Novel BN polymorphs transformed from the smallest BNNTs under high pressure

[Display omitted] •Novel boron nitride polymorphs.•Phase transition of (3,3) BNNTs under high pressure.•High elastic properties and Vickers hardness. Three novel boron nitrides (α-, β- and γ-BN) have been proposed as the candidates of BN family. By means of density functional theory (DFT) calculations, we find the three BN polymorphs exhibit high elastic properties and Vickers hardness which is even better than some metal nitrides and carbides. The α-BN and β-BN are both mechanical and dynamically stable at zero pressure. But the γ-BN is dynamically unstable due to soft mode occurring in phonon dispersion relations and can only exists under high pressure. Accurate hybrid functional calculations show that the α-BN is an insulator and the β-BN is a wide bandgap semiconductor with the potential applications in optoelectronic devices. We predict the dynamically stable α-BN and β-BN can be obtained by compressing (3,3) BN nanotubes (BNNTs) over 8GPa. Lower transition barriers also show the feasibility to synthesize the α-BN and β-BN by high pressure methods. In addition, the special structure of β-BN implies it can be used as a precursor to obtain a multiporous BN by pulling A¯ (3,3) BNNTs out from the β-BN.