Chemical trends in the structural stability of binary crystals

The role of chemical and pressure changes on the structural stability of simple binary crystals is examined. We have considered crystals of the form $A^N$ $B^{8\mathrm{-}N}$ in three experimentally accessible structures: zinc blende, rocksalt, and β-Sn. The total energy of a reference crystal in these structures was calculated via a pseudopotential method as a function of charge transfer and pressure. We construct a phase diagram in pressure–charge-transfer space which enables us to predict the global behavior of this family of crystal structures. The structural boundaries of this phase diagram are in good agreement with experiment and the pressures predicted for structural transitions are in semiquantitative agreement with experiment. From our calculations we are able to define an ionicity scale based on charge transfer which predicts the critical ionicity for separating the zinc-blende and rocksalt structures. Our charge-transfer scale is consistent with the Phillips ionicity scale and provides a microscopic justification for the dielectric scale. Finally, for the case of the rocksalt structure, we can associate the band gap of this material with its structural stability relative to the zinc-blende structure. Specifically, as a function of charge transfer we find a metal-insulator transition occurs near the point when the rocksalt structure becomes stable versus a zinc-blende structure.