Discontinuous freezing-out of local order

Abstract

A “soft” impurity in a host crystal undergoing a second-order displacive phase transition may induce a freezing-out of local order at temperatures above the bulk transition temperature [1]. We show that in MFA this local phase transition is of first order, i.e. the local order appears discontinuously, and the stability limit of the disordered phase against local ordering lies below the stability limit of the locally ordered phase, if the representation of the impurity coordinate contains a third-order invariant, but the space-group representation of the soft optical phonon does not. This situation occurs for example for anE _g -type impurity coordinate coupling to a soft zone-edge or zone-corner phonon driving an antiferrodistortive transition. The theory is applied to the specific case of a Jahn-Teller impurity in a non-Jahn-Teller host crystal.