Covalent-Ionic and Covalent-Metallic Transitions of Tetrahedrally CoordinatedANB8−NCrystals Under Pressure

A microscopic analysis of the differences in Gibbs free energy between covalent, ionic, and metallic forms of $A^N{B}^{8-N}$ compounds is described. The analysis explains the success of the spectroscopic theory of chemical bonds in predicting the covalent-ionic transition at zero pressure as a function of $A$ and $B$. It also suggests that bond charge may play the role of a microscopic order parameter which determines the magnitude of the differences in energy between ionic and covalent structures at zero pressure and temperature.