The elastic and bonding properties of the sulvanite compounds: A first-principles study by local and semi-local functionals

The elastic properties of the compound family of the sulvanite Cu3TMX4Cu3TMX4 (TM=V, Nb, Ta; X=S, Se) have been calculated using first-principles total energy calculations within the density functional theory along with the local density and the generalized gradient approximations. The calculated elastic properties are the bulk modulus (B), the elastic constants (c11, c12 and c44), the Zener anisotropy factor (A), the isotropic shear modulus (G) and the Young modulus (E). By means of these quantities we have also computed other thermodynamic properties such as the average sound velocity (s  ) and the Debye temperature (ΘD)(ΘD). The calculated values of the elastic properties led to the conclusion that these compounds are brittle and fragile. We have also calculated the electron localization function, which exhibits the bonding characteristics in these compounds, showing that the Cu–X bond as well as the TM–X bond have a covalent character, but there is also present some ionicity in these compounds due to the Cu–TMX4 bond.