HIGH-PRESSURE STRUCTURAL PHASE-TRANSFORMATION IN GALLIUM NITRIDE

Under normal conditions GaN crystallizes in the wurtzite structure. At high pressure (30-50 GPa) GaN undergoes a structural phase transformation to the rocksalt structure. The total energy of both structures as well as of the zincblende structure is calculated, for different unit cell volumes, using first-principles non-local pseudopotentials. For the wurtizite structure we obtain a = 3.126 angstrom, c = 5.119 angstrom and an internal parameter u = 0.3767. In the rocksalt structure we get a = 4.098 angstrom and the zincblende lattice constant is found to be a = 4.419 angstrom. At low pressure the wurtizite structure has the lowest energy. At 55.1 GPa there is a phase transformation to the rocksalt structure. No transition to the zincblende structure is observed.