Elastic, electronic and optical properties of cubic antiperovskites SbNCa3 and BiNCa3

The structural, elastic, electronic and optical properties of ANCa3 (A = Sb and Bi) compounds with the cubic antiperovskite structure have been investigated using a full relativistic version of the full-potential augmented plane-wave plus local orbitals method based on the density functional theory, in conjunction with both the local density approximation and the generalized gradient approximation (GGA). For reliable description of energy band gap values, another form of GGA developed by Engel and Vosko (GGA-EV) has been applied. The calculated structural properties, namely equilibrium lattice constant, bulk modulus and its first-order pressure derivative, are in good agreement with the available theoretical and experimental results. We have determined the full set of first-order elastic constants, shear modulus, Young's modulus, Poisson's ratio and Debye temperature of these compounds. Band structures reveal that these compounds are direct energy band gap semiconductors. The analysis of the site and momentum projected densities shows that bonding is of covalent-ionic nature. The obtained energy band gap values using GGA-EV are larger than those obtained within LDA and GGA. The optical constants, including the dielectric function, optical reflectivity, refractive index and electron energy loss, are calculated for radiation up to 15 eV. This is the first quantitative theoretical prediction of the optical properties for these compounds that requires experimental confirmation.