March , 1959
NOTES
of the zone movement, which is in agreement with the theoretical predictions. However, the maximum concentration difference obtained experimentally was much less than that predicted by the theory. Discussion The conditions for the numerical evaluation of the theoretical separation were selected in order to limit the matrix size and the volume of the calculations. These conditions imply that ten discrete fractional crystallizations occur as the molten zone moves through the length of the sample. A larger matrix would more accurately represent the true conditions in the process and would also result in a larger theoretical separation of the isotopes. The effects of the isotopic equilibrium
Recent papers4s5 have indicated that certain rare earth-transition metal oxides possess perovskite-type structures which are ideal for the study of indirect magnetic exchange mechanisms. The perovskite cell contains cations which occupy the B sites and a,re arranged in a simple-cubic array with an oxygen ion at the center of the edges of the cube. The systems presented in this paper were studied in order to determine whether an ordering takes place on the B sites of rare earth-iron-chromium, lanthanum-iron-cobalt oxides. It was expected6 that in lanthanum-iron-chromium oxide, an ordering of the cations on the B sites into alternate (111) planes would result in an overlap of the oxygen p orbitals with empty chromium eg orbitals on one side, half filled iron eg orbitals on the other. It is believed that such a configuration would result in a ferromagnetic Fe-0-Cr interaction. Ordering in alternate (111) planes of diamagnetic C O + ~ in lanthanum-iron-cobalt oxides would produce a' different number of Fe+3 ions in the various (111)~ planes. Since the Fe-0-Fe interaction is antiferromagnetic, this would produce a measurable ferrimagnetic moment. However, no magnetic moments were observed for these systems a t liquid helium temperature. It is therefore concluded that the transition metal ions occupy the B sites; in a random fashion.
HzO
+ DzO
2HD0
011 the
calculations were avoided by basing all the analytical data upon standards prepared from pure (99.8%) DzO and pure HzO. Thus the experimental concentration profiles do not imply that the deuterium is bound as DzO or as HDO, and the value of the equilibrium separation factor used in the matrix calculations likewise avoids this implication. The interface at which freezing occurs is moving into a liquid film which has a lower deuterium concentration than that calculated for the liquid zone as a whole. Adequate stirring of the liquid zone increases the effective separation factor. Smith and Posey found that vigorous agitation and a slow freezing rate were necessary in order to approach the value of a for an equilibrium process. For nonequilibrium conditions they found values of a as low as 1.0007. The absence of agitation in the zone-refining apparatus probably contributed significantly to the great quantitative difference between the theoretical and theexperimentalconcentr* tion profiles. In the spiral apparatus the .freezinginterface was in a vertical portion of the tubing. Because of the temperature. gradient immediately above the freezing interface there was the possibility that a density inversion in the liquid would contribute a mixing effect and thus improve the observed separation. However, the separation does not appear to have been significantly improved by this arrangement. Possibly ultrasonic techniques might be applied in order to agitate the liquid zone. The zone-refining process for the separation of heavy water from ordinary water is so slow a t present that its application may be limited to theoretical studies or to the procurement of ultra-pure samples of D20 or of HzO. Acknowledgment.-Mr. Charles E. Bailey's assistance with the matrix calculations is gratefully acknowledged. PREPARATION AND PROPERTIES OF T H E SYSTEMS LnFe,Crl AND LaFexCol BY
AARONW O L D '
AND WILLIAM
CROFTa
Contribution from the Departments of Chemislru. Lincoln Laboratory,l Lexington, Mass., and Radio Corpovation o f America, Needham Heights, Mass. Received October I S , 1968
447
Experimental Samples of LnCr,Fel-,Oa (Ln = La, Nd, Sm, Y) were prepared by treating the rare earth oxide with varying molar mixtures of iron(II1) oxide and chromium(II1) oxide in air for 96 hours. The samples were reground several times during the heating period. Lanthanum-iron-cobalt oxides, LaCo,Fel-,Oa were prepared by heating lanthanum oxide with varying molar mixtures of iron(II1) oxide and cobalt(I1) carbonate at l l O O o in air for 96 hours and a t 1300° for 18 hours. Standard analytical techniques were used t o determine the amount of rare earth, iron, chromium and cobalt present in these compounds. The rare earth-ironchromium oxides were fused with sodium peroxide and the chromium, present as chromate, was extracted with hot water. The rare earth and iron hydroxides, formed by the fusion, were dissolved in hydrochloric acid and the iron and cobalt were separated from lanthanum by precipitation of the latter with ammonium oxalate. The saturation magnet.ization a t 4'K. was measured by bienyuk and Dwight using a modified vibrating-coil magCELLDIMENSIONS OF Composition
LaFeOa LaCrO, NdFeOa NdCr01 SmFeOI SmCrOa YFeOa YCr03
ao
5.548 5.480 5.456 5.411 5.393 5.367 5.278 5.245
TABLE I THE SYSTEMS LnFezCr(l-x)Oa bo
5.557 5.507 5.585 5.471 5.595 5.496 5.584 5.518
co
7.851 7.758 7.761 7.702 7.714 7.649 7.603 7.540
Volume
(La)
242.05 234.12 236.5 228.0 232.76 225.63 224.01 218.22
(1) The research in this docunient was supported by t,he Army, Navy and Air Force under contract with the Massachusetts Institute of Technology. (2) Staff Member, Lincoln Laboratory, Lexington, Massachusetts. (3) Semi-Conductor and Materials Division, Radio Corporation of America, Needham Heights, Massachusetts. (4) J. B. Goodenough, Phys. Reu., 100, 564 (1955). ( 5 ) A. Wold, R. J. Arnott and J. B. Goodenough, J . A p p l . Pliys., 39, 387 (1958). (6) J. B. Goodenough, private communication.
