Pressure-induced structural transitions in BN fromab initiometadynamics

Abstract

We report here results of ab initio metadynamics simulations of structural transitions in boron nitride at high pressures. Transitions starting from $s{p}^{2}$ bonded (graphite-like) structures are studied in a temperature range from 300 to 3000 K and pressures from 20 to 31 GPa. Rhombohedral boron nitride (r-BN) was found to directly transform at all temperatures into cubic boron nitride (c-BN). Hexagonal boron nitride (h-BN) transforms at $Tl700$ K into wurtzite boron nitride (w-BN). At higher temperatures we found a possible transformation pathway resulting in the fully tetrahedrally ($s{p}^{3}$) bonded metastable structure. This structure is tetragonal ($P{4}_{2}/mnm$) and is an analog of the ``bct C4'' ($I4/mmm$) structure recently discussed for carbon. The $P{4}_{2}/mnm$ structure has been predicted theoretically for BN but so far not reported experimentally. We calculate structural, elastic, and electronic properties of this structure and discuss the transition mechanism. We also study the transitions at extreme pressures in the tera-pascal range starting from $s{p}^{3}$ bonded c-BN and w-BN structures.