136
'
H. R. HOEKSTRA, S. STEGEL, L. H. FUCHS AND J . J. KATZ
~701.
lio
data of Mitchel1"Jalong with an estimated entropy at 298°K. of 22 e.u. for Mg,N&).
of oxygen in the flow gas which reacted with Mg(g) formed in vaporization and thus caused excessive vaporization of the sample and an apparently high TABLE I11 equilibrium constant. Also, slight reaction of the VAPORIZATION OF MgsN2 sample with the porcelain boats may contribute (a) Flow vapor pressure data (N, as carrier gas) t o this deviation. Total w t . , T ,OK. loss,a mg. Time, min. - log Kb The effusion measurements scatter below the 1337 44-64 60-60 6.41 theoretical curve but approach i t more closely in 1355 75-103 60- I 00 5.67 those studies in which small effusion holes were 1403 1 14-225 38-68 4.22 used. This behavior is characteristic of a molecule 1420 143-232 32-02 3.84 or atom with a low accommodation coefficient 1452 30-42 3.52 155-180 being present in the effusion cell. Nz(g) would ap1506 262-317 18-23 2.29 pear likely to have a very low accommodation coef(b) Effusion vapor pressure datac ficient on Mg3N2(s). N2(g) is an extremely stable Total wt. Area of molecule and practicalIy no N(g) atoms would be loss, effusion hole, Time, o$ mg. 0m.a min. - log Kd formed a t 1400" by thermal dissociation; yet, in 1118 4.7 1.20 X 10-2 90 20.7 Mg3Nz(s), which crystallizes in an anti-Tlz03 ar8.8 2.39 X 1 0 - 3 60 15.8 1158 rangement, N atoms are distributed one at a time 105 17.8 1184 28.0 1.26 X through the lattice. 1203 38.6 2.39 X 10-3 GO 13.2 Thus, one concludes that Mg3N2(s) vaporizes by 1229 123.7 1.20 X 80 14.7 a complex mechanism which apparently involves 1248 85.9 8.58 X 10-3 60 13.7 Mg2(g) in one of the early steps; a t equilibrium, 1254 150.2 1.26 X 90 15.0 the vaporization products are Mg(g) and NZ(g). a Measured a t three flow rates: 340, 155 and 80 ml./min. Acknowledgments.-We wish to thank Mr. * Calculated assuming zero flow rate and decomposition to George Leroi who carried out most of the BzOs vapor the elements. Eleven additional runs with the large orifice are shown in Fig. 5. d Calculated assuming decomposition pressure runs in the platinum system. This research to the elements and including corrections for difference in was in part conducted under contract No. AF molecular weights of effusing species and the non-ideal knife 33(616)-338, with the United States Air Force, the edge of the effusion hole. sponsoring agency being the Aeronautical Research The high points observed in the flow measure- Laboratory of the Wright Air Development Center, ments can be explained by assuming a small amount Air Research and Development Command.
THE URANIUM-OXYGEN SYSTEM: UO2.6 TO U30, BY H. R, HOEKSTRA, S.SIEGEL,L. H. FUCHS AKD J. J. KATZ Chemistry Division, Argonne National Laboratory, Lemonl, Illinois Received October 28, 1964
Investigations under way on the uranium-oxygen system in the composition range U02.5 .to UaOe indicate that U Z Odoes ~ not exist; U02.serepresents the lower limit of the monophasic region below USO~. Composltion limits of the orthorhomblc U02.sphase appear to be U 0 2 . sto ~ U02.e6. Transition to the USOSphase occurs above the composition U02.e~. Single crystals of the UOz.6 phase and U30shave been prepared. A metastable (8) modification of Us08 has been observed on oxidation of the UO2.6 phase. High temperature X-ray studies have not as yet shown the existence of ~ U S Oas S a high temperature form of UaOs,but do show the formation of a hexagonal structure above 400'.
Oxide systems of metals which exhibit variable valency are often complicated by the occurrence of non-stoichiometric compounds and the existence of more or less extensive regions of solid solution. One of the more complex of such systems is that formed between oxygen and uranium. Three oxides of the element have been known for many years: UOz, u s 0 8 and U03. In recent years UO, p-UO2 (UOZX), UzOs and UBOI?,as well as a t least three compounds have been claimed, alin the region U02,3-U02.~ though the existence of some of these has not been definitely established. In addition U 0 3 has been shown to exist in four crystalline modifications. Extensive regions of solid solution are encountered between UOZ and UO,. One of the areas in which conflicting results are encountered is in the composition range U02,6to U3Oa. Tensimetric work by Biltz and Muller' (1) W.Biltz and H. MUller, 2.ano~u.allgem. Chena.. 163, 257 (1927).
