Abstract
We reconsider and interpret the mechanical properties of the recently proposed allotrope of carbon, -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 -carbon a high Vickers hardness of 61 GPa comparable to that of superhard cubic boron nitride (-BN), we find that -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 -carbon, we discuss the physical reasons behind the failure of Gao et al.'s and imnek and Vacká's (SV) models in predicting the hardness of -carbon, residing in their improper treatment of the highly anisotropic distribution of quasi--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.
- Received 15 June 2011
DOI:https://doi.org/10.1103/PhysRevB.84.121405
©2011 American Physical Society