Crystal Chemistry and First Principles Studies of Novel Superhard Tetragonal C7, C5N2, and C3N4

Tetragonal C7, C5N2, and C3N4, characterized by mixed tetrahedral and trigonal atomic hybridizations, have been devised based on crystal chemistry rationale and structural optimization calculations within density functional theory (DFT). Substitution of C(sp2) and C(sp3) in C7 for nitrogen yields α-C5N2 and β-C5N2, respectively, both of which are superhard, cohesive, and stable mechanically (elastic properties) and dynamically (phonon band structures). tet-C3N4 with both nitrogen sites within the C7 structure was found to be cohesive and classified as ductile with a Vickers hardness of 65 GPa. Due to the delocalization of π electrons of the sp2-like hybridized atoms, metallic behavior characterizes all four phases.