Thermodynamic stability and properties of boron subnitrides from first principles

We use the first-principles approach to clarify the thermodynamic stability as a function of pressure and temperature of three different alpha-rhombohedral-boron-like boron subnitrides, with the compositions of B6N, B13N2, and B38N6, proposed in the literature. We find that, out of these subnitrides with the structural units of B-12(N-N), B-12(NBN), and [B-12(N-N)](0.33)[B-12(NBN)](0.67), respectively, only B38N6, represented by [B-12(N-N)](0.33)[B-12(NBN)](0.67), is thermodynamically stable. Beyond a pressure of about 7.5 GPa depending on the temperature, also B38N6 becomes unstable, and decomposes into cubic boron nitride and a-tetragonalboron- like boron subnitride B50N2. The thermodynamic stability of boron subnitrides and relevant competing phases is determined by the Gibbs free energy, in which the contributions from the lattice vibrations and the configurational disorder are obtained within the quasiharmonic and the mean-field approximations, respectively. We calculate lattice parameters, elastic constants, phonon and electronic density of states, and demonstrate that [B-12(N-N)](0.33)[B-12(NBN)](0.67) is bothmechanically and dynamically stable, and is an electrical semiconductor. The simulated x-ray powder-diffraction pattern as well as the calculated lattice parameters of [B-12(N-N)](0.33)[B-12(NBN)](0.67) are found to be in good agreement with those of the experimentally synthesized boron subnitrides reported in the literature, verifying that B38N6 is the stable composition of a-rhombohedral-boron-like boron subnitride