Upon increase in pressure, both the group IV elements and III-V semiconductors display a broadly similar behaviour. The diamond and zincblende structures transform to a sixfold coordinated metallic structure similar to that of either -Sn or NaCl. Experimental results[21, 22] and the above calculations establish that depressurisation of silicon and germanium result in dense metastable crystalline phases (although not true of carbon where distortion of the tetrahedral bonding is extremely unfavourable). As yet, there has been no experimental evidence showing the existence of such dense phases in III-V semiconductors. For this reason ab initio calculations are now performed on GaAs, InAs and AlSb to investigate the nature of these materials in the BC8 structure. The ST12 structure is not examined as a possible high density structure in the III-V's since it contains rings of odd numbers of atoms requiring like species bonding which is unfavourable.
Firstly the diatomic equivalent of the BC8 structure, SC16, is described and then the energetics of it is considered at various volumes(pressures) to investigate as to whether the structure is even metastable. Also to be truly metastable the structure must be able to support small displacements of the atoms from their equilibrium sites. Therefore a non-zero temperature molecular dynamics simulation is performed to examine the optic point phonon modes. The bonding is then considered by looking at the band structure and the (fully ab initio) ionicity of SC16 GaAs.