Structural, electronic and mechanical properties of sp3-hybridized BN phases
Motivated by the discovery of new phases of carbon under cold high-pressure compression, we perform a global structure search of high-pressure phases of boron-nitride (BN). Ten new bulk phases are identified, each being energetically more stable than the graphite-like hexagonal BN (h-BN) under high pressures．All the ten high-pressure phases can be viewed as stacking of buckled h-BN layers. Some of these solid structures may be fabricated through cold high-pressure compression of h-BN films. According to the buckling of the h-BN layers, the new BN phases are classify into three groups. The atomic structures, relative stabilities, electronic structures and mechanic properties are studied in details. Strong dependence of the relative stability, band structure and mechanic property on the buckling of h-BN is observed. Computed electronic band structures suggest that most high-pressure BN phases are insulator with wide and indirect band gaps. Calculated elastic constants and hardness suggest that several BN structures are superhard materials with potential applications in materials science and engineering. The computed transition paths indicate that direct transition from h-BN to four of the new sp3-hybridized BN structures, or to w-BN or bct-BN, are likely to occur through cold compression. For the other five new BN structures, although deeper local minima exist in the transition path, their formation through cold compression of h-BN is still plausible due to the low transition barrier from the deeper local minima to the targeted structure.