction potential for the

cluster is expected to give a better U center absorption

energy than the previous two calculations. The charge inside

the Cl sphere is expected to increase also. A calculation

using this ionic correction potential for each region has

not yet been done.

The twenty-eight-center cluster model shown in Figure

2.3 has also been used for the U center calculation in KC1.

In this cluster the hydrogen ion is surrounded by three

shells of ions. The first shell consists of six K ions and

the second shell consists of twelve Cl ions and the third

shell consists of eight K+ ions. The radii used for the

atomic spheres and the a values used in various regions

are the same as those used in the smaller cluster

calculations.

A transition state calculation using this large cluster

has been carried out. In the calculation the averaged

correction potential given in Equation (3.13) is used. The

calculated U center absorption energy is 0.48 Ry which is

about 12% higher than the observed value.

In this large cluster transition state calculation, the

hydrogen alg levels of both spins are below the group of

levels which arise from the chlorine 3p states, while in the

previous twenty-center cluster calculations the hydrogen al

level with spin up (occupation number is 0.5) is above the

chlorine 3p levels and the other hydrogen alg level is below.









Furthermore, the potentials in various atomic spheres are

found to be deeper in the twenty-eight-center case than in

the twenty-center case with different magnitude. Thus, large

differences are found between the two cluster calculations

in potentials and eigenvalues. This is also shown in the

calculated U center absorption energies. The reason for the

fluctuation of the calculated energies from small cluster

calculation to the large cluster calculation is believed to

be the use of a constant correction potential inside the

entire cluster and also the use of a constant potential in

the intersphere region. As mentioned before, the ionic

correction potential for each region of the cluster can be

found using Equation (3.16) and Equation (3.17). Thus, it

is interesting to see the effects of this form of the correc-

tion potential used in the calculation.

A twenty-eight-center cluster calculation of the ground

state of the U center using the above mentioned correction

potential has been done and the results show no spin-polarized

effect in the eigenvalues and potentials as would be expected

in a closed shell system. The diagram of the calculated

eigenvalues is shown in Figure 4.4. The U center ground

state is an alg orbital arising from the hydrogen Is state

with energy equal to -0.545 Ry. Below this level there is

a group of fifteen levels (see Appendix B) from the second

nearest-neighbor (2nn) Cl 3p levels. These levels will

form the valence band in a band picture of the crystal when









more and more ions are added to the cluster under considera-

tion. The discussion of this valence band and comparison with

other calculations and experimental data will be given in

section 4.5. Below the valence levels, the first group near

-1.34 Ry consists of ten levels which arise from the 3nn

K 3p levels. The width of this group of levels is very

narrow because of the relatively small interactions with

other states outside this group. Between -1.42 Ry and -1.48

Ry there are twelve levels. Five of them are from the 2nn

Cl- 3s levels and the rest are from the Inn K+ 3p levels.

Those orbitals mainly concentrated in the Inn K ions have

somewhat higher energy as compared to those concentrated in

the Cl ions. Since these levels are close in energy, they

are actually a mixture of the functions from the two groups

of ions. Several unoccupied orbitals are also shown in

Figure 4.4. The lowest of the unoccupied states is an alg

orbital. For a U center transition, the lowest unoccupied

tlu orbital is the first allowed excited state.

The calculated electronic charges in each region of

the present cluster are shown in Table 4.7. In the same

table the potential near the boundary of each atomic sphere

is also given. As one can see, these potentials are approxi-

mately the same for the atomic spheres. This indicates that

the correction potentials for the atomic spheres calculated

from Equation (3.16) are adequate. For the case of KC1 the

correction potentials used in the intersphere region,











SPIN UP


SPIN DOWN


e
alg
tlu -------~-
alg --------


0.0



-0.1



-0.2



-0.3



-0.4



-0.5



-0.6


(3nn)



(Inn) -
(2nn) . . .


Figure 4.4.


The electronic structure of the U center
ground state in KC1 from the twenty-eight-
center cluster calculation.


e

tlu
alg


alg

(2nn) .. . .
-( n


-0.7




-0.8



-1.3



-1.4




-1.5




-1.6


Cl- 3p


K+ 3p


K+
Cl-


--~-~~----~
---~---














TABLE 4.7

The electronic charges and the potential in each region
from the twenty-eight-center cluster calculation
of the ground state of the U center in KC1.


