Phase transitions in ferroelastic and co-elastic crystals

Abstract Phase transitions in ferroelastic and other crystals with large elastic anomalies (i.e. co-elastic crystals) follow Landau-type behaviour over extended temperature intervals. No or only weak anomalies occur near the transition point and Landau-type behaviour occurs down to the saturation temperature below which the order parameter becomes independent of temperature. It is shown that the thermodynamic order parameter is only in exceptional cases the spontaneous strain and that coupling between several order parameters is an essential feature of many ferroelastic and co-elastic phase transitions. The relevant microstrucrures and the influence of lattice imperfections is briefly reviewed.