- Email: [email protected]
The uranium-oxygen system at high pressure
J. inorg,nucl.Chem., 1970,Vol. 32, pp. 3237 to 3248. PergamonPress. Printedin Great Britain
T H E U R A N I U M - O X Y G E N SYSTEM AT H I G H PRESSURE* HENRY
R. H O E K S T R A ,
STANLEY
S I E G E L and F R A N C I S
X. G A L L A G H E R
C h e m i s t r y Division, A r g o n n e N a t i o n a l L a b o r a t o r y , Argonne, Ill. 60439
(Received5 May
1970)
Abstract-Phase relationships existing in the uranium-oxygen system at temperatures to 1600°C and
pressures to 60 kb are reported. The compounds identified in the system include UO2, U40~, U~60~r, UsO~, U20~ and UO3. Properties of new phases have been studied by X-ray diffraction, thermal and i.r. analysis. Temperature and pressure stability ranges for three crystal forms of U~O~are outlined. The crystal symmetry of a-UzO5 is unknown,/3-U205 is hexagonal and y-UzO5is monoclinic. Marked changes in the relative stability of uranium oxides are shown to occur at high pressure, and are correlated with the respective oxide structure types. INTRODUCTION DETAILED studies at atmospheric pressure have shown that the uranium-oxygen phase diagram in the UO.,-UO3 composition range is rather complex. At least 16 separate crystalline phases have been identified a m o n g 9 uranium oxides; a number of these phases are undoubtedly metastable. Figure I illustrates the phase diagram as a function of temperature in the U O 2 - U O 3 composition interval. We note a wide range in thermal stability ranging from the refractory dioxide to compounds such as U8019 (UO2.aT) and a m o r p h o u s UO3 which have very limited thermal stability and doubtful t h e r m o d y n a m i c stability. Extensive solid solution of oxygen in UO2 occurs at rather moderate temperatures, a minor solid solution region is o b s e r v e d at U409 and another at U802~ (UO2.6). X-ray p o w d e r patterns of four of the nine c o m p o u n d s - U O . , , U409, U16037 (UO2.a) and UsO19 (UO.~.37) are very similar in appearance, and the phases all have relatively high densities ( - 11 g/cm3). At higher oxygen-to-uranium ratios a rather abrupt change to a lower density structural configuration is observed. Bonding in these " u r a n y l - t y p e " oxides is unsymmetrical, i.e. each uranium forms two short coilinear primary bonds with oxygen and 4 - 6 w e a k e r secondary bonds in or near a plane normal to the primary bonds. U80.,1, UaO8, Uv, Oa5 (UO.,.9) and UO3 exhibit this unsymmetrical bonding and the highly unstable U.,O5 phase [3] p r e p a r e d by solution of U308 under controlled conditions also appears to belong in this group. T a b l e 1 summarizes the crystal data available on the uranium oxides. The absence of structural details for most of the uranium oxide phases is due in large measure to the p o o r X-ray scattering p o w e r of oxygen relative to uranium and the resultant inability to determine oxygen positions by powder methods. Limited thermal stability precludes the synthesis of single crystals of most phases. Insufficient resolution in the p o w d e r pattern has prevented effective use of neutron diffraction techniques in structural studies on uranium oxides. *Work performed under the auspices of the U.S. Atomic Energy Commision. 3237
3238
H . R . H O E K S T R A , S. SIEGEL and F. X. G A L L A G H E R
.~ a--
N
8 © {"4
0
~b
0
,.d
¢.~ X
0 0
The uranium-oxygen system at high pressure
50otuox/ I000
3239
UOz+x + UeO21-z
--U6021
--U409-y
-i- z
.¢_ U409 + UsO21-z UsOo + 7-U03 E
1
50O
:51 0 2"0
I 2.1
I 2"2
II 2"3
:1
~1 oo h 2-4 2.5 2.6 Oxygen-uranium ratio
I [
I 2.7
I
II
2.8
2'9
I 3"0
Fig. I. Phase diagram-UO2-UO3.
The addition of a third variable, pressure, can be expected to produce some rather significant changes in phase relationships among the uranium oxides. High pressure increases the stability of close packed structures relative to those of low density. Thus fluorite-type phases should exhibit enhanced stability at high pressure and the upper composition limit of this structure-type may be shifted to a higher oxygen-to-uranium ratio. Increased stability has been observed [2] in the high pressure study of UOz.37, while a possible shift in the fluorite-uranyl boundary is suggested in a high pressure study on UaO8 [11 ]. Conversely, high pressure is expected to decrease stability of the more open uranyl-type structures. The synthesis of a new and more dense configuration of UO3 which still retains elements of uranyl bonding has also been reported[10]. We will refer to this phase as zeta UO3. In the present investigation we have studied phase relationships over the entire UO.,-UO3 composition range at temperatures ranging from 400 to 1600°C and at pressures from 15 to 60 kbars. These pressures represent the effective working range of our tetrahedral anvil apparatus. EXPERIMENTAL All of the oxide compositions studied at high pressure were obtained from two starting materials, UO2 and UO3. The uranium dioxide had been analyzed spectroscopically and found to contain less than 100 ppm of any other metal. The exact oxygen content of the phase, UOz.os*o.o,was obtained by measuring the weight change in a sample oxidized to U308 in air at 800-900°C. U3Os used in many of the sample mixtures was prepared as described above. The y-UO3 supply had been prepared from uranyl nitrate by the fluidized bed process. Spectroscopic analysis of this phase also showed no metal impurity above 100 ppm. Oxygen content, as determined by conversion of the trioxide to U308 at 11. W. B. Wilson,J. inorg, nucl. Chem. 19,212 (1961).
