Three tetragonal superhard sp3 carbon allotropes

Four tetragonal anisotropic superhard carbon phases, T-C-8, T-C-12, T-C-20 and T-C-28 were generated by RG(2) program, T-C 8 has been reported and named bet-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-C-20 >T-C-8>T-C-12>T-C-28, 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-C-20 with hardness of 90.8 GPa is more stable than bet-Carbon and graphite under 20 GPa, suggesting that it is likely to be transformed by cold compressed graphite.

Automated summary

Among the four tetragonal anisotropic superhard carbon phases, T-C-20 is the most stable and hardest, with strained four-membered rings being detrimental to the stability and mechanical properties of carbon phases.