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UNITED STATES ATOMIC ENERGY COMMISSION
THE STRUCTURE OF URANIUM HYDRIDE AND DEUTERIDE
R. E. Rundle
Iowa State College
This document consists of 8 pages.
Date Declassified: February 10, 1947
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.THE STUIUCTURE OFPMRANIUM HYDRIDEAND DEUTERIDE
:* Bsy RL E. Rundle
Uranium metal reacts with hydrogen to form only one hydride.* This hydride, established as a
compound by X-ray diffraction,' has been found chemically to have the composition UH,.' The hydride
of uranium, if existing data on other metallic hydrides is reliable, is unique in that it does not fall in
the class of volatile hydrides, salt-like hydrides, or interstitial solution hydrides. It is a metal-like
hydride of perfectly definite composition and with a structure completely unrelated to any of the three
forms of uranium meiital. It has a stiuctuire in 'which the bonding between uranium and hydrogen must
play a predominant.role, since metal-metal bonds of any strength are almost completely lacking.
UNIT CELL AND X-RAY DENSITY
Debye-Scherrer powder diagrams of URH, have been made using CuKo radiation, a camera of
5-cm radius, and samples sealed in thin-walled glass capillaries. These diagrams may be inter-
preted in terms of a primitive cubic Lhttice, a = 6.63 A. The density of the hydride has been meas-
ured by helium displacement and found to be 10.95 g/cc?. There are then eight uranium atoms in
the cubic unit.
A precise determination of the unit cell has been made using a symmetrical, self-focusing
powder camera of 5-cm radius and unfiltered CuKa radiation. Samples were prepared from purest
Ames uranium and carefully purified hydrogen (purified by decomposition of UHl.t The samples of
Table 1 were prepared at about I atm hydrogen pressure and at relatively low (200 to 300"C) tem-
peratures. Due to small particle size these samples produced satisfactory but not particularly
sharp reflections in the back reflection region.
Other samples, prepared by W. Tucker and P. Figard, were made at pressures up to 1800 psi and
temperatures up to 500 to 600TC. These samples showed considerable growth in particle size and
produced very sharp maxima in the back reflection region. The lattice spacing, as determined from
these samples (Table 2), is more precise but within the limit of experimental error of the spacing of
the low pressure hydride.
-Uranium metal has been heated with UH, and two phases are maintained. Changes in spacing
of:neither uranium metal nor of hydride were great enough to be detected. For example, a sample
50% U metal and 50% UH, gave a hydride spacing of 6.63010.002 A. Neither hydrides prepared at
high hydrogen pressures nor hydrides prepared with excesses of metal showed any alteration in the
UH, spacing. It must be concluded that the composition of UH, is perfectly definite, that there is no
appreciable solubility of hydrogen or uranium in the hydride at ordinary temperatures and pressures.
We consider the best value of the hydride spacing to be 6.6310 0.008 A. The X-ray density,
calculated In accordance with the recommendations of Jette and Foote,4 is 10.92 g/cc, in excellent
agreement with the 'experimental value obtained by helium displacement.
'Other physical and chemical reports on the hydride will be made in J. Am. Chem. Soc. by
F. Spedding, A. Newton, J. Warf et al.
tThis method of purification, first described by A. Newton, has been shown to be very effective.'
Table 1. Lattice constant of UH prepared at low pressure.
No. of lines Spacing Estimated ertor
(A) (one film)
Sample I 8 6.6323 t 0.0010
Sample 1" 6 6.6324 .0009
*Measured by another observer.
Table 2. Lattice constant of UH prepared at 1800 psi H .
Sample No. lines Spacing Estimated error
(A) (one film)
A 12 6.6308 + 0.0008
B 12 6.6317 .0008
C 12 6.6312 .0005
D 12 6.6319 .0008
E 19 6.6306 .0008
At 12 6.6302 .0014
Bt 12 6.6304 .0009
Ct 12 6.6301 .0017
Dt 12 6.6317 .0011
t Measured by another observer.
LATTICE CONSTANT OF THE DEUTERIDE
The deuteride, U4|, has been prepared at low pressures. Precision diagrams, taken as before
with CuKo radiation, lead to a lattice constant, a = 6.620 .002 A, more than 0.01 A smaller than the
hydride spacing. This change in lattice constant is far greater than the experimental error. The
X-ray density of the deuteride is 11.11 gjcc.
SPACE GROUP AND STRUCTURE.'
