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

We present ab initio calculations of the structural, dielectric, and lattic e-dynamical properties of zinc-blende and wurtzite GaN and AIN under hydros tatic pressure, based on a plane-wave pseudopotential method within the den sity-functional theory. The calculated volume dependence is related to pres sure by means of the Vinet equation of state. A linear-response approach to the density-functional theory is used to derive Born effective charges, di electric constants, and phonon frequencies. The static ionicities, the dyna mic charges, and the dielectric constants are found to decrease with pressu re. whereas the phonon frequencies show an increasing longitudinal-transver se splitting. The softening behavior of the low-frequency E-2 mode and of t he corresponding TA(L) mode is related to strengths of the covalent and ion ic forces. Our results are in agreement with recent Raman measurements.