Superhard BN allotropes with tunable hybridization sp2/sp3 ratios by compressed nanotubes

The superhard BN materials with tunable properties present potential applications in multifunctional devices under extreme conditions. Eight novel BN structures of which band gaps ranging from 2.34 to 6.18 eV due to the tunable hybridization sp2/sp3 ratios, are proposed by the polymerization of BN nanotubes (BNNTs), including two fully sp2 hybridization structures (tP14-BN and tP16-BN), two fully sp3 hybridization structures (hP24-BN and hP48-BN) and four sp2/sp3 hybridization structures (mP44-BN, oI28-BN, mP76-BN and mP84-BN). There are four superhard sp2/sp3 BN (B:N = 1:1) structures are the firstly reported. Notably, the mP76-BN with the hardness of 52.55 GPa is the hardest structure in all the reported sp2/sp3 hybrid BN structures. Moreover, the oI28-BN, mP76-BN and mP84-BN exhibit the high fracture toughness property. This study systematically reveals the transition mechanism of BNNTs under high pressure and theoretically presents an efficient route to modulate the sp2/sp3 hybridization ratios in superhard BN allotropes realizing tunable band gaps for the potential applications in electronic devices or mechanical materials.