First-principles study of the crystal structures and physical properties of H18-BN and Rh6-BN

As the analog of carbon allotropes, new three-dimensional (3D) boron nitride (BN) allotropes have attracted much attention of researchers due to their great importance in fundamental sciences and wide practical applications. Here, based on first-principles density-functional theory calculations, we predict two new stable BN allotropes: One is H18-BN with the P6¯m2 (D3h1) symmetry containing eighteen atoms in the hexagonal unit cell and the other is Rh6-BN with the R3¯m (C3v5) symmetry containing six atoms in the rhombohedral primitive unit cell. The dynamic stabilities of the two structures are examined through the phonon spectrum analysis as well as molecular dynamics simulations, whereas the mechanical properties are analyzed by elastic constants, bulk modulus and shear modulus. From the analysis of the enthalpy evolution with respect to pressure, we find that h-BN can be transformed into either H18-BN or RH6-BN structure under a higher pressure of ∼15 GPa. We also find that both the H18-BN and Rh6-BN allotropes are brittle materials with indirect band gaps of 2.31 and 4.48 eV, respectively. The simulated XRD spectra provide detailed structural information of H18-BN and Rh6-BN for future experimental examinations. Our findings not only greatly enrich the existing structural family of 3D-BN materials but also stimulate further experiments.