Solid-plastic and plastic-isotropic liquid phase transitions on molecular crystals

Crystalline solid-plastic solid and plastic solid-isotropic liquid phase transitions are studied using a modified Pople and Karasz model. The thermodynamic properties of the model are evaluated by the cluster variation method. The variation of positional and orientational order parameters with temperature is studied. We have also obtained theoretical isotherms, volume and entropy changes of transitions at zero pressure and theoretical phase diagrams at high pressures. The results are compared with the original Pople and Karasz theory and the other by a modified Pople and Karasz theory which was done by Chandrasekhar et al.