Powder diagrams of UH, show many absences. These absences, which do not appear even on
the most intense powder diagrams, seem to be systematic and most extraordinary. They Include
reflections with the following forms of Miller indices:
(4n, 4n' + 2, 4n" 4 2); (4n, 4n', 4n" + 2); (4n + 2, 4n' + 2, 2n" + 1);
(2n + 1, 2n' + 1, 2n" + 1); (4n, 2h'+ 1, 2n" + 1)
All other possible reflections have been observed with good intensity on powder diagrams made
with CuKa radiation.
These absences correspond to no special set of equivalent positions in the cubic system, and it
can be shown that no-set of eight equivalent urardium atoms with any parameters in any space group
will lead to these absences. The same can be shown for any two sets of four equivalent positions
for the uranium atoms.
The space groups Oh, 0 and T, provide two equivalent positions plux six equivalent positions
which combined lead to a structure thit requires the absences noted and no others. These posi-
tions are: two atoms in (a, it 000, 5Hi, and six atoms in (c) at;0s, 2 0, 0D2, h0o, 02t, O, (or the
six equivalent positions may be taken -a Irom the positions here given).
The parameterless structure provides good agreement between observed and calculated inten-
sities (Table 3). The observed intensities.were visually estimatedcfrom powder diagrams. Calcu-
lated intensities were corrected for Lorentz, polarization and multiplicity factors. No absorption
correction nor temperature correction was made, so that the ratio of calculated and observed in-
tensities changes with angle, but in a regular fashion. The absences noted demand the parameter-
less structure within very narrow limits, so it seems unnecessary to carry the intensity calcu-
DISCUSSION OF THE STRUCTURE
The metal positions of UH, are shown in Figure 1. The atoms in the six equivalent positions,
(c), form three perpendicular but nonintersecting linear arrays of atoms running parallel to the
three cubic axes. The atoms are spaced at half the cube edge, or at 3.316 A; each (c) atom pos-
sesses two other (c) atoms as nearest neighbors at this distance. This is by far the shortest metal-
metal bond in the compound, and apparently is the only metal- metal bond of any strength in the
In addition, each (c) atom has four nearest (a) atoms as neighbors at 3.707 A. These neighbors
form a tetrahedron flattened out along one two-fold axis. Each (a) atom has twelve nearest neigh-
bors, (c), at 3.707 A. These twelve neighbors have the arrangement of a deformed icosahedron.
The metal- metal distances are known in the a and V (high temperature) forms of uranium. In
the former,' each metal atom has two neighbors at 2.76 A, two at 2.85 A, four at 3.27 A, and four at
3.36 A. In the body-centered, > form of the metal, each metal atom has eight nearest neighbors at
2.97 A.P* Accordingly, the 3.316 A spacing in the hydride represents me..al-metal bonds of a
strength corresponding to the weaker bonds in the a metal structure. Since there are but two such
bonds per (c) atom and none for the (a) atoms, the metal-metal bonding in uranium hydride is rel-
atively unimportant. This might also t ? concluded from the density which decreases from 19 to
about 11 on formation of the hydride.
In no sense of the word can uranium hydride be thought of as an interstitial solution. Its com-
position is definite, the metal arrangement unique and unrelated to that of any metal structure, and
metal-metal bonds are practically nonexistent in this hydride.
In the absence of structural data, however, uranium hydride would doubtless be listed as an
interstitial solution, since in appearance and conductivity it still resembles a metal.
A PROPOSAL CONCERNING THE NATURE OF THE HYDRIDE
As we have seen, the metallic properties of uranium hydride are quite inconsistent with an
ionic structure, and the chemical ana physical properties and the structure of the ;iydride do not
correspond to an interstitial solution. The lack of important metal- metal bonds suggests that the
* Apparently gamma-uranium was obtained by chance earlier.7
Figure 1. Uranium positions in the hydride.
Table 3. Intensities of powder reflections from uranium hydride.
Indices Intensity calculated Intensity observed*
Table 3. (Continued).
(511) (333) / 0 0
(520) (432) 855 S
(521) 538 MS
(440) 506 MS
(522) (441) 0 0
(530) (435, 0 0
(531 0 0
(600) (442) 270 MW
(610) 208 W
(611) (532) 605 MS
(620) 0 0
(621) (540) (443) 0 0
(541) 0 0
(533) 0 C
(622) 175 'W
(630) (542) 544 MS
(631) 349 M
(444) 233 MW
(700) (632) 0 0
(710) (550) (543) 0 0
(711) (551) 0 0
(640) 180 W
(720) (641) 558 MS
(721) (633) (552) 714 S
(642) 0 0
(722) (544) 0 0
(730) 0 0
(731) (553) 0 0
(650) (643) 646 S
(732) (651) 905 VS
(800) 237 W
(810). (7401 (652) 0 0
(811) (741) (554) 0 0
(733) 0 0
S = strong; M = medium; W =weak; V = very
important bonds In the structure are metal hydrogen bonds, and these bonds must be such as to
leave the structure with metallic properties.
