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Fluctuations and some strain related interaction mechanisms in structural phase transitions

Abstract Materials which undergo structural phase transitions with bilinear coupling between the spontaneous strain (e sp i ) and the thermodynamic order parameter (Q) of the type λi e sp i Q show characteristic fluctuation pattern at T > T c. Snapshots of such pattern show tweedlike structures with butterfly shaped structure factors. Some experimental fluctuation patterns are reviewed and compared with the theoretical predictions. At T ≤ Tc , characteristic microstructures often include structural twinning. The relevant energy expressions for order/disorder systems are recalled and their contribution to thermodynamic fluctuations are described. It is anticipated that some of the physical mechanisms may play a role also in improper ferroelastics with coupling of the type λi e sp i Q 2 k where Qk is a component of a degenerate order parameter Q = {Qk}.