3240
H . R . HOEKSTRA, S. SIEGEL and F. X. G A L L A G H E R
900°C was UO3.00_+0.01. Oxygen composition intervals studied were 0.05 atom from UO~.00 to UOe.25, 0.025 atom from UO2.25 to UO~.7oand 0.10 atom from UO2.70 to UOa.0o. The equipment used in nearly all of the high pressure experiments was a tetrahedral anvil apparatus. Details concerning this equipment, the operating procedure, and the sample assembly have been described previously [12]. Briefly, the powdered oxide mixtures - 0-6 g each are weighed into platinum foil cups 6 mm in dia., covered with platinum disks and pressed into pellets. The purpose of the platinum foil is to isolate the oxide samples from reaction with pyrophyllite of the sample assembly, and to minimize oxygen gain or loss during the high temperature-high pressure runs. The platinum container is not completely gas-tight, but oxygen analyses rarely showed more than a 0-03 atom loss during the course of a run. Three pelleted oxide mixtures are usually fitted into each sample assembly. In a typical experiment the pyrophyllite tetrahedral assembly is brought to the desired pressure, the samples are heated to effect reaction, they are then quenched to room temperature, and finally pressure is slowly released. Heating periods range from 8 hr at 400°C to 15 min at 1600°C. Several additional experiments were run in Stellite hydrothermal reactors (1/4 in. bore) designed for operation to 4 kb pressure. Oxide samples for these runs were sealed into thin-walled platinum tubes to prevent oxygen exchange with water, the high pressure fluid. The reactors and samples were heated for several days at 500°C and 4 kb pressure. The products obtained in high pressure runs were investigated by X-ray diffraction, i.r. and thermal analysis techniques. Powder X-ray data were obtained with a Philips 114-59 mm camera using either nickel-filtered copper radiation or vanadium-filtered chromium radiation. Cell parameter calculations using a least squares refinement, were based on CuKc~, 1.54051 ,~. and CrKal = 2.28%2 A. I.R. spectra to 200 cm-' were taken with a Beckman IR-12 spectrophotometer. Nujol mulls were spread on polyethylene disks for the low frequency portion of the spectrum, and on KBr plates for the frequencies above 400 cm -1. The D T A - T G A results were obtained on a Mettler recording thermoanalyzer over the temperature interval 25-1000°C with a heating rate of 6, 10 or 15°C/min and a sensitivity of 20 or 100/~V. Runs were made in an inert atmosphere (argon) or in air on samples weighing 50-80 mg. RESULTS AND
DISCUSSION
Phase relationships. UO2-UO2.25 The results obtained in our studies of the UO2-UO3 composition range at high pressure will be discussed in four approximately equal composition segments. In this first region we found no structural change attributable to high pressure. Samples in the UO2.0~-UO2.2o composition range showed the presence of two face-centered cubic phases with parameters a = 5.468(1) .A and a = 5.447(1) A. Comparison with dimensions of stoichiometric UO2 and U409 as given in Table 1 indicates that a small solid solution area exists at either end of the composition range, but does not permit any conclusions regarding the extent of solubility at high pressure and temperature. Several workers[13, 14] have reported partial success in "quenching in" the UO2+x solution at ambient pressure, but we have experienced little success in similar experiments. Consequently, we believe that the apparent restricted solid solution range observed in our high pressure runs cannot be accepted as evidence against extensive solubility at high pressure and temperature. UO~.2~-UO~.5o It has been pointed out that our earlier work demonstrated the markedly enhanced stability of the U8019 phase at high pressure. The present study was 12. H. R. Hoekstra, lnorg. Chem. 5, 754 (1966). 13. B. E. Schaner, J. nucl. Mater. 2, I l 0 (1960). 14. W . A . Young, L. Lynds, J. S. Mohl and G. G. Libowitz, Rep. NAA-SR-6765 (March 30, 1962).
T h e u r a n i u m - o x y g e n s y s t e m at high pressure
3241
designed to determine conditions of temperature and pressure which lead to the formation of this phase, as well as to investigate the possible existence of additional stable compounds in the UOz.zs-UO2.~0 composition range at high pressure. We find that monoclinic UsO,.~ is obtained over the entire pressure range investigated with the tetrahedral apparatus (15-60kb); at 30 kb the phase is stable to at least 1500°C. Our experiments in the Stellite reactor at 4 kb and 500°C however, indicate that UsO,9 cannot be prepared under these conditions, and is, in fact, unstable towards disporportionation to U409 and U802,. A second phase comparable in stability to UsOla has also been identified in this composition range. Our analysis indicate an oxygen-uranium ratio of 2.31 (U,6037) for this phase. Its symmetry as determined from powder data is tetragonal with a = 5.407(1) ,~ and c = 5.497(1)/~ and c/a = 1.017. The X-ray powder data are summarized in Table 2. The phase appears to be identical with that Table
2. X-ray powder data on u r a n i u m oxides I
d
M F VVW W VW VW M W F F W VVW VVW VW VVW W VVW VW VW VVW F VW W W VW VVW VVW VVW VVW W VW VW
3.122 2.736 2.688 1-921 1.904 1.649 1.629 1.566 1' 3720 1.3505 1.2486 1-2397 1.2236 1.2094 1-1149 1-1049 1-0560 1-0458 1.0396 0.9630 0.9544 0.9245 0.9172 0.9139 0.9074 0-9023 0.8677 0.8560 0.8324 0.8270 0.8161 0.7846
hkl
U 16037 111 002 200 202 220 113 311 222 004 400 313 331 204 420 224 422 115 333 511 404 440 315 513 531 424 442 206 602,620 335 533 622 444
3242
H.R.
H O E K S T R A , S. S I E G E L and F. X. G A L L A G H E R
Table 2 (Contd.)
I
d
M M S W VVW W S M M
/3-UzO~ 3"278 3"191 2"935 2"625 2"323 2-051 1 "900 1 '822 1"646
VVW W VVW VVW VW F W B-M
1"632 1'597 1 '541 1'4719 1 '3955 1"3346 1"3168 1"2422
B-M
1'2224
VVW F VVW
1"1984 1-1643 1"1259
VVW
1'0987
VW VW VVW F VVW VW VW VVW VW F F VW W W
1"0839 1 '0433 1 '0282 0"9923 0"9525 0"9501 0"9154 0'9134 0"9069 0"8966 0"8810 0"8645 0-8543 0-8249
VW VW F VVW VW
0"8234 0"8192 0"8115 0"8006 0'7881
hkl
100, 004 101 102 103 104 105 110 106 008, 114, 200 107,201 202 203 108,204 205 109 206 118,207 210,211 1,0, 10 212 213 208,214 1,0, 11 215 0,0, 12 300, 301 302,216 304 2, 0, 10 218 220 221 310 311 312 313 314 315 316 0,0, 16 228,400 401 402 403 404 405
The uranium-oxygen system at high pressure
3243
Table 2 (Contd.) I
d
M VVW VW W
y-U205 3.107 2.713 2.683 1.911
VVW W W W VVW VVW F VW B-M
1.893 1.640 1.630 1.622 1-565 1.557 1-3641 1-3486 1.2451
VVW VW W
1.2332 1.2187 1-2104
VVW VVW B-W
1.2023 1.1129 I. 1086
VW VW VVW VW
1-1009 1-0518 1.0490 1.0424
VW W VVW F W W M M W W VW W VVW VW
1.0391 0.9613 0-9593 0.9511 0-9226 0.9209 0.9184 0.9159 0-9104 0.9083 0.9063 0.9042 0-9015 0.8986
hkl
11]-, 111 020 200, 002 220,202 022 202 131 311,113 311,113 222 222 040 400, 004 331, 331, 313, 133, 133 313 240,042 420,402, 024,204 402,204 242 242,422 224 422,224 151, 151 333 333, 51]115 511,115 440, 044 404 404 351,153 351,153 531,135 531,135 513,315 442,244 424 442,244 600,006 424
reported in annealing studies of U8019 by Anderson[15]. Little solid solubility as evidenced by shift in line positions could be detected for U1nO37. N o additional compounds were identified to the 2.5 oxygen-uranium ratio. 15. J. S. Anderson, Bull. Soc. chim. Fr. 20, 781 ( 1953).
