Anisotropic ideal strengths of superhard monoclinic and tetragonal carbon and their electronic origin

The mechanical and electronic properties, and the atomic deformation mechanism of recently reported monoclinic (C2/m) and body-centered-tetragonal (I4/mmm) carbon polymorphs are studied by first-principles methods. The calculated elastic moduli and ideal strengths suggest that both polymorphs have low compressibility and are superhard, but their relatively lower ideal strength as compared with diamond indicates that they are intrinsically weaker because of differences in bond lengths and concomitant fluctuation of valence charge density. Analyses of electronic structure and atomic deformation mechanism demonstrate that the polarity of the bonds, which results from the fluctuations, is responsible for the lower strength.