Longitudinal phonons in gallium phosphide at high pressures

Frequencies of the zone-centre and zone-boundary phonon modes in GaP are evaluated ab initio using the density-functional theory within the local-density approximation, with atomic numbers as the only experimental input. The norm-conserving pseudo-potentials are applied with Ceperley-Alder exchange and the plane-wave expansion formulation is used for calculation of total energy. Variation in the LO(X) and LA(X) frequencies at high and very high compressions is predicted. Eigendisplacements determined for the TO(X) and TA(X) modes are compared with the predictions provided by different phenomenological models which were developed for the lattice dynamics in the past.