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High-hardness metal compounds have huge potential applications in multifunctional devices used under extreme conditions. Here, an ab initio evolutionary algorithm was used to systematically search for structures in the Re3C system with high hardness and metallicity under the pressure of 0–300 gigapascal (GPa). We discovered a structure (namely, C2/m) with the lowest enthalpy at 154 GPa and two metastable phases (namely, P63/mmc and C2/m- α), which could maintain thermodynamic and mechanical stability at zero pressure, and they could be quenched to ambient conditions. More importantly, the hardness of the metal compounds (21.4 GPa for C2/m, 25.2 GPa for P63/mmc) exceeds that of TiN (18.7 GPa), the material for commercial machine tools. In addition, our analysis shows that the coexistence of Re–C bond and Re–Re bond with strong coupling, and avoiding the antibonding state of 5d-electron of Re–Re bond near the Fermi level, could be the reasons why the structure has high hardness and metal properties at the same time. [ABSTRACT FROM AUTHOR]

By employing an ab initio evolutionary algorithm, we predict two novel metastable structures whose energies are just higher than g-SiC but lower than other two-dimensional (2D) SiC siligraphenes reported so far. One is composed of 4-6-8 C–Si rings named tho-SiC siligraphene and the other one is composed of 5-6-8 rings named pho-SiC siligraphene. They are almost equal in energy, but interestingly, our calculations demonstrate that tho-SiC siligraphene is a semiconductor like g-SiC, while pho-SiC siligraphene is a 2D metal. Additionally, unlike g-SiC which is very easy to switch from a direct band gap to indirect band gap semiconductor and exhibits homogenous isotropy characteristic under biaxial strains, tho-SiC siligraphene keeps a consistently direct band gap from 1.02 eV to 1.98 eV as the uniaxial strain increases from −9% to 9% and it shows strong anisotropic electronic structures. The obtained calculation results indicate that tho-SiC siligraphene may be a better candidate than g-SiC in the application of light-emitting devices in the future. [ABSTRACT FROM AUTHOR]