Ab initio structural, electronic and optical properties of orthorhombic image

Orthorhombic image is studied using density-functional theory (DFT) considering both the local density and generalized gradient approximations, LDA and GGA, respectively. The electronic band structure, density of states, dielectric function and optical absorption are calculated. Two very close indirect (image) and direct (image) band gap energies of 1.68 eV (2.31 eV) and 1.75 eV (2.41 eV) were obtained within the GGA (LDA) approximation, as well as the effective masses for electrons and holes. Comparing with orthorhombic CaCO3 (aragonite), the substitution of carbon by germanium changes the localization of the valence band maximum of the indirect transition, and decreases by almost 2.0 eV the Kohn–Sham band gap energies.