Hardness of T-carbon: Density functional theory calculations

We reconsider and interpret the mechanical properties of the recently proposed allotrope of carbon, T-carbon [Sheng et al., Phys. Rev. Lett. 106, 155703 (2011)], using density functional theory in combination with different empirical hardness models. In contrast with the early estimation based on Gao et al.’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 et al.’s and ˇSimůnek and Vackář’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 et al. and SV models based on the concept of the superatom is suggested, which indeed yields a Vickers hardness of about 8 GPa. DOI: 10.1103/PhysRevB.84.121405 PACS number(s): 64.60.My, 64.70.K−, 71.20.Mq Recently, on the basis of first-principles calculations Sheng et al. proposed a carbon allotrope which they named T-carbon.1 Strictly speaking, its actual stability needs a very large negative pressure which is far beyond currently availabl