AN X-RAY-DIFFRACTION STUDY ON THE KINETICS OF THE CATION REDISTRIBUTION IN THE SPINEL NIAL2O4

A study of the relaxation kinetics in the partially inverse NiAl2O4 sp inel after temperature jumps T-1 --> T-2 is reported. The change of th e lattice parameter was measured by high temperature X-ray diffraction at T-2 (695-760 degrees C) using NiAl2O4 powder samples equilibrated with NiO in air. For the temperature range of T-1 under study (1100-14 50 degrees C), the kinetics can be well described by an equation which is formally equivalent to a third-order rate law. Changes of the diff raction line widths occur during the relaxation which are due to elast ic stresses, strongly dependent on the initial temperature T-1. Compar ison with recent results from optical spectroscopy indicates that the process observed by lattice parameter measurements is not the homogene ous exchange reaction between the two cation sublattices. The nature o f the various relaxation phenomena occurring on different time scales is discussed taking concentration inhomogeneities of structure element s and elastic stress fields on different length scales into account.