<b><i>Ab initio</i></b>calculation of the lattice dynamics and phase diagram of boron nitride

We present a detailed study of the lattice dynamics and of the phase stability of cubic zincblende (c-BN) and hexagonal (h-BN) boron nitrides. The phonon-dispersion relations at different densities are calculated using a first-principles force-constant method. The calculated eigenfrequencies and phonon Gr\"uneisen parameters are in good agreement with experimental findings. From the electronic and vibrational energies as a function of volume we calculate the phase $(p,T)$ diagram of boron nitride in a quasiharmonic approximation. At low temperature c-BN is the stable modification; the c-BN/h-BN coexistence line intersects the temperature axis at 1440 K. In experiments this temperature lies between 1200 and 1800 K. Anharmonic corrections improve the agreement between the calculation and experiment for high pressure and temperature.