Why are the conventionally-assumed high-pressure crystal structures of ordinary semiconductors unstable?

Recent high-pressure X-ray experiments show that, contrary to traditional e xpectations and numerous calculations, the NaCl structure is not present in covalent semiconductors, the diatomic beta -Sn structure is absent in all compound semiconductors, and the CsCl structure is not seen in ionic semico nductors. We explain these systematic absences in terms of dynamical phonon instabilities of the NaCl, beta -Sn, and CsCl crystal structures. Covalent materials in NaCl structures become dynamically unstable with respect to t he transverse acoustic TA[001] phonon, while ionic compounds in the beta -s n structure exhibit phonon instabilities in the longitudinal optical LO[00 xi] 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 InA s, we find that the CsCl phase is dynamically unstable at high pressures wi th respect to TA[xi xi0] phonons. Analysis of the soft normal modes via "is otropy subgroup" suggests two candidate structures that will replace the Cs Cl structure at high pressure: the tP4 (B10) InBi-type and the oP4 (B19) Au Cd-type. Experimental examination of these predictions is called for.