Phase relations in the Fe3O4-ZnFe2O4-Fe2O3 system at 1, 100 K have been determined by the X-ray microanalysis and X-ray diffraction studies of the quenched samples. The results show that in the Fe304-ZnFe2O4 system magnetite (Fe304) and zinc ferrite (ZnFe2O4) form a complete series of solid solutions which is in equilibrium with the Fe2O3 that is practically pure.
The activities were measured by an e. m. f. method using the stabilized zirconia solid electrolytes. Both the activities of magnetite and zinc ferrite in the Fe3O4-ZnFe2O4 solid solutions coexisting with the Fe2O3 exhibit slightly positive deviations from Raoult's law at 1, 100 K. The activity coefficient at infinite dilution, γ0ZnFe2O4 at 1, 100 K was estimated at 2.2.
Lattice constants for the Fe3O4-ZnFe2O4 spinel solid solutions in equilibrium with the Fe2O3 obey Vegard's law. It was also found that the excess Gibbs energy of mixing in the Fe3O4-ZnFe2O4 solid solutions coexisting with the Fe203 at 1, 100 K showed small values. These two results are interrelated in the similar behavior, which suggest that activities are closely connected with crystal structures.
