A PHENOMENOLOGICAL APPROACH TO ORDERING KINETICS FOR PARTIALLY CONSERVED ORDER PARAMETERS

The kinetics of cation ordering in complex crystal structures, such as minerals and ceramics, is discussed with regard to the applicability of time-dependent Ginzburg-Landau theory (TDGL). Deviations from the p redictions made by TDGL may arise due to partial conservation of conti nuous and non-uniform order parameters at low temperatures. Under thes e conditions the movement of areas with large order parameter gradient s ('walls') provides a local alternative to the continuous ordering ki netics described by TDGL. A simple phenomenological description of ord er parameter kinetics taking these constraints into consideration can be obtained. The underlying assumption is that the geometry of spatial order parameter variation close to equilibrium resembles that of a ph ase-separated system. The resulting kinetics differs substantially fro m the predicted TDGL behaviour.