Region Q Va


Intersphere 9.49 -0.35

Outer 0.05

H- (C) 1.74 -0.67

K+ (Inn) 17.96 -0.79

Cl- (2nn) 17.33 -0.67

K+ (3nn) 17.88 -0.65


near


values given for sphere regions are the potentials
the sphere boundaries.








hydrogen sphere, Inn sphere, 2nn sphere and 3nn sphere are

0.125, 0.13, 0.148, 0.079 and 0.23 Ry, respectively. As

one can see the use of a constant correction potential for

the entire cluster is certainly inappropriate.

A transition state calculation using the twenty-eight-

center cluster and a separate correction potential for each

region has been tried, but the calculation has not converged

due to the difficulty in tracking the hydrogenic alg orbital

which oscillates about the chlorine valence levels. From

the correction potentials in the intersphere region and in

the hydrogen sphere, it is expected that the calculated U

center absorption energy will still be too large. Further

discussions on the present correction potential for the

cluster will be given in section 4.5 where the cluster

calculation is compared with the energy band calculation and

other results.



4.2 The F Centers in Alkali Halides

Calculations of the peak energy of the F center absorp-

tion band in several alkali halides have been carried out

using the spin-polarized MS-Xa-SCF method. In these calcula-

tions, the model used for the F center is the eight-center

cluster shown in Figure 2.1. A few calculations using the

twenty-center cluster and the twenty-eight-center cluster

have also been done in the case of KC1.

The F center calculations are similar to the U center








work discussed in the previous section. In fact, the

potential from the U center calculation is used as the

starting point for the present F center self-consistent

calculation. For an F center, the central sphere of the

cluster is occupied by an electron. The a value used for

the local exchange terms in that region is chosen to be the

same as the a value in the surroundings. Thus, for the

eight-center cluster, the a values in all regions are the

same as Schwarz' aHF values (see Table 3.1) for the first

nearest-neighbor cation. For larger clusters (in KC1) the

a value in the central sphere is chosen to be 0.72 which is

the same value used for the rest of the cluster. The radii

used for the atomic spheres are the X-ray corrected radii

(see Table 3.2). In most of the calculations the averaged

correction potential defined in Equation (3.13) is used.

The ground state of the F center in KC1 using the

eight-center cluster has been calculated and the eigenstates

are shown in Figure 4.5. The diagram is similar to that for

the U center eigenstates diagram shown in Figure 4.1. But

the spin-polarization effect caused by the unpaired F center

electron is clearly shown in the present case. The F center

electron occupies an alg orbital with spin up at -o.183 Ry.

The probability for this electron in the central sphere

region is about 0.517 from the present calculation and the

probability in the intersphere region is about 0.357. Thus,

the F center ground state orbital is localized in the anion












SPIN UP

SU


SPIN DOWN


0.00






-0.10-






-0.20-






-0.30






-0.40-






-1.26


-1.27


-1.28 -


-1.29 -


(Inn)


- i-- K 3p


Figure 4.5. The electronic structure of the F center
ground state in KC1 calculated by using
the eight-center cluster.


S(F)









vacancy although it is not as localized as the U center

ground state orbital in the central sphere of the cluster.

On the other hand, the first allowed excited state of the

F center (which is the tlu orbital with spin up at -0.029

Ry) is relatively extensive. Thus, the probabilities for an

electron in this orbital to be found in the intersphere

region and outer region of the present cluster are 0.464 and

0.432, respectively. The calculated potential for the spin

up electron in the anion vacancy is nearly flat. This gives

some justification for the simplified potential well model

for the F center mentioned in section 2.4. However, the

actual problem is more complicated as one notices from the

extensiveness of the excited state wave function of the F

center.