on the thermal decomposition of uranium oxides indicated that UO2.a represents the lower limit of the one phase area extending from U03,since equilibrium oxygen pressures were found to be constant from that point to the composition UOz.3. They found no evidence for the existence of a compound at U2OS. The results obtained by Rundle, et aZ.,2 however, seemed to show conclusively that U Z O ~ does exist, and that its structure is closely related to that of Ua08,but with a larger unit cell. Ignited mixtures of UOz and U308 having the empirical composition U02.62 or above showed no trace of the fluorite uranium dioxide phase on X-ray films. This work was carried out with small quantities of the oxides (in X-ray capillaries) and there may have been some doubt as to the exact composition of the ignited mixture. Grgnvold and Haraldsen3 re(2) R. E. Rundle, N. C. Baenziger, A. 8. Wilson and R. A. McDonald, J. Ana. Chenz. Soc., 7 0 , 99 (1948). (3) F. Gr0nvold and H. Haraldsen, Nature, 162, 69 (1948).
THEURANIUM-OXYGEN SYSTEM:UOz.sTO U30s
Feb., 1955
ported the lower limit of the monophasic region a t U O ~ . while ~ ~ , Alberman and A n d e r ~ o n as , ~ well as Hering and Perio,b confirmed Rundle's result. More recently Milne6 reported that his single crystal studies on Ua08 showed that its structure was hexagonal rather than orthorhombic as Zachariasen' and Gr@nvoldshad found (see also Brooker and Nuffieldg). The value of Milne's work is somewhat in doubt since no complete chemical analysis was reported, and the source of the U ~ O Scrystals was unknown. This paper is a preliminary report in our investigation of the phase relationships of the uraniumoxygen system between U02.6and U308.
films of these samples showed the upper limit of the U02.6 phase to be a t U O Z . ~ ~Above . this composition X-ray patterns show line shifts and intensity changes in the maxima suggesting that this represents the transitional region to U308. Although there is an indication that the cell size of the UOz.Bphase increases with increasing oxygen content ~ ~UOz.Bs,the errors of measurement are suffifrom U O Z .to ciently high to preclude a quantitative measurement. A comparison of calculated lattice dimensions by several observers is given in Table I for USOSand the UOz.e phase (Rundle's UZOE). Good agreement is obtained in all instances except for Milne's U308 (recalculated for orthorhombic symmetry) thus strengthening the belief that his sample was not pure u&.
TABLE I LATTICEDIMENBIONS u308:
Experimental Determination of Oxygen-Uranium Ratio.-As indicated by the previous discussion, an accurate determination of the oxygen-uranium ratio of the compounds under investigation is required. The composition of the compounds prepared in our work has been determined by two methods. The first involves decomposition of the oxide with bromine trifluoride, L e . UOz 2BrFa +UFO Brz 02 The apparatus and procedure involved have been described in detail elsewhere.10 Results of several hundred runs on uranium oxides, as well as a number of other metal oxides, have shown that the composition may be determined to the nearest 0.01 atom of oxygen. The composition of uranium oxides may also be determined accurately by oxidation to U3Os at 750". Repeated observations indicate that the product formed when a lower oxide is heated to this temperaturein air or in oxygen a t a pressure of 150 mm. or above is exactly uSo8. At lower oxygen pressures the composition will be slightly low in oxygen depending on the previous history of the sample and the oxygen pressure utilized. A reaction system similar to that described for work on the thermal stability of alkaline earth diuranates'l will permit determination of composition to better than the nearest 0.01 atom of oxygen. The UOz.s Phase.-As was shown above, previous investigations had indicated that a monophasic region extended U O Z . ~and UOz.8. The structure at the composition below U308 to somewhere between the composition range UOz.5 was found to be related to the Us08 structure, but t,he point a t which the change from one to the other took place was unknown. Clarification of these items was undertaken by heating mixtures of UOZ and UaOs at approximately 0.02 oxygen atom intervals between U02.6 and U02,e5 to 1200' for several weeks in evacuated quartz tubes. The samples were weighed into platinum cups which slipped into the quartz tubes thus preventing the possibility of contamination of the sample by contact with the quartz tube. Samples having the composition UOZ.SS (as confirmed by subsequent analysis) showed traces of the fluorite phase, while U02.57 did not, thus placing the lower limit of the monophasic region at U 0 2 . 6in~ agreement with Gr6nvold's results. It must be remembered that these are X-ray observations a t room temperature-the phase may extend to lower compositions a t 1200' with the apparent limit of solubility depending upon the sample cooling rate. Only high temperature X-ray work can give a definite answer to this question. The upper limit of the U02.6 phase was determined by heating US08 in the evacuated tensimetric apparatus. Quenching of the oxide from 1000" to room temperature gave oxide compositions ranging down to U02.64. X-Ray
+
+
+
(4) K. B. Alberman and J. 8. Anderson, J . Chem. SOC.S u p p l . , 2 , 303 (1949). (5) H. Hering and P. Perio, Bul. SOC. chim. France, 351 (1952). (6) I. H.Milne, A m . Mineral., 86, 415 (1951). (7) W.H. Zachariasen, Manhattan Project Report, CP1249. (8) F. Grplnvold, Nature. 168, 70 (1948). (9) E.J. Brooker and E. W. Nuffield, A m . Mineral.,87, 363 (1952). (10) H.R. Hoekstra and J. J. Kata, Anal. Chem.. 26, 1608 (1953). (11) H.R. Hoakstra and J. J. Thtz, J . A m . Chem. Soc., 7 4 , 1683 (1982).
