Influences of carbon concentration on crystal structures and ideal strengths of B2CxO compounds in the B-C-O system

The search for novel superhard materials with special structures and improved thermal stability and hardness remains considerably experimental and theoretical challenges. Recent reports proposed that higher carbon content in ternary B2CxO compounds, which are isoelectronic with diamond, would lead to increased strength and hardness. This notion was derived from the calculated elastic parameters and empirical hardness formulas based on structural and electronic properties of the equilibrium structures. In present work, we introduce three potential ultra-incompressible and thermodynamically stable B2CxO (x >= 2) phases via a systematic particle swarm optimization algorithm structure searches. By evaluating the trends of the crystal configuration, electronic structure, and mechanical properties as a function of the C concentration, it is found that the high carbon concentration benefits the formation of the sp(3) C-C covalent bonds and leads to the enhanced elastic moduli and ideal strengths in these B2CxO compounds. Studies of strain-stress behavior at large deformation, however, indicate that all these B2CxO compounds possess substantially lower ideal shear strengths than those of diamond and c-BN, suggesting that they may not be intrinsically superhard.