The calculated F center absorption energies, using the

transition state procedure on several alkali halides, are

given in Table 4.8 along with the experimental results and

Ivey's values from Equation (2.1). Symbols used in this

table are the same as those used in Table 4.2 and are

explained in section 4.1. As one can see, the calculated

absorption energies in NaC1, NaBr, and KC1 are in good

agreement with the observed values. In Figure 4.6 the F

center energies given in Table 4.8 are plotted against the

nearest-neighbor distance of the crystal.

As discussed in the case of the U center absorption,

Ivey's relation gives the averaged experimental results and






















tI
0)

*Hl
rd

.,I




H
4J



0
SU





ar-
cnU

0) 4















h I
wr 0


0
0)m




4-i
O




HO
W 0 Q)



















Cd

0



0



E-H
E-4^


H-1
4 (4 4
E 0 0
MMM M


H


X


rd
v-


r11
&-i fa
*i-i rd (
i-l Z


4-


-I



0










-H
4-i














ro
Cd




0


4-i




















i 0
- H
4-I
(0










-H H
crd












i-H
0)o












-1-
[f 11







(U C,

m- Qa)















0.38


A MS-Xa
0.35-
0 EXP
SIVEY'S RELATION
0.32-


0.29


S0.26
z

P 0.23 -


< 0.20- A


0.17


0.14-



4.0 5.0 6.0

NEAREST-NEIGHBOR DISTANCE (A.U.)



Figure 4.6. The optical absorption energies of the
F center as a function of the nearest-
neighbor distance of the crystal.









in the present (F center) case, Ivey's relation gives a good

approximation to the observed values for all alkali halides

shown in the graph. On the other hand, although the present

calculated F center energies for most of the alkali halides

are in good agreement with the experimental values, the

energy calculated for the LiF crystal is 0.231 Ry which is

about 0.13 Ry below the experimental value. If one checks

the similar U center calculation shown in Figure 4.2, one

finds essentially the same differences (compared to Ivey's

result) near the small nearest-neighbor distances of the

crystal. This indicates that the errors in both cases

probably come from the same source. The possible causes are

the neglect of ionic overlap (caused by the muffin-tin

assumption) in the LiF case and the inadequacy of the

correction potential calculated from point charges as the

interionic distance becomes small.

Above all, the calculations using the eight-center

cluster give good results on the F center absorption

energies for most of the ionic crystals. There are other

effects associated with the problem that are omitted in

the present calculation. For example, the lattice distortion

effect and the polarization effect are expected to be

more important in the present case than in the U center

case. The displacement of cations toward the vacancy will

increase the F center transition energy. But the severity

of the above mentioned effects is not known. Effects








associated with the exchange parameters and other parameters

have been discussed in the previous section; that discussion

is also applicable to the present case.

From the ground state calculation of the F center in

KC1, the wave function of the F center is obtained. With

knowledge of the value of the wave function at the neighboring

nucleus, the contact interaction term can be calculated

(see, for example, Seidel and Wolf, 1968). In the present

case the calculated value is about five times greater than

the observed value. The discrepancy may be the consequence

of using a finite size cluster.

The electronic structure of the ground state of the

F center in KC1 has also been calculated in the context of

the twenty-center cluster. The averaged value of the correc-

tion potential at the centers of all the atomic spheres is

used as the correction potential inside the cluster. Thus,

VI(r) equals -8.32/d Ry for all r smaller than Rout*

Other parameters used in the calculation are the same as

those in a similar U center calculation except that the a

value for the central sphere is 0.72 in the present case.

The eigenstates calculated from this twenty-center

cluster are shown in Figure 4.7. A comparison with the

eight-center cluster result shows that the inclusion of the

second nearest-neighbor Cl ions introduces a band of 15

levels at about -0.55 Ry for each spin. These two bands

arise from the atomic 3p levels of the twelve chlorine ions

and have the same width (0.047 Ry). The F center electron










SPIN DOWN


0.0



-0.1



-0.2



-0.3



-0.4



-0.5



-0.6



-0.7



-0.8



-1.3



-1.4



-1.5



-1.6


tlu
alg

-1 T

alg (F)












(2nn) _
















(Inn)
(2nn) --


Figure 4.7.


The electronic structure of the F center
ground state in KC1 calculated by using
the twenty-center cluster.


