Why Are the Conventionally-Assumed High-Pressure Crystal Structures of Ordinary Semiconductors Unstable?

Abstract

Recent high-pressure X-ray experiments show that, contrary to traditional expectations and numerous calculations, the NaCl structure is not present in covalent semiconductors, the diatomic β-Sn structure is absent in all compound semiconductors, and the CsCl structure is not seen in ionic semiconductors. We explain these systematic absences in terms of dynamical phonon instabilities of the NaCl, β-Sn, and CsCl crystal structures. Covalent materials in NaCl structures become dynamically unstable with respect to the transverse acoustic TA[001] phonon, while ionic compounds in the β-Sn structure exhibit phonon instabilities in the longitudinal optical LO[00ξ] branch. The latter lead to predicted new high pressure phases of octet semiconductors. For InSb, we find no phonon instability that could prevent the CsCl phase from forming, but for the more ionic GaP, GaAs, InP, and InAs, we find that the CsCl phase is dynamically unstable at high pressures with respect to TA[ξξ0] phonons. Analysis of the soft normal modes via “isotropy subgroup” suggests two candidate structures that will replace the CsCl structure at high pressure: the tP4 (B10) InBi-type and the oP4 (B19) AuCd-type. Experimental examination of these predictions is called for.