Pressure-induced boron nitride nanotube derivatives: 3D metastable allotropes.

The high-pressure behaviors of large-diameter single-walled boron nitride nanotubes (BNNTs) are studied by the first-principles method. One sp³-hybridized and three sp²/sp³-hybridized BN allotropes are obtained via compressing large diameter BNNTs. Due to the restricted movement of nonequivalent B and N atoms, the large BN nanotubes have a chance to form B-B and N-N bonds between intertubes under pressure, in addition to the common B-N bonds. The electron localization function and Mulliken's population analysis indicate the covalent nature of the B-B and N-N dimers. The electronic band structure and density of state calculations show a local conducting feature of tP24-BN and superhard semiconducting character of the other three allotropes with indirect band gaps of 1.28 - 3.13 eV. [ABSTRACT FROM AUTHOR]