3244
H . R . HOEKSTRA, S. SIEGEL and F. X. G A L L A G H E R
It should be remembered that the cell dimensions quoted for both monoclinic U80~9 and tetragonal U16037 refer to a small pseudo-cell derived from the fluorite structure and containing only 4 uranium atoms. The true cell is some multiple of the small cell just as the cell parameter of U409 is four times its pseudo-cell. Whether the true cell of U16037 and U80~9 is a similar multiple of the small cell can only be answered by more detailed work at some future time. UO2.50-UOz.7~ As the oxygen-uranium ratio is raised to and above 2.50, the fluorite-type structure is replaced by less dense structures. Our first experiments in the UO2.50UO2.75 composition range were arranged to confirm the existence of the y phase reported by Wilson[11]. Formation of this phase from a-U308 at high pressure was confirmed but additional lines due to a second phase invariably appeared in the powder pattern. We have identified this phase as the high pressure modification of UO3[10]. In accord with this finding is the statement by Wilson that two reflections in his powder pattern "appear when y is first formed at lower temperatures, but are not observed at the higher temperatures". These reflections can be shown to coincide with two relatively strong lines (111 and 122) of ~-UO3. Additional experiments with varying mixtures of UO~ and U308 have established that the true composition of Wilson's y phase is U205 rather than U308. Thermal analysis (see below) has confirmed the formula. All further reference to the phase in this paper will be to a-U2Os. Pycnometric measurements give a density of 10.5 g/cm 3 for a-U2Os. We have not succeeded in indexing its powder pattern; our X-ray data for the phase are in good agreement with Wilson's table. Experiments to determine the minimum temperature required to prepare a-U205 from the UO2 + U3Os mixture indicate that the phase does not form after 8 hr at 300°C and 30 kb pressure, but is formed at 400°C. X-ray powder patterns of a-U308 which has been heated at 300°C show little change in the 001 reflections, but the remaining lines are quite diffuse. Apparently the primary bonds along the U - - O - - U - - O chains are not appreciably affected, but the longer and weaker secondary bonds are nearing their "breaking point". At 400°C crystal rearrangement does occur and U205 is formed. Although 30 kb pressure is required to bring about the reaction U308 ~ a-U20~ + ~-UO3 at 500°C, only 15 kb are required to prepare a-U205 via the reaction UO., + U308 --> 2a-U205 . We find that a second crystal modification (fl-U2Os) is formed at 40-50 kb pressure and at temperatures in excess of 800°C. This phase is not the cubic high pressure form of U3Os reported by Wilson, since fl-U205 is indexible as a hexagonal cell with a = 3.813(1)• and c = 13.18(1),~,; X-ray powder data are presented inTable 2. Two molecules in the unit cell give an X-ray density of 11.15 g/cm3; pycnometric densities obtained with small samples range from 10.76 to 11.38 g/cm 3. Evidently o~ and fl-U~O5 represent a transition between the fluorite and uranyl structure types.
T h e u r a n i u m - o x y g e n s y s t e m at high p r e s s u r e
3245
The indexing of fl-U205 as given in Table 2 is incomplete. The c-axis value is a near multiple of the b orthohexagonal value, resulting in a coincidence of many reflections. Hence, the correct assignment of Miller indices will require a knowledge of the structure. However, the symmetry and cell dimensions given have been deduced from single crystal studies. Based on limited Weissenberg and precession data, we find reflections hkl present, hhl with 1 = 2n, and no limitations on hOl. This leaves C4,, D~a, C4v, D4h and D4h as possible space groups. However, oxygen scattering is so weak that a structure determination is required in order to assign the true space group. At the highest pressure (60 kb) attained in this investigation, samples prepared at temperatures below 800°C crystallize as c~-U205 while those prepared at higher temperatures sometimes appeared as fi-UzO~ and sometimes as a third form of this compound, y-U2Os. The powder pattern of y-U205 has been indexed as a monoclinic cell with a = 5.410 ( 1) ,~, b = 5.481 ( 1) ,~, c = 5.410 ( 1) ,~ and/3 = 90.49 °. The calculated X-ray density for a cell containing two molecules is 11.51 g/cm 3 (measured density = 11.36 g/cm 3) which establishes this phase as an extension of the fluorite-type structure brought about at high pressure. The complete pressure-temperature phase diagram at U205 as it has been developed from our experiments is shown in Fig. 2. It is possible that y-U20~ may be the cubic form of U3Os reported by Wilson, but the lower symmetry of the phase is readily apparent in well-crystallized samples. No other compounds have been identified in this portion of the high pressure phase diagram; UsO.,, and U308 are stable at relatively low pressures, but disproportionate to c~ or /3-U20~ and 6-UO~ as pressure is increased. The boundary runs about 15 kb above and parallel to that shown in Fig. 2 for the formation of U205 from U80,9 and U8021. UOz.75-UO3.00 At ambient pressure the uranium-oxide phase diagram (near U(VI)) has been
Uranium-Oxygen System at U205 70
i
i
I
'
'
i
,
I
60
'
'
i
,
I
r u2o,
~_ O
o -~i~
5O --- 40 ._a
o
~ 30
0
I
0
+ + BUz
:l:-/~
0
•
a2O
0
0
I0 0
l
/I
•
UsOt9 + Ua021
i
i
I 500
i
L L i
I I000
Temperature
,
in *C
i
i
,
I 1500
Fig. 2. P r e s s u r e - t e m p e r a t u r e p h a s e relationships at U20.~.
