JOURNAL O F T H E A M E R I C A N CHEMICAL S O C I E T Y (Registered in U. S. Patent Office)
(0 Copyright, 1959, by the American Chemical Society) NUMBER 8
APRIL 28, 1959
VOLUME 81
PHYSICAL AND INORGANIC CHEMISTRY [CONTRIBUTION FROM
THE
DEPARTMENTS OF CHEMISTRY OF
THE
UNIVERSITY OF MICHIGAN AND
THE
UNIVERSITY OF OSLO]
Low Temperature Heat Capacity and Thermodynamic Functions of Triuranium Octoxidel BY EDGAR F. WESTRUM,JR.,
AND
FREDRIK GR$NVOLD
RECEIVED NOVEMBER 10, 1958 The heat capacity of UIOShas been measured over the range 5 to 350°K. A small A-type anomaly was observed with a maximum at 25.3"K. and an entropy increment of 0.56 e.u. per mole of UaOa. The transition is presumed by its similarity with that in UOp to be of magnetic origin. Thermodynamic functions have been calculated by numerical quadrature of the heat capacity and the values of C,, So,H" - H:, and -(F" - H : ) / T at 298.15"K. are: 56.87 cal. deg.-' mole-', 67.534 cal. deg.-l mole-', 10216 cal. mole-' and 33.572 cal. deg.-l mole-', respectively.
Low temperature heat capacity measurements of the structurally-related uranium oxides U022 and U40g3have shown the presence of a A-type transition only in the former, associated with an antiferromagnetic ordering4of the uranium atoms below 30°K. Therefore i t seemed of interest to study other uranium oxides to gain further insight in the conditions under which such transitions occur, and for this purpose the heat capacity of U308has been measured in the low temperature range. There is, furthermore, a need for precise data on the thermodynamic properties of U308,as the only existing heat capacities are average values from drop calorimetric data by Russell6 over three regions in the range 82.7 to 314.6"K., one very old set of measurementse in the range 0 to 100" and a more recent one7in the range 25 to 100". Reasonable extrapolations or interpolations have been made, however, on the basis of the data obtained2 for UOz and UO,, but they are likely to be in some error because of the rather different structures for the oxides and the occurrence of A-type transformations. The existence of an additional (1) This work was supported in part by the Division of Research, U. S . Atomic Energy Commission. (2) W. M . Jones, J. Gordon and E. A. Long, J . Chem. Phys.. 2 0 , 6 9 5 (1952). ( 3 ) D . W. Osborne, E. F. Westrum, Jr., and H. R. Lohr, THIS JOURNAL, '79, 529 (1957). (4) A. Arrott and J. E. Goldman, Phys. Reo., 108, 948 (1957). (5) A. S. Russell, Physik. Z . . 1.3, 59 (1912). (6) J. Donath, Ber. Chem. Ges., 12, 742 (1879). (7) T. M. Snyder (Reported by J. J . Katz and E. Rabinowitch in "The Chemistry of Uranium, Part I." McGraw-Hill Book Co., New York, N. Y.. 1951, p. 273).
crystalline form of U308, a metastable orthorhombic @-modificationsobtained by oxidation of UO2.u in oxygen to 750°, has been claimed. Subsequently evidence for still another modification with cubic structure has been obtainedg by heating U308 and silica in a sealed gold capsule a t about 700" under a pressure of 4000 atm. The structure of the ordinary orthorhombic us08 (or a-U3OB)studied here is not as closely related to either th.at of a - U 0 3 or Reo, as supposed by earlier investigators, in that the oxygen atoms do not surround the uranium in the form of staggered hexagons and deformed hexagonal double pyramids or octahedra with anion vacancies. Recent neutron diffraction work by Andresen13 has shown that two-thirds of the uranium atoms are surrounded by seven oxygen atoms a t the corners of a pentagonal double pyramid and one-third by six oxygen atoms a t the corners of an octahedron. X-Ray single crystal studies by Chodura and Ma1yl4indicated the presence of two different kinds of U30, crystals, one ( 8 ) H. R. Hoekstra, S . Siegel, H. L. Fuchs and J . J . Katz, J . Chem. Phys., 69, 136 (1955). (9) R . Collette, as reported by R. M. Berman, Ani. Mineral., 44, 707 (1957). (10) W. H. Zachariasen, Manhattan Project Report CK-2667 (1947). (11) N. C. Baenziger, U. S . Atomic Energy Commission Declassified Document, AECD-3237, October 15, 1948. (12) F. Gr#nvold, Nolure, 168, 70 (1948). (13) A. F. Andresen, A c f o Crysf.,11, 612 (1958). (14) B. Chodura and J. Maly, Prepublication print P/2099, Second United Nations Intern. Conference for the Peaceful Uses of Atomic Energy, Geneva, 1958.
