Metal-insulator transition and the stability of rocksalt and zinc-blende crystal structures

The stability of the rocksalt versus the zinc-blende crystal structure for binary solids is calculated using total-energy methods as a function of charge transfer. Covalent binary solids are metallic and unstable in the rocksalt structure as compared to the zinc-blende structure. If one starts with a covalent rocksalt structure, as a function of increasing charge transfer, the rocksalt structure undergoes a metal-insulator transition. We find the thermal band gap (${\Gamma }_{15}-X_1$) in the rocksalt structure, which controls this transition, is related to the energy of relative stability between the rocksalt and zinc-blende structures. With increasing ionicity, the rocksalt structure can be thought of as being stabilized against the zinc-blende structure by a Madelung term, although not in a simple fashion.