SPIN UP


alg
tlu
--------_-- t^u


































K 3p
-- --- C- 3s
__________- Cl 3s









occupies an alg orbital at -0.254 Rv. As one can see from

the diagram, the spin polarization effects of the unpaired

F electron on the Cl- 3p band and the K+ 3p and Cl- 3s

bands below are relatively small as compared to the unoccupied

levels. It is also found that the F electron level is closer

to the unoccupied levels than in the eight-center cluster

case.

A transition state calculation shows that the F center

absorption energy is only 0.124 Ry which is about 26% below

the experimental value. This is the same sort of situation as

that which happens in the twenty-center U center calculation.

The addition of the twelve Cl ions has apparently pushed

the ground state level of the color center too high. It is

believed that a more accurate correction potential for the

large cluster is essential, especially in the intersphere

region.

The ground state of the F center in KC1 has also been

calculated using the twenty-eight-center cluster. In the

calculation, two kinds of correction potential were used.

The first is defined in Equation (3.13); it is the averaged

correction potential inside the cluster. The electronic

structure which results from this calculation is shown in

Figure 4.8. The second correction potential is defined in

Equation (3.16) and Equation (3.17); in this scheme each

region has a separate correction potential. The calculated

electronic structure is shown in Figure 4.9. A comparison











SPIN DOWN


tlu
alg




alg


-F)


-0.1


-0.2


-0.3


-0.4


-0.5


-0.6


-0.7


-0.8






-1.3


-1.4


-1.5


-1.6


alg
tlu

alg





















_K Cl 3p








C 3s

K+ 3p


Figure 4.8.


The twenty-eight-center cluster calculation
of the ground state of the F center in KC1
using the averaged correction potential.


0.0 r-


(2nn) ----____








(2nn) E- L-Lf--wu^
(3nn) ----
(Inn) .. . .


SPIN UP











SPIN DOWN


tlu
ag


alg


alg


(F)


(2nn)
ali~=i~_i~i~--=-- 3---


=-- -". C- 3p
--=-- Cl 3


(3nn) - -




(2nn)
(Inn)


K + 3p



C1 3s
K+ 3p


Figure 4.9. The twenty-eight-center cluster calculation
of the ground state of the F center in KC1
using the separate correction potential.


0.0



-0.1 -


-0.2



-0.3


-U.4 -



-0.5 -


-0.6




-0.7




-0.8


-1.3



-1.4



-1.5



-1.6


r


SPIN UP









of the two calculations yields an energy difference between

the F center level and the first unoccupied tlu level of

0.178 and 0.182 Ry, respectively, for the two cases. These

energies are very close to the F center absorption energy

in KC1. The gap between the first unoccupied level and the

top of the valence band with spin up is 0.397 and 0.432 Ry

respectively. The slightly higher value for the second case

is a result of the correction potential used for the 2nn

atomic sphere being smaller than those for the central

sphere and intersphere region. The effect of the separate

correction for each region in the second calculation can

also be seen from the lower bands as compared to the first

calculation. In Figure 4.8 the highest of the three bands

shown is the 2nn Cl 3s band whereas in Figure 4.9 the highest

band is found to be the 3nn K+ 3p band.

A comparison of the electronic charges inside each

region and a comparison of the calculated potentials for

a spin-up electron in the intersphere region and near each

sphere boundary (which result from several F center ground

state calculations in KC1) are both shown in Table 4.9. The

electronic charge inside each ion sphere is seen to increase

with the increase of the size of the cluster. On the other

hand, the charge in the intersphere region is seen to de-

crease from the twenty-center case to the twenty-eight-center

case. The twenty-eight-center cluster has a total charge of

+1, thus, a relatively small correction potential is needed











TABLE 4.9


A comparison of the electronic charges and the
in each region of the cluster between several
ground state calculations in KC1.


