Immiscibility in the NiFe2O4–NiCr2O4 spinel binary

Abstract

The solid solution behavior of the Ni(Fe1−nCrn)2O4 spinel binary is investigated in the temperature range 400–1200°C. Non-ideal solution behavior, as exhibited by non-linear changes in lattice parameter with changes in n, is observed in a series of single-phase solids air-cooled from 1200°C. Air-annealing for 1 year at 600°C resulted in partial phase separation in a spinel binary having n=0.5. Spinel crystals grown from NiO, Fe2O3 and Cr2O3 reactants, mixed to give NiCrFeO4, by Ostwald ripening in a molten salt solvent, exhibited single-phase stability down to about 750°C (the estimated consolute solution temperature, Tcs). A solvus exists below Tcs. The solvus becomes increasingly asymmetric at lower temperatures and extrapolates to n values of 0.2 and 0.7 at 300°C. The extrapolated solvus is shown to be consistent with that predicted using a primitive regular solution model in which free energies of mixing are determined entirely from changes in configurational entropy at room temperature.