First-principles calculations on crystal structure and physical properties of rhenium dicarbide

Structural stability, elastic behavior, hardness, and chemical bonding of ideal stoichiometric rhenium dicarbide (ReC2) in the ReB2, ReSi2, Hex-I, Hex-II, and Tet-I structures have been systematically studied using first-principles calculations. The results suggest that all these structures are mechanically stable and ultra-incompressible characterized by large bulk moduli. Formation enthalpy calculations demonstrated that they are metastable under ambient conditions, and the relative stability of the examined candidates decreases in the following sequence: Hex-I>Hex-II>ReB2>Tet-I>ReSi2. The hardness calculations showed that these structures are all hard materials, among which the Hex-I exhibits the largest Vickers hardness of 32.2 GPa, exceeding the hardness of α-SiO2 (30.6 GPa) and β-Si3N4 (30.3 GPa). Density of states and electronic localization function analysis revealed that the strong C–C and C–Re covalent bonds are major driving forces for their high bulk and shear moduli as well as small Poissonʹs ratio.