First-Principles study of Elastic properties, Hardness and Debye temperature of O-BN under pressures

An orthorhombic structure boron nitride (O-BN, space group: Pbam) was studied by first-principles calculations. The structural, elastic properties, Possion's ratio, elastic anisotropy, hardness, wave velocity and Debye temperature of O-BN were investigated by density functional theory method with the ultrasoft pseudopotential scheme in the frame of the generalized gradient approximation (GGA). The results for the equilibrium structural parameters of O-BN, c-BN and w-BN are consistent with experimental values and other theoretical results. It is found that O-BN has a bulk modulus of 365 GPa, a shear modulus of 362 GPa, a Young's modulus of 816 GPa, the hardness of 59.2 GPa, a large Debye temperature 1842 K and a small Possion's ratio 0.13 under zero temperature and zero pressure; O-BN has elastic anisotropy, and shear anisotropy of shear plane {100}, {010} and {001} increases with the increase of pressures. When the pressure increases, the compression along the b-axis is much larger than those along the a-axis and b-axis in the basal plane, and the compression along the c-axis is the smallest; O-BN can satisfy the criteria for mechanical stability of orthorhombic phase, so it is mechanically stable. (paper)