First-principles investigations of the pressure-induced phase transformations and properties of crystalline and amorphous AlN

Abstract

One-step first principles molecular-dynamics (FPMD) simulations were carried out to investigate structural transformations in wurtzite-type AlN under compression and decompression. The hexagonal transformation path $P{6}_{3}mc\text{\ensuremath{-}}P{6}_{3}/mmc\text{\ensuremath{-}}Cmmm\text{\ensuremath{-}}Fm\overline{3}m$ was identified during compression at $P=50$ GPa and $T=500$ K. The inversion of this path of transformation was established at the decompression of compressed rock-salt AlN at 1500 K. The plausible origin of these transformations is established. The occurrence of intermediate and metastable phases was predicted by taking into account their dynamical and elastic instability, and for this purpose, phonon spectra and elastic constant calculations were performed. FPMD simulations were used to generate amorphous AlN and to investigate its behavior under pressure. The sequence of phase transitions: low-density amorphous---high-density amorphous---rock-salt type was revealed at $P=40$ GPa, and the thermodynamic and mechanical properties of crystalline and amorphous AlN were compared. It was found that the one-step FPMD simulations is a viable scheme to predict phase transitions in crystalline and amorphous AlN with higher precision compared with other FPMD approaches with step-wise pressure changes.