3246
H.R.
HOEKSTRA,
S. S I E G E L
and F. X. GALLAGHER
shown to be very complex. In contrast the high pressure diagram is remarkably simple. No evidence has been found for the existence of the UO.,.9,, phase at 15 kb pressure or above, and ¢-UO3 is the only form of the trioxide encountered in the 15-60 kb interval. (The high pressure form of UO3 was not obtained, however, at 4 kb and 500°C in hydrothermal reactor experiments.) Attempts to determine the upper temperature stability limits for ¢-UO3 are difficult because sample "blowouts" occur frequently when assemblies containing UO3 are maintained at 1200°C or above for more than a few min. However, ¢-UO3 has been identified as a constituent of samples prepared at temperatures to 1500°C. Figure 3 summarizes our p-T data on the uranium oxides. I.R. spectra The i.r. spectra of the fluorite-type uranium oxides are rather simple. Uranium dioxide has a single strong absorption near 350 cm -1 arising from U - O stretching vibrations in the lattice[l 6]. Oxidation of UO,, to U409 and U80~9 broadens the absorption envelope as the single peak splits into several maxima in the 250-500 cm -1 frequency range [2]. Further oxidation of uranium to the uranyl-type oxides gives more complex spectra as the absorption maxima arising from vibrations of the primary and secondary bonds become separated e.g. U-O~ at 740 cm -a and U - O . at 530, 495 and 440cm -~ in U308 (Fig. 4). Bending vibrations occur below 300 cm-L The i.r. spectra of the three U205 phases (Fig. 5) demonstrate the gradual elimination of internal (group) vibrations in the uranyl region and assimilation of the various stretching and bending modes into a single broad absorption peak characteristic of the fluorite-type structure. 2-0
2.1
2'2
2'3
2.4
/i
15oo
2.5
l' o -
2.6
r
2"7
2-8
2-9
I
I
I
: i J !
E
....../,/'/" 2.0
,"......
,
,
2.1
2.2
/,
2.5
L
,
, ,
//[
2.4 2.5 2.6 Oxygen-uronium rotio
F i g . 3. P r e s s u r e - t e m p e r a t u r e 16. T . S h i m a n o u c h i ,
II II If
i I I
...........l...........t...L._....¢..4.....I .........,t..I .......J........................................ ," •. . . . . I.,4 I "( /" "a ,.[i { ,'' / ," ,/17 f I," I,/ ," I /" II f" .,r / r II /" /" I ,,"" I ,,,"" .' ."
® 500
I-
/
500
I I I
I I
M. Tsuboi and T. Miyazawa,
4/O I000 .u
1ooo
o
3"0 1500
I
I
I
2.7
2,8
2.9
phase relationships:
3-o
UO2-UO3.
J. c h e m . P h y s . 3 5 , 1 5 9 7 ( 1 9 6 1 ).
( 60
The uranium-oxygen [
I
system
(
at high p r e s s u r e
I
I
v
I
d
3247
r
~.
I
i ~u(3//~/ / ~ I
// I000
900
800
I
1
I[ I
i"~ \ l~/t
I
I
700 600 500 400 WAVENUMBER CM -I
I
I]
300
200
Fig. 4. I.R. s p e c t r a o f U 3 0 ~ a n d ~ - U O a .
The high pressure form of UO3 has been shown[10] to retain uranyl bonding with 2 U - - O bonds at 1.83 A and the remaining 5 bonds ranging from 2.21 to 2.56 in length. With such a range of bond lengths the i.r. spectrum becomes rather complex. Figure 4 illustrates the observed spectra for ~-UO3. The characteristic strong absorption in the uranyl region is evident.
I000
I
I
I 900
I 800
[
I
I I I I 700 6 0 0 500 4 0 0 WAVENUMBER CM -I
Fig. 5.1.R. s p e c t r a o f U 2 0 5 p h a s e s .
I 500
200
3248
H.R. HOEKSTRA, S. SIEGEL and F. X. GALLAGHER
Thermal analysis The uranium oxide phases prepared at high pressure, particularly the three U205 modifications, were investigated by a combined D T A - T G A technique. Thermogravimetric data on duplicate runs with each U~O5 phase were consistent but the differential thermal analyses were not sufficiently reproducible to merit their use for quantitative enthalpy change measurements. Sample packing and crystallinity are believed to be the variables largely responsible for this difficulty. Thermal analyses were used, however, to determine stability towards phase change, disproportionation, and oxidation. Samples were first heated in an argon atmosphere to measure thermal stability, then in air to determine stability toward oxidation. The oxidation experiment also provided a convenient confirmation for the oxygen-uranium ratio in the sample. In argon a-U20~ showed a very small endothermic effect centered at 500°C. Static sealed tube experiments confirmed the disproportionation of a-U205 to U~O9 and UsO2,. Hexagonal fl-U205 shows two small exotherms centered at 225°C and 375°C when heated in an argon atmosphere. X-ray powder patterns on fl-U205 samples which had been heated to 300°C and to 450°C reveal that the first exotherm involves a minor change in the powder pattern and that the second exotherm results from a disproportionation of U205. Monoclinic y-U20~ disproprotionates exothermally in argon at 310°C. We note that in each instance decomposition of U.,O5 occurs by disproportionation rather than retracing the 3' -~ fl -~ a-U20~ sequence. Oxidation of all three U205 phases is exothermic, a-U205 gained 1.0_+ 0. l per cent (theoretical is 0.96 per cent) in weight during oxidation to U3Os. The exotherm maximum occurs at 470°C when samples were heated at 6°C/min and at 500°C at a 15°C/min heating rate. The hexagonal fl-UzO~ was less stable toward oxidation, with its exotherm centered at 285°C. The measured weight increase was 1-! _+0.2 per cent. Monoclinic y-U20~ is the only phase to disproportionate prior to oxidation. Samples heated in air show two exotherms with no weight change associated with the first; the second exotherm at 360°C is accompanied by a 1.1 -+ 0.2 per cent weight increase. The exotherm associated with oxidation of UsO,9 to U3Os is centered at 425°C, the observed 1.7(1) per cent measured weight gain compares well with a theoretical 1.69 per cent. Oxidation of U,6037 occurs at 525°C with a 2.0(0.1) per cent weight gain; the theoretical gain is 2.05 per cent.
