X-ray observation of the structural phase transition of aluminum nitride under high pressure

The III-V compound AlN has been studied under pressure to 30 GPa by x-ray diffraction in a diamond-anvil cell. At 22.9 GPa the wurtzite-to-rocksalt phase transition takes place, accompanied by a volume reduction of 17.9%. The structure of the high-pressure phase and the amount of volume reduction are in agreement with a recent calculation of Gorczyca et al. [Solid State Commun. 79, 1033 (1991)]. On the other hand, the bulk modulus of the wurtzite phase (${\mathit{B}}_0$=207.9±6.2 GPa) agrees well with a calculation of Ching and Harman [Phys. Rev. B 34, 5305 (1986)]. The axial ratio c/a in the wurtzite phase monotonically decreases wtih increasing pressure from the atmospheric value of 1.60.