Immiscibility in the nickel ferrite–zinc ferrite spinel binary

Immiscibility in the trevorite (NiFe2O4)–franklinite (ZnFe2O4) spinel binary is investigated by reacting 1:1:2 molar ratio mixtures of NiO, ZnO and Fe2O3 in a molten salt solvent at temperatures in the range 400–1000 °C. Single phase stability is demonstrated down to about 730 °C (the estimated consolute solution temperature, TcsTcs). A miscibility gap/solvus exists below TcsTcs. The solvus becomes increasingly asymmetric at lower temperatures and extrapolates to stoichiometric parameters = 0.15, 0.8 at 300 °C. A thermodynamic analysis, which accounts for changes in configurational and magnetic ordering entropies during cation mixing, predicts solvus phase compositions at room temperature in reasonable agreement with those determined by extrapolation of experimental results. The delay between disappearance of magnetic ordering above TC=590∘C (for NiFe2O4) and disappearance of a miscibility gap at TcsTcs is explained by the persistence of long-range ordering correlations in a quasi-paramagnetic region above TCTC.