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[67].
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.