potential
F center


8-center 20-center 28-centera 28-centerb


Qint 1.32 10.14 9.78 9.56

Qout 0.07 0.41 0.06 0.05

QF 0.56 0.43 0.62 0.61

QK+ (Inn) 17.85 17.95 17.98 17.97

QC- (2nn) 17.19 17.29 17.33

QK+ (3nn) 17.89 17.88

Vint -0.18 -0.42 -0.32 -0.35

V F -0.59 -0.58 -0.72 -0.76

VK+ (Inn)c -0.57 -0.73 -0.80 -0.81

VC1-(2nn)c -0.56 -0.59 -0.67

VK+ (3nn)c -0.71 -0.65


All energies are in Rydberg.
aUse correction potential in Equation (3.13).
bUse correction potential in Equations (3.16) and (3.17).
CValues given are potentials near the sphere boundary.








in the calculation as compared to that in a smaller cluster

calculation. It seems that this large cluster should give

better results for the present problem. The calculated F

center absorption energy in KC1 turns out to be 0.222 Ry

and 0.223 Ry using the two different kinds of correction

potential mentioned before. These energies deviate from the

observed value by about the same amount but opposite direc-

tion of that found in the twenty-center calculation. This

oscillation of deviation with increasing number of shells

can also be seen in the position of the F center level in

various diagrams shown in Figures 4.5, 4.7 and 4.8. The

difficulty in the present case is traceable to the fact that

the F center excited state is not localized while the F

center ground state is fairly well localized in the central

region of the cluster. Thus, a small deviation of the rela-

tive magnitude between the constant potential in the inter-

sphere region and the potential inside the central sphere

(which is nearly constant for the case of the F center)

will affect directly the calculated transition energy.

Clearly, a more accurate evaluation of the correction poten-

tial between spheres is needed. One method is to integrate

the potential in the intersphere region directly using the

method discussed in Appendix A or to use the calculated

averaged value inside the cluster combined with Equation

(3.17). Estimates have been done and they indicate that the

calculated transition energy will be improved in both the

twenty-center and twenty-eight-center cases.









From the above discussion, we know that the eight-

center cluster calculations give reasonably good results on

the F center absorption energies. But the calculation of

the contact term for the spin interaction between the F

center electron and the neighboring ions indicates that the

small size of the cluster has compressed the wave function

of F electron and gives a contact term too high compared

to the experimental value. Thus, larger clusters are needed

to see the improvement on the F center wave function. From

the ground state calculation of the large cluster, the

contact term is improved, although the value is still too

high. On the other hand, the transition energy is found to

be worse than that obtained in a small cluster calculation.

Improvement of the correction potential has been discussed

before, but there are also other difficulties which perhaps

are associated with the large cluster. Thus, calculations

with a cluster of pure crystal have been done. A study of

these calculations can give some information about the

present cluster approach. Results of these calculations will

be given in section 4.5.



4.3 The U, Center in Potassium Chloride

The calculation of the U2 center in KC1 has been done

using the cluster shown in Figure 2.2. In this cluster a

hydrogen atom is surrounded by four K ions and four Cl ions.

Since this cluster is electrically neutral, no Watson sphere









is needed in this case. The parameters used in the calcula-

tion are the same as those used in the previous U center

calculations except for the radius of the interstitial

hydrogen sphere which is choose to be as large as possible

without overlapping the other spheres. By this criterion,

the radius of the hydrogen sphere is 2.04 atomic units.

The calculated orbital energies of the ground state

of this center in KC1 are shown in Figure 4.10. The hydrogen

ground state is an a, orbital with spin up in the present

calculation. This hydrogenic al orbital is mixed with the

al orbital from the neighboring chlorine ions. From the

charge distribution of the orbitals, it is found that the

most hydrogen-like al orbital with spin up is below the group

of levels from the Cl 3p levels while the unoccupied

hydrogenic al orbital with spin down is above the chlorine

levels. Thus, in this spin-unrestricted calculation, we

know that the system with two electrons in the hydrogen Is

levels plus a hole in the chlorine 3p levels has a higher

energy than that which has filled chlorine 3p levels and one

electron in the hydrogen Is level. This situation was

unclear from the energy considerations of Cho et al. (1966),

although the same conclusion was reached.

In finding the excitation energy of this center, there

are two possible forms of transition. The first possibility

is an electron excited from the highest occupied a symmetry

orbital, which is a mixture of the hydrogen Is level and the











0.0-





-0.1-





-0.2-





-0.3






G> -0.4

z



-0.5





-0.6-





-0.7-


Figure 4.10.


