A theory of melting of molecular crystals .2. Phase diagrams and relations with solid state transitions

The Pople and Karasz theory of melting of molecular crystals is extended to include the third energy parameter mentioned in our previous paper.(1) The modified form of the Pople and Karasz theory is used to study solid-solid transitions and to obtain complete phase diagrams for transitions under pressure. It is found that solid-solid and melting transitions become second-order under certain conditions. The entropy of melting as a function of the ratio of the solid transition to boiling temperature, the theoretical phase diagrams as well as the melting entropy versus the solid transition entropy are compared with available experimental data and as well as the Pople and Karasz theory.