Influence of disorder in crystal structure on ferroelectric phase transitions

A new model of ferroelectric phase transitions in disordered crystals is developed. The model takes into account the nonequivalence of the structural environment of identical ions, which alters the balance of forces governing ferroelectric structural instability. In contrast with its predecessors, the new model can be used for qualitative and, in many cases, quantitative predictions of the temperature range of the diffuse ferroelectric phase transition and the value of the disorder-induced shift of the average transition temperature as a function of the degree of disorder and chemical composition. This conclusion is confirmed by a comparison of the theoretical results with an abundance of known experimental facts.