High-pressure properties of wurtzite- and rocksalt-type aluminum nitride

Electronic and ground-state properties of both wurtzite- and rocksalt-structure AlN are evaluated in the local-density-functional formalism using nonlocal norm-conserving pseudopotentials. For the wurtzite structure we obtain a=3.129 Å and c=4.988 Å and an internal parameter of 0.3825, which deviates considerably from the ideal value. In the case of the rocksalt structure we get a=4.032 Å. Both modifications are large-band-gap semiconductors with a direct gap in the wurtzite structure and an indirect gap in the rocksalt structure. The transition pressure from hexagonal to cubic was calculated to be 12.9 GPa, in reasonable agreement with the experimental value.