Kinetics of cation site exchange in mixed garnets

The redistribution rate of cations between the a- and d -sites in quasibinary mixed yttrium—iron gallium and—iron aluminum garnet single crystals has been investigated for temperatures between 773 and 1523 K. The frozen-in cation distributions of quenched and of slow cooled samples were derived from magnetization measurements and are described by a phenomenological kinetic equation. The activation energies derived for the thermally activated process range from 1.3 to 4eV and are related to the anion vacancy concentration of the crystals. The zero defect intrinsic a-dbarrier height is 4.5 eV as inferred from the observed linear relation between the frequency factor and the activation energy of the Arrhenius equation. This empirical relation, known as the compensation law, has been further applied to interpret reported intra-sublattice site exchange effects of cations on the c-sites and cation self-diffusion data of YIG. The inferred c-c barrier height of 5.5 eV agrees within the error limits of ± 0.1eV with the enthalpy of formation, 5.4 eV, and the enthalpy of solution, 5.3 eV, derived with equilibrium models based on the ionic fluid (Temkin) model and using experimental results of the kinetics, the cation distribution and the solubility of garnets. Apparently the c-c bonds form the strongest binding scaffold of the garnet structure, which is manifested by a relation to the melting point. While premelting around the anion vacancies assisted by their thermal fluctuations reduces the effective barrier height for cation migration, the apparently uneffected melting point of the compound itself is reflected by the compensation law.