Three tetragonal superhard sp3 carbon allotropes

Abstract

Four tetragonal anisotropic superhard carbon phases, T-C8, T-C12, T-C20 and T-C28 were generated by RG2 program, T-C8 has been reported and named bct-Carbon. The calculated relative enthalpy, elastic constants and phonon dispersion spectra show that these structures are stable. Their lattice failure modes are mainly tensile. Thermodynamic stability, thermal performance and hardness are all in the order T-C20 >T-C8>T-C12>T-C28, which is contrary to the proportion of the four membered ring in the corresponding structure, suggesting that strained four-membered ring is detrimental to the stability and mechanical properties of carbon phases. Among them, T-C20 with hardness of 90.8 GPa is more stable than bct-Carbon and graphite under 20 GPa, suggesting that it is likely to be transformed by cold compressed graphite.