Design and theoretical study of novel multifunctional 3D-BC2N polymorphs

Highlights: • By compressing well-mixed CNT and BNNT arrays, nine novel BC₂N phases were constructed. • 3D (6,6)-Ⅱ, 3D (4,4)-Ⅱ, 3D (8,8), 3D (6,6)-Ⅰ and 3D (8,0) are energetically more favorable than previously proposed diamond-like β-BC₂N. • 3D (4,4)-I, 3D (4,4)-II, 3D (6,6)-I, 3D (6,6)-II, and 3D (8,8) are superhard phases with indirect band gap; 3D (6,0) and 3D (8,0) are superhard phases with direct band gap; 3D (10,0) is semi-metallic; 3D (12,0) is a low density phase. We constructed a series of novel 3D-BC₂N structures that can be obtained by compressing well-mixed carbon nanotube and boron nitride nanotube arrays. Sequential theoretical calculations indicated a variety of electronic and mechanical properties, such as superhardness, tunable band gap, and ductility, for these novel 3D-BC₂N polymorphs. In specific, 3D (6,6)-Ⅱ, 3D (4,4)-Ⅱ, 3D (8,8), 3D (6,6)-Ⅰ and 3D (8,0) are energetically more favorable than previously proposed diamond-like β-BC₂N; the hardness of 3D (6,6)-Ⅱ can reach a remarkably high value of 72 GPa; 3D (6,0) and 3D (8,0) are direct gap superhard phases; and 3D (10,0) has a semimetallic nature.