Superhard high-pressure structures of beryllium diborocarbides

Abstract

Two novel high-pressure structures of BeB2C2 with space groups R3m and P-4m2 are firstly reported using a swarm methodology on crystal structure search. Both two phases comprise peculiar 3D sp3 B–C bonded frameworks resulted from the strong out-of-plane twisting of the graphite-like sp2 B–C layers in ambient-pressure Pmmn structure under compression. Enthalpy difference calculations reveal that the phase transition from Pmmn to R3m structure happens at 13.7 GPa with a striking 16.4% volume drop. The R3m and P-4m2 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 R3m and 56.5 GPa for P-4m2) 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 R3m phase and the breakdown of BC4 tetrahedrons in the P-4m2 phase with the breaking of B–C bonds are responsible for their tensile-induced and shear-induced lattice instability, respectively.