BN Polymorphs in Hexagonal 2–7 Stacking Orders: First-Principles and High-Throughput Study

BN polymorphs are important basic materials in superhard materials, as well as in other industrial fields and in microelectronics. The ground-state phase of BN polymorphs has a 3C stacking order. In addition to 3C, eight BN polymorphs (2H, 4H, 5H, 6H-I, 6H-II, 7H-I, 7H-II, and 7H-III) are produced by a random sampling strategy combined with group theory and graph theory (RG2) in this work. It is found that the stack order of 2–7H BN polymorphs is basically similar to that of 3C BN, although with a slight difference. The calculated total energy of these 2–7H BN polymorphs is only 4–17 meV/atoms higher than that of 3C BN, and they are all dynamically and mechanically stable. In addition, their thermal stability at 1000 K is also studied by ab initio molecular dynamics (AIMD) techniques. A combination of tensile stress and hardness is sufficient to prove that BN is a superhard material in 2–7H BN polymorphs. The band gaps of 2–7H BN polymorphs are in the range of 6.19–6.98 eV, and they can be considered as promising ultrawide-bandgap semiconductors. Finally, the anisotropy in Young’s modulus and X-ray diffraction (XRD) patterns of 2–7H BN polymorphs are also investigated in this work