Moreover, though the melting point of the hydride is unknown it is certainly fairly high for a
compound with such weak metal- metal bonds. At high hydrogen pressures the hydride has been
taken above 600C, and it is certain that the melting point is much higher. In addition, since the
metal breaks up into a powder on formation of the hydride, it must be that the hydride is brittle.
The particles of the hydride produced at high temperatures were large enough to confirm this
The properties cited certainly must mean that covalent bonding of hydrogen to individual
uranit.-n atoms, leading to a molecular crystal, is out of the question. It is also inconsistent with
the two very different types of uranium atoms in the structure.
The physical properties of the hydride, such as melting point and brittleriess, are in keeping
with a valence-type compound, i.e.. a continuous structure held together by covalent bonds.* Since
metal- metal bonds are absent or weak, the continuous structure can be provide only by metal-
hydrogen bonds, and by these only if hydrogen bridges'metal atom to metal atom.
To provide electrical conductivity it is also necessary to have a continuous structure. Without
good metal- metal bonds this, too, can only be provided by a metal- hydrogen- metal bridge struc-
Finally, the nearest metal metal distances in the hydride are, except for the 3.316 A spacing,
too long to be metal metal bonds and too short to provide space for hydrogen between metal atoms
except in a bridge-type structure. Indeed, if one tries to find sensible places for hydrogen without
the use of a bridge structure, the uranium hydride structure appears incomprehensible.
The structure can be understood in terms of electron deficient "hall bonds" of the type recently
proposed to explain the bonding in the boron hydrides, aluminum alkyl dimers, .and certain inter-
stitial carbides. nitrides, and metallic oxides."0 In accordance with this proposal, hydrogen may use
its Is-orbital for the formation ol two bonds (but with one electron pair for the two bonds). Since
s-orbitaJs lack directional properties, we should expect the U-H-U bond angle to be 180" because
of the small size of hydrogen. Circumstances under which electron deficient "half bonds" are to be
expected are described elsewhere.O,io In this case we should expect them if uranium has fewer
valence electrons than stable bond-orbitals. That this condition is reasonable for uranium hydride
we shall attempt to demonstrate.
If we now consider which bonds in the structure must be the bridge bonds, U-H-U, the result
is quite simple. The 3.316 A distance, as we have seen, corresponds to a metal- metal bond. The
3.707 A spacing is quite satisfactory for the bridge bonds, and all other distances are too large for
such bonds. There are 12 such bridge bonas to each uranium atom of the set (a), or 24 per unit
cell. There are then 24 hydrogens per unit cell, making the overall formula UI4,.
In the structure proposed here each atom of type (a) is bonded to twelve atoms of type (c) by
hydrogen bridges. Each atom of type (ci is bonded directly to two other type (c) atoms and by hy-
drogen bridges to four atoms of type (a).
This structure is consistent with the use of six orbitals of uranium for bond formation for
both types of uranium atoms in the structure. The configuration about (c) uranium atoms is that
of a tetrahedron flattened along one two-fold axis and with two extra bonds directed along the
shortened two-fold axis. There are twelve bonds from type (a) atoms. These bonds are directed
toward the corners of a nearly regular icosahedron (see Figure 2). UI (a) type atoms use but six
bond orbitals then they must use orbitals which are directed in two directions and must use each
orbital to form two "hall bonds." If this is th3 case it is understandable that the six orbitals
chosen for bond formation by the two types of atoms are unlike. II uranium can furnish at least
six stable bond-orbitals and wit; lurnish at most four valence electrons in hydride formation, wbOch
seems reasonable, then conditions for "half bonds" are fulilled., .
Of course, the structure is not entirely clarified by the proposal made here. The types of
orbitals used for bond formation by uranium are not clear in case of either (a) or (c) type atonsq If,
as Pauling has suggested," when uranium furnishes lour valence electrons, the 7s-orbital coKtains
an electron pair, than the orbitals available (or bond formatior are the five 6d-orbitals, the three
p-orbitals and quite possibly the 5f-orbitals.j To obtain better bonding it would doubtlessbe possible
*This terminology follows that of F. Seitz.'
t The bond is drawn directly through hydrogen to indicate one electron pair, since U-H-U
would normally imply two electron-pair bonds.
i The elements in this part of periodic table have rare-earth-like properties, suggesting the
stability of the 5f-orbitals.'2
to shift the electrons from the 7s-orbital to other almost equally stable levels. The best six hybrid
bond-orbitals available to uranium are difficult to obtain from the wealth of atomic orbitals avail-
- able.z Thisis,a problem beyond the scope of the proposal made here, and that it is difficult io solve
is not a detraction from the present proposal.