1777
1778
EDGARF, WESTRUM, JR.,
orthorhombic, the other monoclinic in the form of twins. These structures have a close relationship t o that proposed by Andresen,') confirming the slight movement of some of the uranium atoms from special t o more general positions, but superstructure reflections were observed which require a doubling of the c-axis. The scattering ratio of uranium t o oxygen is considerably greater for neutrons than for X-rays; hence, the coordinates of the oxygen atoms found by neutron diffraction are considered to be more reliable in establishing the main features of the U808structure. UIOSSample.-The UIOs sample was obtained by purify-
ing uranyl nitrate hexahydrate, A.R., following essentially the procedure used by Honigschmid.'d This UsOa was reduced t o U G by heating it .in dry, purified hydrogen gas at 500' until the formation of mater ceased. The temperature was raised t o 1200" and the sample kept at this temperature for 4 hr. before cooling to room temperature. It then w a s oxidized in air a t 800" t o constant weight, transferred to a vitreous silica tube which was evacuated and sealed, heated at 800" for seven days and gradually cooled t o room temperature over a period of two months. Spectrographic analysis of the U ~ O Ssample showed the AI 60, B < presence of these impurities (in p.p.m.): 0.08, Cu 3, Fe 10, hlg 30, S i 20 and Si 200. The weight increase upon oxidation of UO: to UaOa corresponded t o the theoretical value within 0.01 O/U atom, and X-ray powder photographs showed only the presence of the UaOa-phase described earlier.'$ Experimental Technique .-The Mark I cryostat and the technique employed in low temperature adiabatic calorimetry are described e1~ewhere.I~The copper calorimeter (laboratory designation W-9) has a capacity of 90 cc.; it is gold-plated inside and out and has six vanes. The heat capacity of the empty calorimeter was determined separately, using the same thermometer and heater and exactly the same amount of indium-tin solder for sealing. The calorimeter, thermometer and heater represented from 15 to 39% of the total heat capacity observed. Corrections for a slight difference in the amount of Apiezon-T grease on the empty and loaded calorimeter were applied. The platinum resistance thermometer (laboratory designation A-3) has been calibrated by the Kational Bureau of Standards, and the temperatures are judged t o correspond with the thermodynamic temperature scale within 0.1"K. from 5 to 10"K., within 0.03"K. from 10 t o 90°K., and within 0.04"K. from 90 to 350°K. Precision is considerably better, and the temperatures are probably correct to 0.001"K. after corrections for quasi-adiabatic drift. The calorimeter was loaded with sample and evacuated, and helium was added a t 4 cm. pressure a t 25' to provide thermal contact between sample and calorimeter. The mass of the calorimetric sample mas 267.912 g.
Results The heat capacity determinations are listed in Table I in chronological order, expressed in terms of the thermochemical calorie, defined as 4.1840 absolute joules. The ice point was taken to be 273.15"K. and the atomic weight of uranium as 238.07. The data are presented in terms of one mole of U308, i.e., 842.21 g. An analyticallydetermined curvature correction mas applied to the observed values of S H / A T . The approximate temperature increments usually can he inferred iron? the adjacent mean temperatures in Table I. The heat capacity w r s u s temperature curve is shown in Fig. 1 over the low temperature region, where a X-tylpe transition is encountered a t about 23°K. This transition resembles that found in (15) 0. Honigschmid, 2. a7rorg. oilgem. Chem.. 226, 489 (1936). (16) F . Grdnvold. J . I n o r g . Kiccl Chern . 1, 357 (1955). (17) E. F. I r e s t r u m , J r . , and A. P. Beale. J r , t o be published.
