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; ..... ; I" UNITED STATES ATOMIC ENERGY COMMISSION,
UF,, A NEW FLUORIDE OF URANIUM
S Aristid V. Grosse
This document consists of 2 pages. Assi VOS
Date of Manuscript: April 1944
Date Declassified: June 13, 1944
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Technical Information Division, Oak Ridge Directed Operations
Oak Ridge, Tennessee
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There are two known fluorides of uranium, with widely different physical and chemical properties.
Uranium tetrafluoride, UF, has been known for a long time. It is a green solid, melts at about 1,0000C,
is insoluble in water, and chemically is rather inert. Uranium hexafluoride, UF,, discovered by O. Ruff*
In 1909, is a very reactive colorless gas or very volatile crystalline solid. It melts at 69, has a vapor
pressure of I atmosphere at 56and is thermally stable up to at least 400C. It is the most volatile
compound of uranium and is the only uranium compound available for methods of isotope separation
where a gas vapor is required.
Uranium pentafluoride, of. the empirical composition UF,, was discovered in the course of an
investigation which had as its object to effect an exchange of uranium between uranium hexa- and tetra-
fluoride. tThis was of interest in connection with enriching UF, depleted in U 235 in the course of
isotope separation, by means of UF, of natural isotope composition.
It was first observed that when powdered anhydrous UF4 (obtained by dehydrating the crystalline
2-UF4 5H0) Is heated to 80 with liquid colorless UFe, in a glass tube, it remains apparently insoluble,
but gradually loses its original bright green color and assumes a greyish tinge. At the same time the
UF, remains perfectly colorless and clear. On distilling off all the UF,, a spongy-looking, dirty grey-
green solid remains. On adding a few drops of water to this powder the original green color is re-
stored, and simultaneously a clear yellow solution of hexavalent uranium is formed.
These facts indicated the possibility of compound formation between the two fluorides.
This possibility became a certainty when UF, was heated with excess liquid UF, to 95 to 100C for
periods ao 20 to 100 hours. It was found that UF, reacts with UP, according to the equation
UF, + UF4 2 UF,
forming a new fluoride of uranium. This compound is a greyish to white solid; some preparations
appear to be crystalline. The pentafluoride readily disproportionate into UF, and UF, on heating,
particularly under reduced pressure, and may be better formulated as a complex:
S This complex does not noticeably disproportionate at 70 and 2 to 5 mm HIg pressure. Since this tem-
S perature is above the sublimation and melting point of UF,, any excess of the latter may be readily
separated from the complex by heating in a vacuum. However, the complex readily dissociates into
its constituents on heating for periods of 1 to 2 hours at a temperature range of 175 to 300 at 2-mm
pressure; the UF, elimination from the complex is complete at 300 to 4001.
O. Ruff and A. Heinzelmann, Berichte 42: 495 (1909): Zeit. Anorg. Chem. 72, 63, (1911).
t See A. Grosse's paper on "Exchange of Uranium between UF, and UFs.'
UF,, A NEW FLUORIDE OF URANIUM
By Aristid V. Grosse
2] MDDC- 1041
The pentafluoride is violently decomposed by water in accordance with the equation
2Ul~ + 2 H,0 UF, + UOF, + 4 HF.
The green insoluble UF, can be easy separated from the yellow solution of uranyl fluoride. This
affords a simple analytical method for determining the tetra- and hexavalent uranium content of the
complex. However, the pentafluoride is not as sensitive to moisture in air as the hexafluoride is.
Preparation by Direct Addition
A typical experiment in the preparation of UF, by direct addition of UF, was made as follows:
2.0 millimoles, or 0.628 g, of anhydrous UF,, e.g. 2UF4. 5H1O, were heated with 15.0 millimoles, or
5.280 g, of UF, (resublimed over KFt) for 100 hours to 100*C in a copper reaction vessel of about 25-cc
capacity. (The vessel was made of copper tubing with silver-soldered copper ends. A brass valve, of
the type used in refrigerators, was silver-soldered on at the top. The whole apparatus weighed about
30 g and had a free volume of about 25 cc. It readily withstood the few atmospheres pressure generated
by excess UFe and could be easily weighed on an analytical balance. For drying, filling, and emptying
the UFl, it was attached to the usual type of vacuum line.)
After heating, the excess of UF, was pumped off at 100C and 2-mm pressure: 12.9 millimoles, or
4.53 g, of unreacted UFe were recovered. The UF, retained 2.1 moles, or 0.738 g, of UFe, so that the
composition of the pentafluoride was
The slight excess of UF, may be due to either Insufficient pumping, or to stronger adsorption of the
last amounts, or to both.
On heating the pentafluoride for 2 hours to 300"C at 2-mm Hg, 0.67 g UF,, or 90 per cent, of the
complex's UF6 content was recovered. To eliminate the residual UF,, higher temperatures and/or
longer heating periods were necessary.
It is possible that the pentafluoride may also be prepared by dehydro fluorinating the little-defined
complex between UF, and hydrogen fluoride of the composition
UF,, x HF.
This unanalyzed complex was obtained by O. Ruff, as an intermediate in his unsuccessful attempts to
prepare UF, from the reaction of UC1, with hydrogen fluoride.
The pentafluoride was discovered in the course of studying the exchange of uranium between UF4
and UFP during the period from November 1940 to February 1941, at the Department of Physics of
Acknowledgment is due to the John Simon Guggenheim Memorial Foundation for the grant of a
fellowship. Acknowledgments are also due to Dr. E. Booth for mechanical help and to A. Kant for
tThe purification of UFe by means of alkali fluorides, which permits its easy handling in glass
vessels will be described in a separate paper.
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