THE KINETICS OF THE MONOCLINIC TO MONOCLINIC PHASE-TRANSITION IN BAAL2GE2O8-FELDSPAR

BaAI(2)Ge(2)O(8)-feldspar (BAGF) undergoes an order/disorder phase tra nsition (C2/m<->I2/c) with T-tr=1690 K. The kinetic behaviour and equi librium properties of Al, Ge cation ordering within a tetrahedral fram ework have been studied by isothermal annealing experiments in the tem perature range between 0.5 and 0.9 T-tr. The evolution of the order pa rameters with time and temperature was followed by X-ray single crysta l and powder diffraction methods and by phonon spectroscopy. The kinet ic behaviour of the long-range order parameter is contrasted with thos e of short-range correlations as measured by Hard Mode Infrared Spectr oscopy. Possible mechanisms causing the observed kinetic and equilibri um behaviour are discussed.