Theoretical study on novel orthorhombic ternary monocarbides M0.5Re0.5C (M=V, Nb, Ta) from first-principles calculations

Abstract

A new type of ternary monocarbides M0.5Re0.5C (M = V, Nb, Ta) with outstanding mechanical properties were firstly reported by introducing the Re into their well-known hard parent compounds MC to tune the valence electron concentration (VEC). By an efficient structure searching method, a universal orthorhombic structure with space group of Cmc21 has been uncovered and demonstrated to be the ground-state phase of these three monocarbides, which can be sustained up to the ultrahigh pressure of 300 GPa for V0.5Re0.5C and Nb0.5Re0.5C. The thermodynamic and dynamical stabilities of the predicted phases were confirmed by the formation enthalpy and phonon spectra calculations. The structural, mechanical as well as electronic properties of M0.5Re0.5C in comparison with their parent compounds MC were thus fully investigated using first-principles density functional theory (DFT) calculations. Compared to their parent compounds MC, results on the mechanical calculations indicated that they not only possess larger Vickers hardness and ideal indentation shear strength values but also exhibit much smaller elastic and ideal strength anisotropy which are advantageous to their potential applications. Detailed analysis of electronic structures reveals that the origin of the improved mechanical properties of M0.5Re0.5C is mainly attributed to the enhanced hybridization of metals-d and C-2p orbitals by locating the fermi level at the valley of the pseudogap.