tP40 carbon: A novel superhard carbon allotrope*

In this work, a novel carbon allotropetP40 carbon with space groupP4/mmmis proposed. The structural stability, mechanical properties, elastic anisotropy, and electronic properties oftP40 carbon are investigated systematically by using density functional theory (DFT). The calculated elastic constants and phonon dispersion spectra indicate that thetP40 phase is a metastable carbon phase with mechanical stability and dynamic stability. TheB/Gratio indicates thattP40 carbon is brittle from 0 GPa to 60 GPa, whiletP40 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 oftP40 carbon at different pressures are estimated and plotted, suggesting that thetP40 carbon is elastically anisotropic. The calculated hardness values oftP40 carbon are 44.0 GPa and 40.2 GPa obtained by using Lyakhov-Oganov's model and Chen's model, respectively, which means that thetP40 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 thetP40 carbon is an indirect and wider band gap semiconductor material.