T H E U R A N I U M - O X Y G E N SYSTEM AT H I G H PRESSURE* HENRY
R. H O E K S T R A ,
STANLEY
S I E G E L and F R A N C I S
X. G A L L A G H E R
C h e m i s t r y Division, A r g o n n e N a t i o n a l L a b o r a t o r y , Argonne, Ill. 60439
(Received5 May
1970)
Abstract-Phase relationships existing in the uranium-oxygen system at temperatures to 1600°C and
pressures to 60 kb are reported. The compounds identified in the system include UO2, U40~, U~60~r, UsO~, U20~ and UO3. Properties of new phases have been studied by X-ray diffraction, thermal and i.r. analysis. Temperature and pressure stability ranges for three crystal forms of U~O~are outlined. The crystal symmetry of a-UzO5 is unknown,/3-U205 is hexagonal and y-UzO5is monoclinic. Marked changes in the relative stability of uranium oxides are shown to occur at high pressure, and are correlated with the respective oxide structure types. INTRODUCTION DETAILED studies at atmospheric pressure have shown that the uranium-oxygen phase diagram in the UO.,-UO3 composition range is rather complex. At least 16 separate crystalline phases have been identified a m o n g 9 uranium oxides; a number of these phases are undoubtedly metastable. Figure I illustrates the phase diagram as a function of temperature in the U O 2 - U O 3 composition interval. We note a wide range in thermal stability ranging from the refractory dioxide to compounds such as U8019 (UO2.aT) and a m o r p h o u s UO3 which have very limited thermal stability and doubtful t h e r m o d y n a m i c stability. Extensive solid solution of oxygen in UO2 occurs at rather moderate temperatures, a minor solid solution region is o b s e r v e d at U409 and another at U802~ (UO2.6). X-ray p o w d e r patterns of four of the nine c o m p o u n d s - U O . , , U409, U16037 (UO2.a) and UsO19 (UO.~.37) are very similar in appearance, and the phases all have relatively high densities ( - 11 g/cm3). At higher oxygen-to-uranium ratios a rather abrupt change to a lower density structural configuration is observed. Bonding in these " u r a n y l - t y p e " oxides is unsymmetrical, i.e. each uranium forms two short coilinear primary bonds with oxygen and 4 - 6 w e a k e r secondary bonds in or near a plane normal to the primary bonds. U80.,1, UaO8, Uv, Oa5 (UO.,.9) and UO3 exhibit this unsymmetrical bonding and the highly unstable U.,O5 phase [3] p r e p a r e d by solution of U308 under controlled conditions also appears to belong in this group. T a b l e 1 summarizes the crystal data available on the uranium oxides. The absence of structural details for most of the uranium oxide phases is due in large measure to the p o o r X-ray scattering p o w e r of oxygen relative to uranium and the resultant inability to determine oxygen positions by powder methods. Limited thermal stability precludes the synthesis of single crystals of most phases. Insufficient resolution in the p o w d e r pattern has prevented effective use of neutron diffraction techniques in structural studies on uranium oxides. *Work performed under the auspices of the U.S. Atomic Energy Commision. 3237
3238
H . R . H O E K S T R A , S. SIEGEL and F. X. G A L L A G H E R
.~ a--
N
8 © {"4
0
~b
0
,.d
¢.~ X
0 0
The uranium-oxygen system at high pressure
50otuox/ I000
3239
UOz+x + UeO21-z
--U6021
--U409-y
-i- z
.¢_ U409 + UsO21-z UsOo + 7-U03 E
1
50O
:51 0 2"0
I 2.1
I 2"2
II 2"3
:1
~1 oo h 2-4 2.5 2.6 Oxygen-uranium ratio
I [
I 2.7
I
II
2.8
2'9
I 3"0
Fig. I. Phase diagram-UO2-UO3.
The addition of a third variable, pressure, can be expected to produce some rather significant changes in phase relationships among the uranium oxides. High pressure increases the stability of close packed structures relative to those of low density. Thus fluorite-type phases should exhibit enhanced stability at high pressure and the upper composition limit of this structure-type may be shifted to a higher oxygen-to-uranium ratio. Increased stability has been observed [2] in the high pressure study of UOz.37, while a possible shift in the fluorite-uranyl boundary is suggested in a high pressure study on UaO8 [11 ]. Conversely, high pressure is expected to decrease stability of the more open uranyl-type structures. The synthesis of a new and more dense configuration of UO3 which still retains elements of uranyl bonding has also been reported[10]. We will refer to this phase as zeta UO3. In the present investigation we have studied phase relationships over the entire UO.,-UO3 composition range at temperatures ranging from 400 to 1600°C and at pressures from 15 to 60 kbars. These pressures represent the effective working range of our tetrahedral anvil apparatus. EXPERIMENTAL All of the oxide compositions studied at high pressure were obtained from two starting materials, UO2 and UO3. The uranium dioxide had been analyzed spectroscopically and found to contain less than 100 ppm of any other metal. The exact oxygen content of the phase, UOz.os*o.o,was obtained by measuring the weight change in a sample oxidized to U308 in air at 800-900°C. U3Os used in many of the sample mixtures was prepared as described above. The y-UO3 supply had been prepared from uranyl nitrate by the fluidized bed process. Spectroscopic analysis of this phase also showed no metal impurity above 100 ppm. Oxygen content, as determined by conversion of the trioxide to U308 at 11. W. B. Wilson,J. inorg, nucl. Chem. 19,212 (1961).
