Elastic and thermodynamic properties of Mo2C polymorphs from first principles calculations

Transition metal carbides have unique physical and chemical properties and been widely used in engineering parts that need to work under high temperatures and pressures. o-Mo₂C, h-Mo₂C and t-Mo₂C are three critical molybdenum carbides polymorphs while remaining are largely unknown in their mechanical anisotropy, hardness and thermal properties. In this work, we investigated systematically the mechanical and thermodynamic properties of these three candidate carbides using first principles calculations based on density functional theory. Our results showed that the bonds in these compounds were mainly of metallic and covalent type. The Gibbs free energy analysis showed thermodynamically stable structures for all the three carbides. Their shear moduli were estimated to range from 149.1 to 153.4 GPa and hardnesses were expected to be less than 20 GPa. Young׳s moduli were analyzed to have more anisotropic features than bulk modulus for all the three compounds. In addition, heat capacities were calculated to predominate by phonon excitations at high temperature but electron excitations at low temperatures near 0 K.