AND
FREDRIK GR$NVOLD
VOl. 81
TABLE I HEATCAPACITY OF U,O, IN CAL. DEG.-' MOLE -1 Molecular weight = 842.21; 0°C. = 273.15'K. T.OK. CD Series I
95.80 103.64 112.78 122.11 131.23 140.23 148.i6 157.14 165.91 175.01 1M.09 193.22 202.34 211.63 221.06
26.78 28.85 31.13 33.29 35.28 37.08 38.68 40.16 41.57 43.03 44.40 45.68 46.91 48.12 49.29
Series 11 211.07 223.37 232.88 242.51 231.93 261.09 270.15 279.15 288.06 296.92 305.72 314.45 323.32 332.11 340.03 346.86
48.41 49.43 50.62 51.76 52.72 53.64 54.39 55.28 3.10 56.77 5'7.41 58.07 58.66 59.20 59.67 60.07
Series I11 24.80 26.78 29.16 31.62 31.3
6.321 1.977 5.483 6.178 6.S20
T,O K . 36.93 39.63
C.
7.960 8.681
Series IV 13.80 15.07 16.i6 18.43 20.36 22.33 24.21 26.45 29.37 32.59 35.90 39.52 43.56 47.96 52.85 58.24 61.29 70.92 77.69 84.49 91.8'7 99.98
1.174 1.473 1.949 2.516 3.311 4.324 5.737 5.228 5.542 6.461 7.485 8.613 9,960 11.444 13.117 14.93 17.03 19.20 21.38 23.50 25.71 27.90
Series IT 4.77 5.36 6.25 i.32 8.37 9.22 10.11 10.99 11.95 13.06 11.46 12.44 13.59 14.96 16.47
0.078 .111 ,155 ,221 ,314 ,437 ,510 ,633 .794 1.016 G.703 0 8ii 1.126 1.461 1.857
T,O K . 18.09 19.96 21.92 23.07 23.46 23.83 24.18 24.51 24.83 25.13 25.42 25.76 26.12 26.50 26.87 27.23 27.58 28.01
C.
2.395 3.123 4.084 4.770 5.050 5.331 5.639 5.971 6.362 6.922 6.396 4.935 4.873 4.903 4.942 5.001 5.072 5.181
Series I'\ 17.11 18.94 22.17 23.22 24.17 24.73 24.96 25.11 25.24 25.35 25.41 25.49 25.58 25.65 25.73 25.81 25.90 26.00 26.15 26.34 26.48 26.68 27.97
2.059 2.707 4.224 4.885 5.626 6.222 6.493 6.950 8.087 i.675 6.883 5.261 5.175 5.030 4.991 4.893 4.863 4.852 4.867 4.810 4.985 4.880 5.188
UO2, but i t is not as large and its maximum lies about 4°K. lower. Values of C, So,H" - H,", and - ( F " -H;)/T for CaOa a t selected temperatures are listed in Table 11. The enthalpy, entropy and free-energy increments were computed by numerical integration, using graphically-interpolated values of heat capacity. The heat capacity values are considered to have a probable error of about 5y0 at 5"K., 17 a t 10°K. and O.lyO above 25°K. Values below 5°K. were estrapolated with a T 3 function. The effects of nuclear spin and isotopic mixing are not included in the entropy and free-energy values. The estimated probable error in the thermodynamic functions is O.lyoabove 100"K., but some of the values are given to an additional digit for comparison purposes.
April 20, 1959
H E A T CAPACITY AND THERl1ODYXhMIC
FUNCTIONS OF TRIURANIUM OCTOXIDE
1'T'Tg
TABLE I1
MOLAL THERMODYNAMIC FUSCTIOXS OF UaOr ~. -(Fo
CP.
T ,O K . 10 15 20 25 30 35 40 45 50
60 70 80
90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 3 .io 273.15 298.15
SO
Cal. deg. mole
cal. deg. mole
0.49 1.47 3.16 6.i2 5.73 i.21 8.77 10.45 12.15 15.55 18.90 22.10 25.16 27.89 30.46 32.82 35.00 37.01 38.87 40.61 42.24 43.79 45.25 46.62 47.92 49.14 50.30 51.40 52.47 53.49 54.45 55.35 56.21 5i.01 60.25 51.74 56.87
0.196 0.558 1.188 2.192 3.163 4.155 5.219 6.348 7.537 10.052 12.701 15.436 18.218 21.011 23.791 26.544 29.257 31.926 34.543 37.108 39.619 42.078 44.484 46.841 49.148 51.405 53.616 55.779 57.900 60,100 62.013 64.011 65.968 67.887 76,933 62.617 67. 534
Ha - H:
- Ri),
T
cal. -
cal. deg. mole
1.42 6.06 17.23 40.14 66. 78 99.07 139.0 187. 0 243.4 381.9 554.2 i59.3 995.8 1261.1 1553.0 1869.5 2208.7 2568,8 2948.3 3345.7 3760,O 4190.3 4635.5 5095 5568 6053 65.50 7059
0,054
mole
,154 ,326 .557 ,937 1.324 1.745 2.193 2.668 3.417 4.7% 5.945 7.151 8.399 9.673 10.965 12.267 13.577 14.888 16.197 17.501 18.799 20.087 21.367 22.636 23.892 25.137 26.368 27.585 25.916 29.985 31.166 32.332 33.455 39.058 30 360 33,572
8
1 .