SPIN DOWN


3t2


3a1




















2a1

2t2
Itl
it1
le
It2


3t2


3a1










2a1


2t2
it1
(Cl 3p) le
it2


1a1


lal


The electronic structure of the ground state
of the U2 center in KC1.


SPIN UP









neighboring ClI 3p levels, to the first unoccupied t2 orbital.

The second possible form of excitation is to excite an electron

from the highest occupied spin down t2 orbital, which is an

orbital concentrated on the four Cl ions, to the unoccupied

hydrogen al orbital. The excitation energies calculated from

these two kinds of transition are 0.395 and 0.213 Ry, respec-

tively. Comparing these results to the experimental U2

absorption energy, 0.387 Ry (Fischer, 1967), one finds that

the first form of transition is likely to be the process.

In most of the previous calculations on optical transitions

in a U2 center (Mimura and Uemura, 1959; Hagston, 1971), the

absorption process was thought to be a charge transfer

which is equivalent to the second form of the transition in

the present calculation. There is no direct proof of this

charge transfer process in a U2 center absorption. In order

to see the effect of the possible lattice distortion around

the hydrogen atom upon the transition energy, a calculation

of the charge transfer process with the Cl ion and the K

ion displaced outward and inward, respectively, along the

line joining the ion and the hydrogen atom was carried out.

The result shows an even smaller energy for the U2 center

absorption. Thus, from the present calculations the process

of charge transfer cannot account for the observed absorption

energy, and the process with an electron excited from a

hybrid chlorine-hydrogen al orbital to a higher state gives

a good interpretation for the U optical absorption.
2









The proton hyperfine interaction of the U2 center in

KC1 has been investigated by many workers in this field.

Spaeth and Seidel (1971) used the orthogonalized function

method and obtained a relative proton hyperfine constant

shift 5 = 9.27%, where 6 is defined by
aHi aH
6 = (4.1)
aH

and aHi is the proton hf constant for the U2 center while

aH is the hf constant for a free hydrogen atom. Since the

experimentally determined aHi (1378 MHz) is less than the

free hydrogen hf constant (1420 MHz), the relative proton

hf constant shift is equal to -3.01% (Spaeth and Sturm,

1970). To improve the calculated value, they tried to take

into account the van der Waals interaction between the H

atom and its neighboring ions. This has the effect of shifting

electron density away from the proton into the outer region

of the H atom resulting in a value of 6 = -9.73%. Another

model to improve the calculation of 6 is to include the

crystal field effects using the ligand field model. In this

way, Hagston (1970) obtained a value of 6 between -1% and

-2% in a semi-empirical calculation.

In the present calculation, the proton hf constant is

calculated from the spin density at the position of the

proton which can be calculated from the following expression:


( Z 1 r) 2 rLd I 'ir 1(4.2)
Mt blt









where n. is the occupation number of the ith state and

Yit (r) is the wave function of the ith state at position r.

The summation of i is over all states with spin up and the

summation of j is over all states with spin down. Using the

spin density at the proton, the calculated 6 is equal to

-4.5% which is in fairly good agreement with the experimental

value.

