Phenomenological approach of the phase transitions in ABX3-type crystals

Perovskite ABX3-type crystals undergo ferroelastic phase transitions at different temperatures. We suggest a three components order parameter model to describe the successive phase transitions due to condensation of several soft modes. The thermodynamical potential models and phase diagrams of the expansion coefficients are suggested for an adequate Landau-type description of the phase transitions in ABX3. We study the pressure-temperature phase diagrams and the temperature dependence of elasticity of CsPbX3 (X=Cl, Br).