Kinetics of cation distribution in non-stoichiometric nickel gallate spinel

Cations in the spinel structure are distributed over sites of tetrahedral and octahedral coordination. The cation distribution and its kinetics in non-stoichiometric nickel gallate spinel were studied by high-temperature optical spectroscopy combined with the temperature-jump relaxation technique. It is found that up to 1200 °C the optical absorption spectra of the spinel are dominated by ligand field transitions of Ni2+ ions on octahedral sites. The kinetics of cation site-exchange were investigated from 850 to 1100 °C by monitoring the temporal evolution of the absorbance due to Ni2+ ions in octahedral sites after sudden changes in temperature. The temperature dependence of cation kinetics shows two regimes, one below and one above about 950 °C with activation energies of about 230 kJ/mol and 60 kJ/mol, respectively. Due to sample preparation, the low-temperature activation energy corresponds to an extrinsic vacancy migration regime. The activation energy of kinetics in the upper temperature regime is discussed in connection with the increasing reactivity of the optical absorber at high temperatures.