Ab initiostudy of phase transformations in boron nitride

The structural properties and phase stability of the four common polytypes of boron nitride, cubic zinc blende (c-BN), hexagonal (h-BN), wurtzite (w-BN) and rhombohedral (r-BN), are studied by ab initio calculations. Electronic energies are calculated using an ultra soft pseudopotential method under the density-functional theory, and phonon dispersions are calculated using the first-principles force-constant method. The $p-T$ phase diagrams of these four boron nitride phases are constructed with the quasiharmonic approximation. Direct compression simulations are then performed to find probable phase transformation paths among these polytypes, with additional energy calculations of plausible transition structures. The c-BN phase is the most thermodynamically stable in ambient conditions among these four polytypes, and the transformation between r-BN and c-BN has the smallest energy barrier. Direct transformation between h-BN and c-BN is far less favorable than indirect transformation, with w-BN or r-BN as an intermediate. The presence of structural defects is a key attribute in reducing the energy barrier of phase transformation. The results in this work offer theoretical clues to experimental data on c-BN film growth, particularly the absence of w-BN.