NOTES
448
Vol. 63
TABLE I1 CELLDIMENSIONS OF THE SYSTEMLaFeZCol-zOs Orthorhombic phase Compoaition
a0
5.548" 5.540 5.542
...
Prepared a t 1100° bo
5.557" 5.554 5.546 Both Phases
...
Prepared at 1300' co
7.851" 7.844 7.839
...
ao
5.548'
... ...
...
...
...
...
5.472
...
...
...
5.483
...
...
...
...
bo
Orthorhombic 5.557"
... ...
co
7.851"
...
...
...
...
5.497 Rhombohedral
7.761 13.196
Hexagonal (rhombohedral) phase 5.469 13.167 ... ... 5.466 13.138 ... 5.446 13.104 ... 5.440 13.098 5.424 13.068 The third decimal place of all lattice constants is less significant because the peaks in the back reflection region of the X-ray patterns are broad.
...
netometer recently described by Dwight, Menyuk and Smith.' No appreciable moment was observed for these compounds a t liquid helium temperature in a magnetic field of 10,000 oe. The lattice symmetry and lattice constants were calculated from spectrometer traces taken with a Philips Norelco Diffractometer, using Ka iron radiation. The results of chemical and crystallographic analysis are summarized in Tables I and 11.
Discussion I n order for a net ferromagnetism t o result from a Fe-0-Cr interact,ion in the rare earth-ironchromium oxides or for ferrimagnetism to result from the antiferromagnetic Fe-0-Fe interaction in lanthanum-iron-cobalt oxide, j t is necessary that there be a preferential ordering on alternate (111) planes of the perovskite lattice of the cobalt or chromium atoms and the iron atoms. Since no magnetic moments were observed at liquid helium temperature, it is concluded that the transition metal ions occupy the B sites in a random fashion. X-Ray diffraction measurements (Table I) have established the solid solubility between LaFe03 and LaCr03, between NdFeOs and NdCr03 between SmFe03 and SmCrO3 and between YFeOa and YCr03. The end member, LnFe03and LnCr03, are orthorhombic; their space group is Dzh(16) Pbnm and there are four formula weight per unit (7)
K. Dwight, N. M e n y u k and D. Smith, J . A p p l .
(1958).
P h y a . , 29, 491
cell. In all cases the substitution of Cr+3for Fe+3 results in a decrease of the unit cell volume. The compounds obey Vegard's rule between each pair, and the unit cell volumes decrease with a corresponding decrease of the rare earth ionic radius. The system LaCozFel-,03 (Table 11) prepared a t 1100" possessed orthorhombic symmetry from LaFe03 to LaCoo.zFeo.bOa. A small decrease in the cell volume was observed upon substitution of cobalt for iron. Between LaC00.~~Fe~.,503 and LaCoo.eFeo.403both an orthorhombic and a rhonibohedrnl phase were present. Pure lanthanum cobalt oxide and compositions to LaCo0.&e0.3603 mere rhombohedral. Samples of the system LaCor Fel- z03prepared a t 1300" showed orthorhombic symmetry: from LaFeO3 to LaC0~.~Fe~.603 and was rhombohedral from LaCo0.6$e0..1503 to pure LaCoOa. The small increase of the unit cell size of LaCoOa, noted in Table 11, is caused by the formation of several per cent. of C O +in ~ the sample heated t o 1300". Acknowledgments.-The authors wish to thank J. B. Goodenough, D. G. Wickham, R. J. Arnott, N. Menyuk and K. Dwight for their invaluable advice and assistance. In addition the authors are grateful to Mr. L. Doctor for carrying out the several analyses of the compounds reported in Tables I and 11.
1
