We show that electronegativity can be used to effectively identify the hardness of crystal materials on the basis of a new microscopic model for hardness. Bond electronegativity is proposed to characterize the electron-holding energy of a bond, which is the intrinsic origin of hardness. Applying this model to $c\mathrm{\text{−}}{\mathrm{BC}}_2\mathrm{N}$ materials, we confirm the proper bond composition of the experimentally observed phase of $c\mathrm{\text{−}}{\mathrm{BC}}_2\mathrm{N}$, in which the bond ratio $N(\mathrm{C}\mathrm{\text{−}}\mathrm{C}):N(\mathrm{B}\mathrm{\text{−}}\mathrm{N}):N(\mathrm{B}\mathrm{\text{−}}\mathrm{C}):N(\mathrm{C}\mathrm{\text{−}}\mathrm{N})$ is $3:3:1:1$. A number of bonds that can or cannot form a superhard material are qualitatively distinguished, which enables us to explore novel superhard materials by screening possible elemental combinations.

• Received 21 January 2008

DOI:https://doi.org/10.1103/PhysRevLett.100.235504