3240
H . R . HOEKSTRA, S. SIEGEL and F. X. G A L L A G H E R
900°C was UO3.00_+0.01. Oxygen composition intervals studied were 0.05 atom from UO~.00 to UOe.25, 0.025 atom from UO2.25 to UO~.7oand 0.10 atom from UO2.70 to UOa.0o. The equipment used in nearly all of the high pressure experiments was a tetrahedral anvil apparatus. Details concerning this equipment, the operating procedure, and the sample assembly have been described previously [12]. Briefly, the powdered oxide mixtures - 0-6 g each are weighed into platinum foil cups 6 mm in dia., covered with platinum disks and pressed into pellets. The purpose of the platinum foil is to isolate the oxide samples from reaction with pyrophyllite of the sample assembly, and to minimize oxygen gain or loss during the high temperature-high pressure runs. The platinum container is not completely gas-tight, but oxygen analyses rarely showed more than a 0-03 atom loss during the course of a run. Three pelleted oxide mixtures are usually fitted into each sample assembly. In a typical experiment the pyrophyllite tetrahedral assembly is brought to the desired pressure, the samples are heated to effect reaction, they are then quenched to room temperature, and finally pressure is slowly released. Heating periods range from 8 hr at 400°C to 15 min at 1600°C. Several additional experiments were run in Stellite hydrothermal reactors (1/4 in. bore) designed for operation to 4 kb pressure. Oxide samples for these runs were sealed into thin-walled platinum tubes to prevent oxygen exchange with water, the high pressure fluid. The reactors and samples were heated for several days at 500°C and 4 kb pressure. The products obtained in high pressure runs were investigated by X-ray diffraction, i.r. and thermal analysis techniques. Powder X-ray data were obtained with a Philips 114-59 mm camera using either nickel-filtered copper radiation or vanadium-filtered chromium radiation. Cell parameter calculations using a least squares refinement, were based on CuKc~, 1.54051 ,~. and CrKal = 2.28%2 A. I.R. spectra to 200 cm-' were taken with a Beckman IR-12 spectrophotometer. Nujol mulls were spread on polyethylene disks for the low frequency portion of the spectrum, and on KBr plates for the frequencies above 400 cm -1. The D T A - T G A results were obtained on a Mettler recording thermoanalyzer over the temperature interval 25-1000°C with a heating rate of 6, 10 or 15°C/min and a sensitivity of 20 or 100/~V. Runs were made in an inert atmosphere (argon) or in air on samples weighing 50-80 mg. RESULTS AND
DISCUSSION
Phase relationships. UO2-UO2.25 The results obtained in our studies of the UO2-UO3 composition range at high pressure will be discussed in four approximately equal composition segments. In this first region we found no structural change attributable to high pressure. Samples in the UO2.0~-UO2.2o composition range showed the presence of two face-centered cubic phases with parameters a = 5.468(1) .A and a = 5.447(1) A. Comparison with dimensions of stoichiometric UO2 and U409 as given in Table 1 indicates that a small solid solution area exists at either end of the composition range, but does not permit any conclusions regarding the extent of solubility at high pressure and temperature. Several workers[13, 14] have reported partial success in "quenching in" the UO2+x solution at ambient pressure, but we have experienced little success in similar experiments. Consequently, we believe that the apparent restricted solid solution range observed in our high pressure runs cannot be accepted as evidence against extensive solubility at high pressure and temperature. UO~.2~-UO~.5o It has been pointed out that our earlier work demonstrated the markedly enhanced stability of the U8019 phase at high pressure. The present study was 12. H. R. Hoekstra, lnorg. Chem. 5, 754 (1966). 13. B. E. Schaner, J. nucl. Mater. 2, I l 0 (1960). 14. W . A . Young, L. Lynds, J. S. Mohl and G. G. Libowitz, Rep. NAA-SR-6765 (March 30, 1962).
T h e u r a n i u m - o x y g e n s y s t e m at high pressure
3241
designed to determine conditions of temperature and pressure which lead to the formation of this phase, as well as to investigate the possible existence of additional stable compounds in the UOz.zs-UO2.~0 composition range at high pressure. We find that monoclinic UsO,.~ is obtained over the entire pressure range investigated with the tetrahedral apparatus (15-60kb); at 30 kb the phase is stable to at least 1500°C. Our experiments in the Stellite reactor at 4 kb and 500°C however, indicate that UsO,9 cannot be prepared under these conditions, and is, in fact, unstable towards disporportionation to U409 and U802,. A second phase comparable in stability to UsOla has also been identified in this composition range. Our analysis indicate an oxygen-uranium ratio of 2.31 (U,6037) for this phase. Its symmetry as determined from powder data is tetragonal with a = 5.407(1) ,~ and c = 5.497(1)/~ and c/a = 1.017. The X-ray powder data are summarized in Table 2. The phase appears to be identical with that Table
2. X-ray powder data on u r a n i u m oxides I
d
M F VVW W VW VW M W F F W VVW VVW VW VVW W VVW VW VW VVW F VW W W VW VVW VVW VVW VVW W VW VW
3.122 2.736 2.688 1-921 1.904 1.649 1.629 1.566 1' 3720 1.3505 1.2486 1-2397 1.2236 1.2094 1-1149 1-1049 1-0560 1-0458 1.0396 0.9630 0.9544 0.9245 0.9172 0.9139 0.9074 0-9023 0.8677 0.8560 0.8324 0.8270 0.8161 0.7846
hkl
U 16037 111 002 200 202 220 113 311 222 004 400 313 331 204 420 224 422 115 333 511 404 440 315 513 531 424 442 206 602,620 335 533 622 444
3242
H.R.
H O E K S T R A , S. S I E G E L and F. X. G A L L A G H E R
Table 2 (Contd.)
I
d
M M S W VVW W S M M
/3-UzO~ 3"278 3"191 2"935 2"625 2"323 2-051 1 "900 1 '822 1"646
VVW W VVW VVW VW F W B-M
1"632 1'597 1 '541 1'4719 1 '3955 1"3346 1"3168 1"2422
B-M
1'2224
VVW F VVW
1"1984 1-1643 1"1259
VVW
1'0987
VW VW VVW F VVW VW VW VVW VW F F VW W W
1"0839 1 '0433 1 '0282 0"9923 0"9525 0"9501 0"9154 0'9134 0"9069 0"8966 0"8810 0"8645 0-8543 0-8249
VW VW F VVW VW
0"8234 0"8192 0"8115 0"8006 0'7881
hkl
100, 004 101 102 103 104 105 110 106 008, 114, 200 107,201 202 203 108,204 205 109 206 118,207 210,211 1,0, 10 212 213 208,214 1,0, 11 215 0,0, 12 300, 301 302,216 304 2, 0, 10 218 220 221 310 311 312 313 314 315 316 0,0, 16 228,400 401 402 403 404 405
The uranium-oxygen system at high pressure
3243
Table 2 (Contd.) I
d
M VVW VW W
y-U205 3.107 2.713 2.683 1.911
VVW W W W VVW VVW F VW B-M
1.893 1.640 1.630 1.622 1-565 1.557 1-3641 1-3486 1.2451
VVW VW W
1.2332 1.2187 1-2104
VVW VVW B-W
1.2023 1.1129 I. 1086
VW VW VVW VW
1-1009 1-0518 1.0490 1.0424
VW W VVW F W W M M W W VW W VVW VW
1.0391 0.9613 0-9593 0.9511 0-9226 0.9209 0.9184 0.9159 0-9104 0.9083 0.9063 0.9042 0-9015 0.8986
hkl
11]-, 111 020 200, 002 220,202 022 202 131 311,113 311,113 222 222 040 400, 004 331, 331, 313, 133, 133 313 240,042 420,402, 024,204 402,204 242 242,422 224 422,224 151, 151 333 333, 51]115 511,115 440, 044 404 404 351,153 351,153 531,135 531,135 513,315 442,244 424 442,244 600,006 424
reported in annealing studies of U8019 by Anderson[15]. Little solid solubility as evidenced by shift in line positions could be detected for U1nO37. N o additional compounds were identified to the 2.5 oxygen-uranium ratio. 15. J. S. Anderson, Bull. Soc. chim. Fr. 20, 781 ( 1953).
