Ab initiostudy of AlN: Anisotropic thermal expansion, phase diagram, and high-temperature rocksalt to wurtzite phase transition

Abstract

We calculate the anisotropic thermal expansion of wurtzite AlN within the quasiharmonic approximation by sampling the volume as a function of two unit cell axes. From the derived Gibbs energy for the low pressure wurtzite phase and the high pressure rocksalt phase, we calculate the phase diagram. By applying ab initio molecular dynamics, we can confirm recent experimental findings regarding a temperature-driven rocksalt to wurtzite backward phase transition. We propose a detailed mechanism for the transition and predict the existence of a $\ensuremath{\beta}$-BeO type high-temperature modification of AlN. Furthermore, we find the $h$-MgO type intermediate structure previously reported for AlN and other compounds which show the wurtzite to rocksalt forward pressure-induced phase transition.