KINETICS AND THERMODYNAMICS OF INTRACRYSTALLINE FE-GA DISTRIBUTION IN Y3(FE, GA)5O12

Garnets of composition Y3Fe5-xGaxO12, with x = 0-5, were synthesized from oxides. Samples with various Ga content were annealed at temperatures between 700-1290-degrees-C; the heating duration varied between 90 s and 1350 h. Cation distribution was measured by Mossbauer spectroscopy at room temperature. The standard free energy change for the exchange reaction Fe3+ (tet) + Ga3+ (oct) <-> Fe3+ (oct) +Ga3+ (tet) is about 20 kJ mol-1, and decreases slightly with increasing Fe content. The specific rate constants for the ordering process were determined according to the Mueller model for order-disorder kinetics. The activation energies for the ordering process between 200-250 kJ mol-1 were calculated from the temperature dependence of the specific rate constants.