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. Lister, 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 . htokcevn, 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).
April 20, 1959
HEATCAPACITY AND THERMODYNAMIC FUNCTIONS OF CU-FERRIC OXIDE
range 240 to 190°K. according to the magnetization data by Guillaud" on pure synthetic samples. The latter author found t h a t the transition temperature decreased with increasing strength of the magnetic field and also a discontinuity in the resistivity of a-FezOs and rather pronounced changes in the expansion coefficient in the range of the anomaly. No latent heat was detected, although the presence of a second-order transition could not be excluded. The transition was studied further by NCel and Pauthenet12 and interpreted by NCel13 by supposing that the ferromagnetism consisted of an isotropic and an anisotropic part. The anisotropic part is only observed above 260"K., i e . , when the magnetic moment is in the (111) plane. Studies of the thermomagnetic properties14 of two natural hematite crystals showed in contrast to those measured by NCel and Pauthenet12 only the anisotropic part of the ferromagnetism at room temperature. On cooling, this ferromagnetism in the (111) plane gradually disappeared from 253 to 173°K. The low-temperature susceptibility values found were in accord with those reported b y MorinlO and Guillaud." Properties of the isotropic and anisotropic ferromagnetism in hematite were measured in the range 200 to 300°K. by Haigh15 and a cyclic hysteresis effect studied in detail. Magnetic resonance measurements on a-Fe203 confirm the presence of a highly anisotropic, weak ferromagnetism which disappears below room temperature. According to the results by ilnderson, et al., l6 on a synthetic single crystal (containing 3Yc Fez+) the resonance absorption corresponding to this anisotropy goes down to zero rather abruptly in the range from 260 to 255"K., which is not easily explained on the basis of a change in the direction of the magnetic moments. Similar measurements by Kumazai, et al.,17on a natural sample showed the transition to take place a t about 220°K. and less abruptly. Some spread in the transition temperature also was observed by neutron diffraction between synthetic ,and natural hematite, l8 but there was no noticeable effect of a magnetic field on the transition temperature or line intensity above and below the transition. Measurements of magnetostriction in hematite in the transition region have shownlg that the magnetostrictive effects are closely related to the parasitic ferromagnetism, both having the same type of dependence on applied field strength and temperature. On the basis of a saturation magnetization of 0.43 e.m.u./g." the uncompensated magnetic moment is 0.006 Bohr magneton per iron atom, and possible explanations given by NCe14 about the origin of the parasitic ferromagnetism is t h a t a(11) C. Cuillaud, J . p h y s . rad., 1 2 , 489 (1951). (12) L . Niel a n d R P a u t h e n e t , C o n p f . end., 234, 2172 (1952). (13) L. N'i.el, Revs. M o d . P h y s . , 25, 88 (1953). . , 2043 ( 1 4 ) H Bizette, R . Chevallier a n d B. Tsai. Comfit. ~ f ' ? ~ d236, (1933). (15) G . Haigh, Phil. M a g . , 2, 877 (1957). (16) P . W. Anderson, F. R . M e r r i t t . J . P. Remeika a n d W. .4. Yager, Phys. Rew., 93, 717 (1954). (17) H. K u m a z a i , H . Ahe, K . Ono, I . Hagashi, J. Shinada and K . l w a n a g a , i b i d . , 99, l l l G (1955). (18) L. M . Corliss. J. M. Hastings a n d J. E . G o l d m a n , ibid., 93, 893 (1954). (19) H. M. A . Urquhart a n d J. E. G o l d m a n , i b i d . , 101, 1443 (1956).
1781
Fe203 is a slightly defect structure or that the sublattices inside a domain are not equivalent, either for geometrical reasons or on account of local fluctuations in composition. According to Li,*O who also has made a semi-empirical analysis of the super-exchange interaction responsible for the magnetic lattice of hematite, the weak ferromagnetism observed in the (111) plane of hematite is identified with the magnetization in domain walls, pinned down by lattice imperfections. The disappearance of this ferromagnetism on cooling is interpreted as due to mutual annihilation and migration of the domain walls to the surface. On heating through the transition, the domain walls are recreated by nucleation and stabilized by the imperfection centers. I t is considered extremely improbable that all the moments would turn 90" a t the same temperature. More recently, Dzyaloshinski21has developed a thermodynamic theory for explaining the weak ferromagnetism in a-Fe203 and concludes that the transition a t about 250°K. is of the first kind. An expression for the entropy change is given but not evaluated. While Dzyaloshinski considers the ferromagnetism due to relativistic spin-lattice and magnetic dipole interactions, Bertaut22 finds that dipole interactions can neither account for the weak ferromagnetism nor the magnetic- anisotropy of hematite. Ferric Oxide Sample.-The ferric oxide for this investigation was prepared by oxidizing "Iron by Hydrogen," Merck, in air a t 1000" until constant weight was attained; this required 8 hr. Iron determination gave 69.91% total iron (theoretical, 69.94Yc) and the ferrous content was found to be zero within the limits of error (0.05%). Spectrographic analysis showed the presence of about 0.0170 Mn, less than 0.017,Al, Co, Mg, Ni and Si, and less than 0.001~,Ca, Cu and Sn. X-Ray powder photographs of the product showed only lines from a-Fez03. Its lattice constants (XCoK,, = 1.78890 A.) were determined at about 25' in a 11.48 cm. diameter camera with asymmetric film mounting. The rhombohedral unit cell dimensions are a = 5.4266 A. and a = 55.556", which agree very well with the values a = 5.4271 k. and a = 55.263" (25") found by Willis and R ~ o k s b y . The ~ ~ sample was found to be slightly ferromagnetic with a moment of 0.36 gauss/g. a t room temperature, i.e., of the same magnitude as reported by earlier investigators. 1-14 The particle size was of the order of 5 p. Experimental Technique.-Mark I cryostat and the technique employed in low temperature adiabatic calorimetry are described elsewhere.*< T h e copper calorimeter (laboratory designation W-10) has a rapacity of 92.8 cc.; it is gold-plated inside and out and has no vanes. The heat capacity of the empty calorimeter was determined separately, u i n g the same thermometer and heater and exactly the same amount of indium-tin solder for sealing and Apiezon-T grease for thermal contact with the thermometer and heater. It represented from 22 to 807, of the total heat capacity observed. ( 2 0 ) Y . - Y .Li, i b i d , 101, 14.7ll r l R . j G ) ; 102, 101.5 110.i8) 121) I . Dzyaloshinski. J . Phvc. C h r w .Solids, 4 , '211 (1938). (22) F.R c r t a u t , C o m p t . reud , 246, 3X35 (19.78). ( 2 3 ) B . T . R.I. Willis a n d H . P . R o o k s b y , P r o r . I'hss S o < , B65, 9:o (1952) (24) E. F. W e s t r u m , J r , , a n d A. F. Beale, Jr., to be published.
FREDRIK GR$NVOLD AND EDGAR F.
1782
100
0
25" t o provide thermal contact between sample and calorimeter.
K.
TEMPERATURE,
300
200
Results
30 A
-
0
PARKS 6 KELLEY,
SPECULAR HEMATIT
PARKS 6 KELLEY,
KAHLBAUM
T H I S RESEARCH
I.
0 W
0.37 .
0 20
0
I W
W
-n
-1
0
I
I
0.2 y
i
The heat capacity determinations are listed in Table I in chronological order and 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 iron as 55.85. The data are presented in terms of one mole of FeeOs, ie., 159.70 g. An analyticallydetermined curvature correction was applied to the observed values of AH/AT. The approximate temperature increments usually can be inferred from the adjacent mean temperatures in Table I.
0
