Predicted crystal structures of carbon nitrides. The compression of graphite-like C₁₁N₄ (a) can obtain the superhard diamond-like C₁₁N₄ (b), and the inset in (b) represents a cage-like part structure.^[138] (c) Superhard sp²-sp³ CN₂ with body-centered tetragonal structure and calculated hardness of 77.4 GPa.^[103] (d) β-InS-type CN structure recently synthesized at HPHT with calculated hardness of 62.3 GPa.^[105,139] The C and N atoms are painted in brown and white colors, respectively.