Melting Curve Anomaly and Solid-Solid Phase Transition at High Pressures

A model is presented in order to account for the melting curve anomaly and the solid-solid phase transition observed at high pressures. In the model, the effective diameter of each constituent atom can take two values. It is assumed that the atoms with larger diameter are in the lower energy state than the atoms with smaller diameter. The population in each states is determined by the condition that the Helmholtz free energy takes the minimum value. The phase diagram resulting from the simplest approximation shows the expected features, i.e., the dropping of the melting temperature by compression and the solid-solid phase transition with or without the critical point.