Ab initio study of phonons in hexagonal GaN

Phonon dispersion relations for the hexagonal wurtzite phase of 2H-GaN are estimated using the local-density approximation with ultrasoft pseudopotentials and plane-wave basis, calculating the Hellmann-Feynman forces and applying the direct method. Ab initio calculations are performed on the rhombohedral supercell, which lowers the crystal space group. We propose a method to restore the proper symmetry. The calculated phonon frequencies at the Gamma point are compared with Raman:and infrared measurements, phenomenological models and other nb initio calculations, and a very good agreement is obtained. Phonon frequencies at high-symmetry zone-boundary points and phonon density of states are deduced as well. [S0163-1829(99)08643-9].