3244
H . R . HOEKSTRA, S. SIEGEL and F. X. G A L L A G H E R
It should be remembered that the cell dimensions quoted for both monoclinic U80~9 and tetragonal U16037 refer to a small pseudo-cell derived from the fluorite structure and containing only 4 uranium atoms. The true cell is some multiple of the small cell just as the cell parameter of U409 is four times its pseudo-cell. Whether the true cell of U16037 and U80~9 is a similar multiple of the small cell can only be answered by more detailed work at some future time. UO2.50-UOz.7~ As the oxygen-uranium ratio is raised to and above 2.50, the fluorite-type structure is replaced by less dense structures. Our first experiments in the UO2.50UO2.75 composition range were arranged to confirm the existence of the y phase reported by Wilson[11]. Formation of this phase from a-U308 at high pressure was confirmed but additional lines due to a second phase invariably appeared in the powder pattern. We have identified this phase as the high pressure modification of UO3[10]. In accord with this finding is the statement by Wilson that two reflections in his powder pattern "appear when y is first formed at lower temperatures, but are not observed at the higher temperatures". These reflections can be shown to coincide with two relatively strong lines (111 and 122) of ~-UO3. Additional experiments with varying mixtures of UO~ and U308 have established that the true composition of Wilson's y phase is U205 rather than U308. Thermal analysis (see below) has confirmed the formula. All further reference to the phase in this paper will be to a-U2Os. Pycnometric measurements give a density of 10.5 g/cm 3 for a-U2Os. We have not succeeded in indexing its powder pattern; our X-ray data for the phase are in good agreement with Wilson's table. Experiments to determine the minimum temperature required to prepare a-U205 from the UO2 + U3Os mixture indicate that the phase does not form after 8 hr at 300°C and 30 kb pressure, but is formed at 400°C. X-ray powder patterns of a-U308 which has been heated at 300°C show little change in the 001 reflections, but the remaining lines are quite diffuse. Apparently the primary bonds along the U - - O - - U - - O chains are not appreciably affected, but the longer and weaker secondary bonds are nearing their "breaking point". At 400°C crystal rearrangement does occur and U205 is formed. Although 30 kb pressure is required to bring about the reaction U308 ~ a-U20~ + ~-UO3 at 500°C, only 15 kb are required to prepare a-U205 via the reaction UO., + U308 --> 2a-U205 . We find that a second crystal modification (fl-U2Os) is formed at 40-50 kb pressure and at temperatures in excess of 800°C. This phase is not the cubic high pressure form of U3Os reported by Wilson, since fl-U205 is indexible as a hexagonal cell with a = 3.813(1)• and c = 13.18(1),~,; X-ray powder data are presented inTable 2. Two molecules in the unit cell give an X-ray density of 11.15 g/cm3; pycnometric densities obtained with small samples range from 10.76 to 11.38 g/cm 3. Evidently o~ and fl-U~O5 represent a transition between the fluorite and uranyl structure types.
T h e u r a n i u m - o x y g e n s y s t e m at high p r e s s u r e
3245
The indexing of fl-U205 as given in Table 2 is incomplete. The c-axis value is a near multiple of the b orthohexagonal value, resulting in a coincidence of many reflections. Hence, the correct assignment of Miller indices will require a knowledge of the structure. However, the symmetry and cell dimensions given have been deduced from single crystal studies. Based on limited Weissenberg and precession data, we find reflections hkl present, hhl with 1 = 2n, and no limitations on hOl. This leaves C4,, D~a, C4v, D4h and D4h as possible space groups. However, oxygen scattering is so weak that a structure determination is required in order to assign the true space group. At the highest pressure (60 kb) attained in this investigation, samples prepared at temperatures below 800°C crystallize as c~-U205 while those prepared at higher temperatures sometimes appeared as fi-UzO~ and sometimes as a third form of this compound, y-U2Os. The powder pattern of y-U205 has been indexed as a monoclinic cell with a = 5.410 ( 1) ,~, b = 5.481 ( 1) ,~, c = 5.410 ( 1) ,~ and/3 = 90.49 °. The calculated X-ray density for a cell containing two molecules is 11.51 g/cm 3 (measured density = 11.36 g/cm 3) which establishes this phase as an extension of the fluorite-type structure brought about at high pressure. The complete pressure-temperature phase diagram at U205 as it has been developed from our experiments is shown in Fig. 2. It is possible that y-U20~ may be the cubic form of U3Os reported by Wilson, but the lower symmetry of the phase is readily apparent in well-crystallized samples. No other compounds have been identified in this portion of the high pressure phase diagram; UsO.,, and U308 are stable at relatively low pressures, but disproportionate to c~ or /3-U20~ and 6-UO~ as pressure is increased. The boundary runs about 15 kb above and parallel to that shown in Fig. 2 for the formation of U205 from U80,9 and U8021. UOz.75-UO3.00 At ambient pressure the uranium-oxide phase diagram (near U(VI)) has been
Uranium-Oxygen System at U205 70
i
i
I
'
'
i
,
I
60
'
'
i
,
I
r u2o,
~_ O
o -~i~
5O --- 40 ._a
o
~ 30
0
I
0
+ + BUz
:l:-/~
0
•
a2O
0
0
I0 0
l
/I
•
UsOt9 + Ua021
i
i
I 500
i
L L i
I I000
Temperature
,
in *C
i
i
,
I 1500
Fig. 2. P r e s s u r e - t e m p e r a t u r e p h a s e relationships at U20.~.
