Superhard high-pressure structures of beryllium diborocarbides. (October 2020)

• Record Type: Journal Article
• Authors: Yan, Haiyan; Chen, Lei; Wei, Zhiting; Zhang, Meiguang; Wei, Qun
• Is Part Of: Vacuum. Volume 180(2020)
• Publication Date: 2020-10
• Subjects: Beryllium diborocarbides -- High-pressure structure -- First principles calculations -- Hardness -- Ideal strength; Vacuum -- Periodicals; 621.55
• DOI: 10.1016/j.vacuum.2020.109617
• Languages: English
• ISSNs: 0042-207X

Abstract

Abstract: Two novel high-pressure structures of BeB2 C2 with space groups R 3 m and P -4 m 2 are firstly reported using a swarm methodology on crystal structure search. Both two phases comprise peculiar 3D sp 3 B–C bonded frameworks resulted from the strong out-of-plane twisting of the graphite-like sp 2 B–C layers in ambient-pressure Pmmn structure under compression. Enthalpy difference calculations reveal that the phase transition from Pmmn to R 3 m structure happens at 13.7 GPa with a striking 16.4% volume drop. The R 3 m and P -4 m 2 phases are all indirect band gap semiconductor demonstrated by the electronic structure calculations. The orientation dependences of the Young's moduli and shear moduli of the these two phases were quantitatively studied. The calculated high hardness (42.7 GPa for R 3 m and 56.5 GPa for P -4 m 2) and large ideal strengths suggest that they are potential superhard materials. Analyses of atomic plastic deformations reveal that the collapse of 3D B–C frameworks in the R 3 m phase and the breakdown of BC4 tetrahedrons in the P -4 m 2 phase with the breaking of B–C bonds are responsible for their tensile-induced and shear-induced lattice instability, respectively. Highlights: Two novel superhard high-pressure structures of BeB2 C2 were firstly reported. Two structures are dynamically stable at ambient pressure with large hardness beyond 40 GPa. The orientation dependences of elastic moduli for two phases were systematically studied. The uncovered P -4 m 2 structure exhibits abnormally high tensile strength at certain direction. 3D sp 3 covalent B–C frameworks are responsible for their excellent mechanical properties.