关键词: 碳, 第一性原理, 超硬材料, 硬度

In this paper, we predicted a superhard carbon phase (Pc-carbon) by using CALYPSO software. The crystal structure belongs to monoclinic system with the space group Pc. We have studied the electronic and mechanical properties of Pc-carbon by first principles calculations. The calculated total energy per atom of Pc-carbon have a minimum value of -8.08 eV, confirming that the optimized structure is stable. And the minimum total energy per atom of Pc-carbon is higher than diamond and graphite, suggesting that the Pc-carbon should be thermodynamically metastable comparing diamond and graphite. There are no imaginary frequency throughout the entire Brillouin zone in the phonon dispersion, confirming the dynamical stability of Pc-carbon up to 100 GPa. The elastic constants of Pc-carbon follow the Born mechanical stability criteria, demonstrating the mechanical stability of Pc-carbon. The calculated B/G value and Poisson's ratio show that Pc-carbon is brittle. The calculated Vickers hardness value of Pc-carbon is 87.6 GPa, which is much larger than the minimal value for a superhard materials (40 GPa), indicating Pc-carbon is a potential superhard material. The Vickers hardness of Pc-carbon is less than that of diamond and M-carbon, but is comparable to that of bct-C₄ and Ibam-C. In addition, the ideal tensile and shear strengths of Pc-carbon (65.8 and 56.5 GPa) are comparable to those of c-BN (55.3 and 58.3 GPa), suggesting that Pc-carbon may have similar tensile and shear resistance to c-BN. The elastic anisotropy index A^U is 0.35, indicating that Pc-carbon is elastic anisotropic; the fractional anisotropy ratio of bulk modulus A_B and shear modulus A_G are 0.010 and 0.032, suggesting that the bulk modulus and shear modulus of Pc-carbon are all elastic anisotropic. The hydrostatic calculations of Pc-carbon indicate that Pc-carbon have excellent incompressibility as the pressure is increased up to 100 GPa. And Pc-carbon is an ultra-incompressible material like other carbon allotropes. The calculated band gap of Pc-carbon is estimated to be 0.99 eV, indicating that Pc-carbon is an indirect band gap semiconductor. The PDOS of Pc-carbon reflects significant sp³ hybridization between atomic orbitals, which leads to the superhard properties of Pc-carbon. Therefore, Pc-carbon is a potential superhard semiconductor material.

Key words: carbon, first principles calculations, superhard materials, Vickers hardness