The calculation of the hf contact term at the nuclei of

the Cl and K ions has also been done using the spin density

obtained from Equation (4.2). Comparing with the experimental

results the calculated hf contact term for the Cl ion is

overestimated and that for the K ion is underestimated and

is negative. In order to see the effect on the distribution

of the spin-polarization due to a particular choice of the

cluster, a different cluster for the U2 center in KCl was

used. This cluster has a K ion at its center with six

Cl ions surrounding it and also has a hydrogen atom located

at (d/2, d/2, d/2) of the cluster, where d is the nearest-

neighbor distance of the crystal. The structure of the cluster

is similar to that shown in Figure 2.1 except that there is

an interstitial atom in the present case and the cluster has

C3v symmetry. The calculated hf contact term for the K+ ion

in this case is improved and has a value near zero. The hf

contact term for the Cl ion is lowered. Thus, overall the

cluster with C3v symmetry has improved results for the hf

contact term compared to the tetrahedral cluster, but the

results are not satisfactory. Since in the present calcula-








tion the spin polarization is mainly within the outer sphere

of the cluster in contrast with the experimental results that

show spin polarization in the third shell of ions, a larger

cluster may be used to improve the results. On the other hand,

the hf contact term of the nearest-neighbors of the inter-

stitial hydrogen can also be calculated approximately using

the one-electron wave function of the unpaired electron from

the tetrahedral cluster calculation. The hf contact term at

the Cl nucleus in the first shell is found to be 18.0 MHz

which is roughly comparable to the experimental value of

23.7 MHz. At the K nucleus, the calculated hf contact term

is 3.1 MHz and the experimental value is 1.0 MHz. These

results are better than the results obtained using Equation

(4.2). But the calculated spin density at the proton position

using the one-electron wave function is found to be too small.

The reason for this low spin density is probably the fact

that the hydrogen-like al orbital is mixed with the al orbi-

tal from the neighboring Cl ions shifting the spin density

away from the proton.



4.4 The U1 Center in Potassium Chloride

The U1 center is similar to the U2 center discussed in

the previous section except that the interstitial impurity is

a negative hydrogen ion in the present case. The study of

this center in KC1 has been done using the tetrahedral

cluster shown in Figure 2.2. A Watson sphere with charge of

+1 was used to enclose the whole cluster.









The calculated orbital energies are shown in Figure

4.11. Contrary to the case of the U2 center, both of the two

hydrogenic al orbitals are above the group of five levels

arising from the Cl 3p levels. The optical absorption

process in this case is thought to be an electron excited

from the hydrogenic al orbital to the first unoccupied t

orbital. The transition state calculation has been done

and shows a transition energy of 0.18 Ry. There are a few

experimental measurements of the absorption band of the

U1 center in KC1 (Delbecq et al., 1956; Rolfe, 1958), but

the peak of this band generally is not clearly defined.

It is estimated that the peak of the absorption band is at

about 0.32 Ry and the edge of this band is about 0.26 Ry.

In the case of this interstitial impurity, the interac-

tion between the hydrogen ion and the neighboring ions is

believed to be greater than that of the previously discussed

centers. Consequently, the displacement of ions with respect

to their equilibrium positions is more important. Thus,

two more calculations with displacementswere carried out.

In these calculations, the chloride ions were allowed to

displace outward along the cubic diagonals and the potassium

ions were allowed to displace inward along the cubic diagonals.

The hydrogen-like level was found to shift down because of

the relaxation of ions surrounding it, and the resulting

transition energies were 0.22 Ry and 0.25 Ry with the ions

displaced approximately 5% and 8% of the nearest-neighbor





S94


SPIN UP


SPIN DOWN


(U,)


---- (Cl- 3p)


Figure 4.11.


The electronic structure of the ground
state of the U1 center in KC1.


3ao


3 t2


3a


0.0-i


-0.1-


-0.2-


-0.3-


-0.4-





-0.5-


t2
It1
le
It2

la1


2t
it1
le
it2

lal


S20,









distance from their crystal equilibrium positions, respec-

tively.

Thus, from the above results, we know that the

displacement of the neighboring ions of the hydrogen played

an important role in the U1 center structure. Since the

actual amount of displacement or other distortions of the

crystal lattice are not known, we can only qualitatively

understand the problem. For more accurate calculations, the

polarization effect due to the extra electron on the hydrogen

and the more general form of potential must be considered.



4.5 Cluster Calculation of a Pure KC1 Crystal

In the previous sections of this chapter we have

discussed the calculation of several color center problems

using clusters of several different sizes. In section 3.5

we have also discussed the correction potential needed to

simulate the potential which arises from the rest of the

crystal when the cluster is embedded in the solid. From the

calculations on U centers and F centers we know that the

orbital energies of the cluster are affected by the correc-

tion potentials which are calculated approximately from

point charges. It is interesting to compare the electronic

structure from a calculation on a pure alkali halide crystal

using the present cluster method with that from an energy

band calculation or experimental data, e.g., band gap, band

width, etc. of a pure crystal. In this way some insight




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