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. SiC₅, BC₅, IrC₅, and WC₅ are superhard materials with Vickers hardness over 40 GPa, of which BC₅ 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.