Pressure dependence of positron annihilation in boron phosphide

We have obtained the valence electron and positron charge densities in boron phosphide from wavefunctions derived in a model pseudopotential band structure calculation. The charge densities are calculated at pressures ranging from 0 to 90 GPa. We find that the positron density is maximum in the open interstices and is excluded not only from the ion cores, but also to a considerable degree from the valence bonds. Electron momentum densities are calculated in the (001-110) plane. The observation that the total positron annihilation rate increases with pressure has been explained in terms of increased positron penetration into the ion cores.