(3
W
n
-
6
I S
W
-1
0
I
4 4
0
l 2
u
0 0
IO 20 TEMPERATURE,
30 OK.
Fig. 1.-Lfolal heat capacity of UaOa from 5 tn 35'K. 0,0, Data from series 111, I V , V and V I are indicated by 1, and A, respectively. The dashed line indicates the estimated lattice heat capacity.
mole-' and O.SG cal. deg.-' mole-I of L-201, respectively. They were evaluated by subtracting IDlS the lattice contribution from the total increment 8107 in the range 10 to 40°K. The lattice heat ca8648 pacity values were estimated on the basis of a linear 9197 change in Debye 0's with temperature from 218 a t 97% 1OoK. to 324 at 40°K. This is indicated in Fig. 1 10321 by a dashed line. The entropy increment asso13256 ciated with the A-type transition is about 21y0of the entropy increment observed per uranium atom 8819 for UO? (0.87 e.u. per mole UO?),?and because of 10316 the proximity of the transition temperatures, one Discussion might be led to assume t h a t the U309 snvple I t is interesting to compare the present results contained UO?. Further consideration shows that with the only existing low temperature enthalpy this is unlikely because (1) the weight increase upon data, obtained by Russell5 in 1912 by drop calo- oxidation of the UO?-sample to U 3 0 a had the exrimetry. The mean values from his work are pre- pected value, (2) X-ray powder photographs of the sented in Table 111. T h e agreement between the sample showed only lines from the L-308-phase and (3) if the sample were deficient in oxygen. L - 4 0 9 TABLE111 should be formed (as is known to be the case under COMPARISOS WITH EARLIER ESTHALPY MEASUREMEXTS the same heating condition^)^ as i t does not shnw a H n - H t t , cal./mole A-type transition. Xelrertheless, a new L-ROqTI, OK. Tz,O K . Russell5 This research sample was made up from uranyl acetate dihydrate 4044 195.0 4052 82 7 which was first ignited over a Bunsen burner and 3997 3906 196.1 273.2 then heated in air to constant weight in an electric 2205 314.6 2322 275.7 muffle furnace a t about 1000" and cooled to room first set of values is very good, while that for the temperature. A sample weighing 142.2% g. other two is poorer. The drop calorimetry value was loaded into the calorimeter as before, and 1s by Donaths in the range 0 to 100" and the NBS runs were made in the region from 21.05 to 2i.SG"K. value' in the range 25 to 100" are considerably which showed the presence of the transition a t the higher than those obtained by extrapolation of the same temperature and the same enthalpy increment within a few tenths of 1c;C. present results. The enthalpy and entropy increments associated R'ith respect to its magnetic properties, UaOs and Lc ions, and with the A-type transition a t 25.3'K. are 12 cal. behaves as if it consists of
-.-
7
FREDRIK G R ~ N V O AND L D EDGARF. WESTRUM, JR.
1'is0
not of U 4 + and U6+ ions,18 on the assumption of spin-only magnetism. The same conclusion was reached by Dawson and Lister, l9 but deviations from the Curie-Weiss law were observed. The U6+ ions have no unpaired electrons, while U5f has one, supposedly in a 5f orbital, which gives rise to the paramagnetism of u308. For spin-only magnetism the magnetic entropy should thus amount to 2R In 2, or 1.84 e.u. per mole u&, but apparently only a fraction of this is associated with the effective magnetic moment of U308,decreases considerably as the temperature is lowered, and only amounts t o 0.6 Bohr magneton per uranium atom a t 25"K., and the magnetic entropy might therefore be spread over a rather large temperature region. In the crystal structure of U308 infinite linear -U-0-Uchains are present in the (18) H. Haraldsen a n d R.B a k k e n , Satirrwiss., 28, 127 (1940). (19) J K . Dawson a n d M . W. L i s t e r , J . Chem. S O L . 2181 , (1950).
