In this work, a novel carbon allotrope tP40 carbon with space group P4/mmm is proposed. The structural stability, mechanical properties, elastic anisotropy, and electronic properties of tP40 carbon are investigated systematically by using density functional theory (DFT). The calculated elastic constants and phonon dispersion spectra indicate that the tP40 phase is a metastable carbon phase with mechanical stability and dynamic stability. The B/G ratio indicates that tP40 carbon is brittle from 0 GPa to 60 GPa, while tP40 carbon is ductile from 70 GPa to 100 GPa. Additionally, the anisotropic factors and the directional dependence of the Poisson’s ratio, shear modulus, and Young’s modulus of tP40 carbon at different pressures are estimated and plotted, suggesting that the tP40 carbon is elastically anisotropic. The calculated hardness values of tP40 carbon are 44.0 GPa and 40.2 GPa obtained by using Lyakhov–Oganov’s model and Chen’s model, respectively, which means that the tP40 carbon can be considered as a superhard material. The electronic band gap within Heyd–Scuseria–Ernzerhof hybrid functional (HSE06) is 4.130 eV, and it is found that the tP40 carbon is an indirect and wider band gap semiconductor material.
Three novel three-dimensional orthorhombic carbon phases are proposed based on first-principles calculations in this work. These phases possess dynamic stability and mechanical stability and are theoretically more favorable in energy compared to most other carbon allotropes. The hardness levels of oP-C16, oP-C20, and oP-C24 are 47.5, 49.6, and 55.3 GPa, respectively, which are greater than those of T10, T18, and O12 carbon. In addition, although oP-C16, oP-C20, and oP-C24 are metals, their ideal shear strengths are also greater than those of common metals such as Cu, Fe, and Al. Due to p
y
electrons crossing the Fermi level, oP-C16, oP-C20, and oP-C24 show metallicity, and their charge densities of the band decomposition suggest that all the conductive directions of oP-C16, oP-C20, and oP-C24 are exhibited along the a- and b-axis, similar to C5.
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