A triatomic carbon and derived pentacarbides with superstrong mechanical properties.

Diamond has the largest hardness of any natural material with an experimental Vickers hardness value of 90-150 GPa. Here, we reported the stable triatomic carbon allotrope with giant hardness closing that of diamond and a family of pentacarbides with superstrong mechanical properties from the state-of-the-art theoretical calculations. The triatomic carbon allotrope can be transformed into a two-dimensional carbon monolayer at a high temperature. We predicted that the triatomic carbon allotrope holds a hardness of 113.3 GPa, showing the potential capability of cracking diamond. Substitution with Al, Fe, Ir, Os, B, N, Si, W, and O element resulted in strong pentacarbides with Young's modulus of 400-800 GPa. SiC(5), BC(5), IrC(5), and WC(5) are superhard materials with Vickers hardness over 40 GPa, of which BC(5) was successfully synthesized in previous experimental reports. Our results demonstrated the potential of the present strong triatomic carbon and pentacarbides as future high-performance materials.