KINETICS OF ORDER-DISORDER PROCESSES IN SPINELS

In spinels, the cations are distributed over sites with octahedral and tetrahedral coordination, respectively. In crystals with a significan t temperature dependence of the cation distribution, the kinetics of t he cation exchange can be studied by means of temperature-jump relaxat ion experiments. An analysis is reported for the kinetics of the catio n exchange between the tetrahedral and octahedral sublattices of spine ls. Several microscopic models for the cation exchange which involve d irect exchange, a vacancy mechanism, and an interstitial mechanism are discussed and compared to experimental data. These data were obtained by means of optical spectroscopy for the relaxation of the cation dis tribution in NiAl2O4 after temperature-jumps in the temperature range between 720 and 890 degrees C.