Phenomenological and microscopic models of sublattice disorder in ionic crystals—I: phenomenological models

The nature of phase transitions which arise from two classes of phenomenological models for the free energy of ionic crystals containing Prenkel disorder of one constitutent is examined. In one class of models, defect-produced energy and excess entropy vary quadratically with defect concentration, while in the second class the energy per added defect is damped by disorder more rapidly than in the first class. The order of phase transitions and their effect on the effective carrier concentration for electrical conductivity are discussed.