[ C O S T R I l 3 L T I O N F R O M THE
DEPARTMEXT OF
Vol. 81
c-direction, a situation which should favor superexchange.20 Since magnetic susceptibility measurem e n t ~have ~ not indicated significant changes in the magnetic properties of U308 around 25°K. no conclusion concerning the mechanism can be drawn at present. Acknowledgments.-The support of the Division of Research of the U. S. Atomic Energy Commission, the interest of Professor H. Haraldsen in this study, the assistance of Miss Liv Gjertsen with the preparation of the compound and the cooperation of Mr. R. A. Berg with the measurements and calculations are acknowledged with gratitude. The spectrographic analysis was kindly carried out by Dr. J. Haaland, JENER, Kjeller, Norway. (20) P. W. Anderson, P h y s . Rev., 79, 350 (1950)
A N N ARBOR,MICHIGAN RLINDERX,SORWAY
CHEMISTRY O F THE UXIVERSITY O F hfICHIG.4N]
a-Ferric Oxide : Low Temperature Heat Capacity and Thermodynamic Functions' BY FREDRIK GR~NVOL 4 NDD EDGAR F. WESTRUM, JR. RECEIVED NOVEMBER 10, 1958 The heat capacity of synthetic a - F e 2 0 8was determined from 5 t o 350'K. No heat capacity anomaly was observed at the magnetic transition about 25OoK'. Thermodynamic functions have been calculated and the values of C,, So,H" - €€:, and - ( F " - H",)/Tat 298.15"K. are: 21.80 cal. deg.-I mole-', 20.889 cnl. deg.-' mole-', 3718.9 cal. mole-' and 8.416 cal. deg.-l mole-', respectively
Ferric oxide exists in two crystalline modifica- have been made5 over three regions in the range 81 tions, alpha and gamma, of which alpha is con- to 317°K. Neutron diffraction6 work on a-Fen03in the range sidered the stable form, but there are indications that it might transform into gamma under special SO to 1000°K. confirmed earlier X-ray data7-9 conditions.2 The heat capacity of a-ferric oxide leading to a rhombohedral unit cell containing two previously has been measured3 over only four formula units, with the oxygen atoms forming a of the octanarrow regions between 90 and 290"K., and it was slightly distorted close packing and considered of interest to extend the measurements hedral interstices filled by iron atoms. The structo lower temperatures and obtain more accurate ture consists of sheets of iron atoms parallel to the thermodynamic data for this compound. More- (1 11) plane and sheets of oxygen atoms in between. over, a-Fe203 is a substance of rather complex In concordance with the view expressed by Neel,+ magnetic proper tie^,^ having a weak ferromagne- the neutron diffraction data indicated the existtism, which disappears below about 250°K., ence of an antiferromagnetic structure. All four iron atoms in the unit cell are located on the space superimposed on its antiferromagnetism. I n the study by Parks and Kelley3 two different diagonal. The neutron diffraction data suggest samples were used, one consisting of large crystals that they are non-equivalent and have relative of specular hematite, the other of a finely divided spin orientation (+ - - +). At room temperature powder prepared from iron oxalate (Kahlbaum). the moments seem to be parallel to the (111) plane The heat capacity of the synthetic sample was con- and directed toward one of the three nearest siderably higher (3.ScJc a t 90"K., 2.47, a t 2T5"K.) neighbors, while a t lower temperatures they are than of the mineral, which was explained on the oriented normal to the (111) plane or, in other basis of its somewhat amorphous state, as inferred words, in the (111) direction. from X-ray powder photographs. The lower This change in the direction of the moments \ d u e s of the specular hematite were adopted, and causes the parasitic ferro- or ferrimagnetism to the resulting entropy a t 298°K. found to be 21.5 disappear below about 250°K. according to the i 0.5 e.u. -1part from these data a series of magnetic susceptibility data by bIorinlO and in the enthalpy measurements on a synthetic sample ( 5 ) .4.S . Russell, P h y s i k Z.,13, 59 (1912). !I:
T h i s w o r k x a i ~ u p l i u r t e c lin pxrt I,? t h e Divisir>n .Atntoro, Guzz. c h i m , 70, 145 (1940). ( 0 ) L-, Y Bel\,and V. I . h t o k c e v n , T v i i d y Do/s. i l k n i l S a i d 2 .9.S.CR 57, S I 9 ( 1 0 1 7 ) . ( I O ) F. J . Rlorin, P h y c . Rev., 78, 819 (1950).
