Electronic structure of the alkaline-earth silicon nitrides M2Si5N8 (M = Ca and Sr) obtained from first-principles calculations and optical reflectance spectra

Results of first-principles band-structure calculations for the ternary alk aline-earth silicon nitrides M2Si5N8 (M = Ca and Sr) are presented. In the structures of M2Si5N8 (M = Ca, Sr and Ba), the N atoms show connections to two (N-[2]) and three (N-[3]) neighbouring silicon tetrahedral centres. Cal culations show that the local electronic structure is strongly dependent on the local chemical bonding. The valence band is dominated by N 2p hybridiz ed with the s, p states of the alkaline-earth-metal and silicon atoms. The upper part of the valence band is dominated by the 2p states of N-[2] atoms , while the N-[3] 2p states lie about 2 eV below the Fermi level. The botto m of the conduction band consists of the N 3s characters hybridized with s orbitals of the alkaline-earth metals, while the s character of Si atoms is higher in energy. Sr2Si5N8 is a semiconductor with a direct energy gap at Gamma, while Ca2Si5N8 is an indirect semiconductor. Optical diffuse reflect ance spectra show an energy gap of 4.9 eV for Ca2Si5N8, 4.5 eV for Sr2Si5N8 , as well as 4.1 eV for Ba2Si5N8, in fair agreement with the calculated val ues.