CALCULATED STRUCTURAL PHASE-TRANSITIONS OF ALUMINUM NITRIDE UNDER PRESSURE

Total-energy calculations within the local-density approximation (LDA) to the density-functional theory are used to study the properties of AlN under pressure. The free energy (at zero temperature) is calculated as a function of pressure for AlN in the wurtzite, zinc-blende, rocksalt, nickel arsenide, anti-NiAs, and beta-tin structures. According to the calculations the wurtzite structure is stable from zero pressure (P) up to almost-equal-to 12.5 GPa, where AlN transforms into the rocksalt phase. The volume reduction is 19%. The volume change agrees with recent experiments, but the transition pressure is lower than observed, presumably due to the LDA. It is argued that the transition pressure in reality is close to 17 GPa. Another transformation is predicted at higher pressures, and the calculations suggest that this structure could be similar to that of NiAs, but with c/a = 1.72. The transition to the NiAs structure would occur between 30 and 40 GPa, and it is accompanied by a very small volume change. The internal parameters (c/a and u) of the hexagonal phases are optimized by total-energy minimization, and their variation with pressure is derived. At P=0 the internal parameters of wurtzite-AlN are (c/a, u) = (1.596, 0.3820). When P is increased c/a decreases monotonically, and u increases, but the interrelation between u and c/a differs from ideality (defined as the case where all bonds would have the same lengths).