Pressure dependence of the dielectric and lattice-dynamical properties of GaN and AlN

We present ab initio calculations of the structural, dielectric, and lattice-dynamical properties of zinc-blende and wurtzite GaN and AIN under hydrostatic pressure, based on a plane-wave pseudopotential method within the density-functional theory. The calculated volume dependence is related to pressure by means of the Vinet equation of state. A linear-response approach to the density-functional theory is used to derive Born effective charges, dielectric constants, and phonon frequencies. The static ionicities, the dynamic charges, and the dielectric constants are found to decrease with pressure. whereas the phonon frequencies show an increasing longitudinal-transverse splitting. The softening behavior of the low-frequency E-2 mode and of the corresponding TA(L) mode is related to strengths of the covalent and ionic forces. Our results are in agreement with recent Raman measurements.