Immiscibility in the Fe3O4–FeCr2O4 spinel binary Original Research Article

A recent thermodynamic model of mixing in spinel binaries, based on changes in cation disordering (x) between tetrahedral and octahedral sites [Am. Mineral. 68 (1983) 18, 69 (1984) 733], is investigated for applicability to the Fe3O4–FeCr2O4 system under conditions where incomplete mixing occurs. Poor agreement with measured consolute solution temperature and solvus [N. Jb. Miner. Abh. 111 (1969) 184] is attributed to neglect of: (1) ordering of magnetic moments of cations in the tetrahedral sublattice antiparallel to the moments of those in the octahedral sublattice and (2) pair-wise electron hopping between octahedral site Fe3+ and Fe2+ ions. Disordering free energies (ΔGD), from which free energies of mixing are calculated, are modeled by where the previously neglected effects are accommodated by: (1) adding a non-configurational entropy term to provide coupling between cation disordering and magnetic ordering and (2) revising the configurational entropy (Sc) analysis. Applying the constraint α=−(2/3)β and regressing the existing database for Fe2+ ion disorder in Fe3O4 gives: σel/R=−0.730±0.081 and γ, the coupling parameter between cation disordering and magnetic ordering, =−0.664±0.075. The revised mixing model predicts a consolute solution temperature and a solvus at 500 °C of n=0.05 and 0.70 for the Fe (Fe1−nCrn)2O4 spinel binary.