137
2 molecules per unit cell.
22. Zachariasen Gr6nvold Hering and Perio Milneb This work
6.70 6.703 6.708 6.69 6.704
b, kX.
11.94 11.91" 11.94" 11.79 11.95
X-Ray density, g./cc.
k.!
8.39 8.40 8.39 8.51 8.39
4.14 4.136 4.141 4.14 4.142
U 0 m phase: 32 uranium atoms per unit cell.
8.26 Rundle 6.72 31.65 8.269 This work 6.738 31.70 Reported as pseudo-cell, of true cell. from hexagonal to orthorhombic symmetry. (1
8.35 8.38 Converted
Preparation of Single Crystals of U308.-Single crystals of uranium oxide having the composition U02.04 were prepared by heating U30s in air a t 1100-1500". At the higher temperature some volatilization occurred with condensation a t the cooler portion of the tube (-1100"). Material suitable for single crystal studies was obtained in several hours at the higher temperatures, while several weeks of heating was required a t 1100-1200". The crystals were sometimes obtained as needle-shaped laths, and sometimes as hexagonal blocks. A serious barrier to the study of these crystals was the fact that they were invariably twinned. Single crystals of USOSwere obtained from U0z.s~by heating the In performing this oxidation it. UOz.64 in oxygen to 750'. was observed that the structure of the U308 ?s given by powder photographs was not always identical with the conventional orthorhombic pattern. The structure of the new U308, as tentatively indexed, has orthorhombic symmetry with the lattice constants a = 7.04 kX., b = 11.40 kX., and c = 8.27 kX. This leads to 4 molecules in the unit cell and an X-ray density of 8.38 g./cc. Work is continuing in order to assure that the symmetry has been assigned correctly and to locate the uranium positions. The new form of USOS(a) was converted to the conventional (or CY) structure by repeating the temperature cycle (to 750"). High Temperature X-Ray Studies.-With the knowledge that the @-formof U30s and the U02.6phase could be produced by suitable quenching from elevated temperatures, experiments were started to determine the phase transformation temperature (CY to a) and the decomposition temperature (@to UOz.a) by means of high temperature X-ray studies. For purposes of technique it was desirable to confine the U308in a closed and evacuated capillary. However, since it was known that a-U308 loses oxygen below 750' under these conditions, a second set of data was obtained using an open-ended capillary, thus permitting the sample to equilibrate with atmospheric oxygen. Upon heating the a-U3OSto 975' in either an open or a closed capil!ary, the P-UaOs form was not observed and the decomposition to U O Z .was ~ ~ never detected. Apparently, the D-U308 must appear near the decomposition temperature of Us08 (approximately 1150") which is beyond the heating range of the X-ray camera. However, some surprising pattern changes were observed in the high temperature runs, and these indicate that the uranium-oxygen phase diagram in the region Us08 is quite complicated. We are currently studying the patterns, Fnd, although the results are very incomplete, we wish to indicate some of these findings at present.
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H. R. HOEKSTRA, S.SIEGEL,L. H. FUCHS AND J . J. KATZ
138
In the temperature range 25 to 365', the a-axis increases in value and the b- and c-axes decrease in value with increasing temperature as indicated by Fig. 1. If the ratio a / b for u308 is plotted as a function of temperature, this ratio increases, so that a t 365" the value is very near 0.577. For hexagonal symmetry, a/b = &/3; hence the us08 is approaching hexagonal or pseudo-hexagonal symmetry. From 445" up to approximately 600", there is a noticeable sharpening of the diffraction pattern, especially a t large angles. Furthermore, the a value has now dropped slightly, and increases slowly with temperature. The c value has not changed, but it too increases with temperature. Thus, there is every indication that a phase transformation has taken place. This new phase can be indexed on a hexagonal basis with a = 6.801 f 0.001 kX., c = 4.128 f 0.001 kX. The cell contains one molecule and the calcuIated X-ray density is 8.41 g . / ~ m . ~ . 6.84
1
6.72
-
6.68
-
6.76 6.74
1
a
1
1
1
1
1
1
1
1
1
-1
-
-
-
