Preparation and Properties of Ferrospinels Containing Ni3+

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Abstract
Nickel‐iron spinels with compositions between NiFe2O4 and Ni2FeO4 have been prepared under oxygen pressures up to 2000 psi and at temperatures between 250° and 1000°C. Good agreement was found between the measured lattice parameters of the resulting spinel phases and the values obtained by extrapolating from the previously determined spinel series between Fe3O4 and NiFe2O4. Spinel compositions with lattice parameters as low as 8.3005 A were obtained; this compares with 8.3885 for Fe3O4 and 8.3394 for NiFe2O4. As is the case of the spinel solid‐solution series between Fe3O4 and NiFe2O4, the composition of these nickel‐rich spinels was found to depend strongly on both the oxygen pressure and the temperature. These relationships have been determined and are briefly discussed. Since it had been shown that there is essentially no measurable deviation from Végard's law between Fe3O4 and Ni2FeO4, lattice parameter measurements were a convenient method of determining the compositions of the spinel phase. The fact that single phase nickel‐iron spinels can be prepared with nickel concentrations greater than NiFe2O4 indicates that Ni3+ replaces Fe3+ in the spinel lattice. Of the several possible configurations for a composition such as Ni2FeO4, there is strong evidence from magnetic moment measurements that on octahedrally ligated spinel sites the preferred arrangement is the [Ni2+ Ni3+] ion pair. The corresponding decrease in moment as Ni3+ substitutes for Fe3+ between NiFe2O4 and Ni2FeO4 agrees very well with that predicted by the Néel antiparallel array theory. A lowering of the Curie temperature was also observed in this compositional range.

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