Comparative study of structural, electronic and optical properties of Ca2Si, Ca2Ge, Ca2Sn, and Ca2Pb

We report ab initio results for the ground-state properties, band diagrams, density of states, and dielectric functions of Ca(2)Z (Z = Si, Ge, Sn, and Pb) both in the orthorhombic and cubic structures. The calculations are performed by means of total energy ultrasoft pseudopotential and full potential linearized augmented plane wave methods within local density and generalized gradient approximations. The estimated difference in the cohesion energy shows the orthorhombic structure to be the stable phase. We also demonstrate that these materials are semiconductors independently of the phase. A direct energy gap and sizable anisotropy in optical spectra characterize orthorhombic Ca(2)Z, whereas a competitive direct-indirect character of the gap and a high value of oscillator strength of the first direct transition are predicted for cubic Ca(2)Z