3246
H.R.
HOEKSTRA,
S. S I E G E L
and F. X. GALLAGHER
shown to be very complex. In contrast the high pressure diagram is remarkably simple. No evidence has been found for the existence of the UO.,.9,, phase at 15 kb pressure or above, and ¢-UO3 is the only form of the trioxide encountered in the 15-60 kb interval. (The high pressure form of UO3 was not obtained, however, at 4 kb and 500°C in hydrothermal reactor experiments.) Attempts to determine the upper temperature stability limits for ¢-UO3 are difficult because sample "blowouts" occur frequently when assemblies containing UO3 are maintained at 1200°C or above for more than a few min. However, ¢-UO3 has been identified as a constituent of samples prepared at temperatures to 1500°C. Figure 3 summarizes our p-T data on the uranium oxides. I.R. spectra The i.r. spectra of the fluorite-type uranium oxides are rather simple. Uranium dioxide has a single strong absorption near 350 cm -1 arising from U - O stretching vibrations in the lattice[l 6]. Oxidation of UO,, to U409 and U80~9 broadens the absorption envelope as the single peak splits into several maxima in the 250-500 cm -1 frequency range [2]. Further oxidation of uranium to the uranyl-type oxides gives more complex spectra as the absorption maxima arising from vibrations of the primary and secondary bonds become separated e.g. U-O~ at 740 cm -a and U - O . at 530, 495 and 440cm -~ in U308 (Fig. 4). Bending vibrations occur below 300 cm-L The i.r. spectra of the three U205 phases (Fig. 5) demonstrate the gradual elimination of internal (group) vibrations in the uranyl region and assimilation of the various stretching and bending modes into a single broad absorption peak characteristic of the fluorite-type structure. 2-0
2.1
2'2
2'3
2.4
/i
15oo
2.5
l' o -
2.6
r
2"7
2-8
2-9
I
I
I
: i J !
E
....../,/'/" 2.0
,"......
,
,
2.1
2.2
/,
2.5
L
,
, ,
//[
2.4 2.5 2.6 Oxygen-uronium rotio
F i g . 3. P r e s s u r e - t e m p e r a t u r e 16. T . S h i m a n o u c h i ,
II II If
i I I
...........l...........t...L._....¢..4.....I .........,t..I .......J........................................ ," •. . . . . I.,4 I "( /" "a ,.[i { ,'' / ," ,/17 f I," I,/ ," I /" II f" .,r / r II /" /" I ,,"" I ,,,"" .' ."
® 500
I-
/
500
I I I
I I
M. Tsuboi and T. Miyazawa,
4/O I000 .u
1ooo
o
3"0 1500
I
I
I
2.7
2,8
2.9
phase relationships:
3-o
UO2-UO3.
J. c h e m . P h y s . 3 5 , 1 5 9 7 ( 1 9 6 1 ).
( 60
The uranium-oxygen [
I
system
(
at high p r e s s u r e
I
I
v
I
d
3247
r
~.
I
i ~u(3//~/ / ~ I
// I000
900
800
I
1
I[ I
i"~ \ l~/t
I
I
700 600 500 400 WAVENUMBER CM -I
I
I]
300
200
Fig. 4. I.R. s p e c t r a o f U 3 0 ~ a n d ~ - U O a .
The high pressure form of UO3 has been shown[10] to retain uranyl bonding with 2 U - - O bonds at 1.83 A and the remaining 5 bonds ranging from 2.21 to 2.56 in length. With such a range of bond lengths the i.r. spectrum becomes rather complex. Figure 4 illustrates the observed spectra for ~-UO3. The characteristic strong absorption in the uranyl region is evident.
I000
I
I
I 900
I 800
[
I
I I I I 700 6 0 0 500 4 0 0 WAVENUMBER CM -I
Fig. 5.1.R. s p e c t r a o f U 2 0 5 p h a s e s .
I 500
200
3248
H.R. HOEKSTRA, S. SIEGEL and F. X. GALLAGHER
Thermal analysis The uranium oxide phases prepared at high pressure, particularly the three U205 modifications, were investigated by a combined D T A - T G A technique. Thermogravimetric data on duplicate runs with each U~O5 phase were consistent but the differential thermal analyses were not sufficiently reproducible to merit their use for quantitative enthalpy change measurements. Sample packing and crystallinity are believed to be the variables largely responsible for this difficulty. Thermal analyses were used, however, to determine stability towards phase change, disproportionation, and oxidation. Samples were first heated in an argon atmosphere to measure thermal stability, then in air to determine stability toward oxidation. The oxidation experiment also provided a convenient confirmation for the oxygen-uranium ratio in the sample. In argon a-U20~ showed a very small endothermic effect centered at 500°C. Static sealed tube experiments confirmed the disproportionation of a-U205 to U~O9 and UsO2,. Hexagonal fl-U205 shows two small exotherms centered at 225°C and 375°C when heated in an argon atmosphere. X-ray powder patterns on fl-U205 samples which had been heated to 300°C and to 450°C reveal that the first exotherm involves a minor change in the powder pattern and that the second exotherm results from a disproportionation of U205. Monoclinic y-U20~ disproprotionates exothermally in argon at 310°C. We note that in each instance decomposition of U.,O5 occurs by disproportionation rather than retracing the 3' -~ fl -~ a-U20~ sequence. Oxidation of all three U205 phases is exothermic, a-U205 gained 1.0_+ 0. l per cent (theoretical is 0.96 per cent) in weight during oxidation to U3Os. The exotherm maximum occurs at 470°C when samples were heated at 6°C/min and at 500°C at a 15°C/min heating rate. The hexagonal fl-UzO~ was less stable toward oxidation, with its exotherm centered at 285°C. The measured weight increase was 1-! _+0.2 per cent. Monoclinic y-U20~ is the only phase to disproportionate prior to oxidation. Samples heated in air show two exotherms with no weight change associated with the first; the second exotherm at 360°C is accompanied by a 1.1 -+ 0.2 per cent weight increase. The exotherm associated with oxidation of UsO,9 to U3Os is centered at 425°C, the observed 1.7(1) per cent measured weight gain compares well with a theoretical 1.69 per cent. Oxidation of U,6037 occurs at 525°C with a 2.0(0.1) per cent weight gain; the theoretical gain is 2.05 per cent.