We reconsider and interpret the mechanical properties of the recently proposed allotrope of carbon, T-carbon, using density functional theory in combination with different empirical hardness models. In contrast with the early estimation based on Gao 's model, which attributes to T-carbon a high Vickers hardness of 61 GPa comparable to that of superhard cubic boron nitride (c-BN), we find that T-carbon is not a superhard material, since its Vickers hardness does not exceed 10 GPa. Besides providing clear evidence for the absence of superhardness in T-carbon, we discuss the physical reasons behind the failure of Gao 's and Simunek and Vackár's (SV) models in predicting the hardness of T-carbon, residing in their improper treatment of the highly anisotropic distribution of quasi-sp3-like C-C hybrids. A possible remedy for the Gao and SV models based on the concept of the superatom is suggested, which indeed yields a Vickers hardness of about 8 GPa.

http://dx.doi.org/10.1103/PhysRevB.84.121405