DIFFUSION-VIBRATION THEORY OF MELTING OF IONIC-CRYSTALS

A diffusion-vibration theory of melting is proposed. As the temperature of the substance is increased, the amplitude of vibration of the atoms about their mean position increases, together with an increase in the equilibrium separation between the constituent atoms. The melting starts when the instantaneous separation between the atoms (r) becomes equal to the equilibrium separation corresponding to the point of inflection in the P.E. versus r curve. As the melting process starts, the temperature becomes constant and hence the diffusion-vibration force balances the interatomic force. The theory is applied to study the melting temperature of ionic compounds, which are strongly bounded substances involving two-term potential energy function. The results of the theoretical study are found to be in good agreement with the experimental ones.