The propoaed.structure provides. uranium hydride with a continuous, valence-type bonding
(Figure 2). which.wilh accouna. for, high melting point and brittleness, and with a resonating system
of "half bonds" which: .Jl-ild lead to high 1polarizability, metallic luster, and high electrical con-
ductivity, much as these properties ar !'und in graphite. Moreover, it provides a satisfactory
eiplalationl'f6r the uranium- uranium distances in the hydride and suggests satisfactory positions
preciselyy the correct number of hydrogen atoms.
The author is quite aware- that it is impossible to defend the proposed structure except insofar
as the proposal provides an explanation of the unusual properties, composition and metal arrange-
ment of the hydride. This proposal is offered here because it seems unlikely that an M-H-M
bridge is unique to uranium hydride. Many other metallic hydrides have rather similar properties,.
and though structural information is not yet complete enough to cite another good example of this
type, it seems to the author that most "interstitial solution" compounds need further study. NScarly
all such compounds, including hydriues, are brittle, hard, and high melting, properties quite inton-
dis lent with the weakening of mdtal- metal bonds unless replaced with better bonds. A number of
meIalic hydrides fulfill the conditions for "half bonds" as described before.10
Twelve nearest neighbors (c) about uranium
atom (a). All twelve distances are equal (3.707A)
and the (c) atoms lie at the corners of a nearly
regular icosahedron. In accordance with the
proposal presented in the paper, each of twelve
bonds is an U-H-U bridge.
Two nearest (c) atoms (3.316A) and four near-
est (a) atoms (3.707A) about any (c) atom. The
shorter bond is an ordinary U-U bond and the
longer bond is a U-H-U bridge, according to
the proposal of this paper. The hydrogen atom
is shown in the longer bond in this case.
Fig. 2. Coordination about uranium atoms in the hydride.
Uranium forms a metallic hydride, U`,, a compound of definite composition, unique metal ar-
rangement unrelated to that of the metal itself, and almost lacking in metal-metal bonds.
The hydride is cubic, a = 6.631 A, with eight uranium atoms per unit cell at positions (a) 000,
Hi., and (c) 0, Or0, 204, 20, 0,. 1 of the space group Oj, O2, or Td. The X-ray density4is
10.92 g/cc. The deuteride spacing is 6.620 A, definitely smaller than for the hydride.
It is proposed that the hydrogens form U-H-U bridges between metal atoms of type (a) and
(c) in the structure, where the bridge contains one electron pair for the two bonds. This struqtzre
accounts for the physical properties, unique metal arrangement, and formula of the hydride, and is
consistent with a recently proposed theory of electron deficient structures.'"0
The author wishes to express his appreciation to the members of the Ames section of the., .
Metallurgical Project for both samples and analytical data. Especial thanks are due A. S. Wilson
for aid in the precise determination of hydride and deuteride spacings, to R. Nottorf, Dr. A.,Newton,
W. Tucker, and F. Figard for special sample preparations and to Dr. I. B. Johns and Dr. F. H.
1. Rundle, R., Metallurgical Project Report, CT-609, p 30, April 1943.
2. Battelle Memorial Institute, Metallurgical Project Report, CT-818, July 1943.
3. Newton, A., J. Warf, 0. Johnson, and R. Nottorf, Metallurgical Project Report, CC-1201,
4. Jette, E. and F. Foote, J. Chem. Phys. 3:605 (1935).
5. Jacobs, C. and B. Warren, J. Am. Chem. Soc. 59:2588 (1937).
6. Wilson, A. and R. Rundle. Metallurgical Project Report, CT-1775, May 1944. F
7. McLennan, J. and R. McKay, Trans. Roy. Soc. Canada, m, 24:1 (1930).'
8. Seitz, F., Modern Theory of Solids, Chapter I, McGraw-Hill Book Co.,New York, 1940...
9. Rundle, R., Electron deficient compounds, J. Am. Chem. Soc: 69:1327-1331 (1947).,
10. Rundle, R., A new interpretation of interstitial compounds, submitted for publication in J. Am.
11. Pauling, L., The Nature of the Chemical Bond, Second Edition, p 413, Cornell University Press,
Ithaca, N. Y., 1945.
12. Chem. and Eng. News 24:161 (1946).
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