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 alkalineearth silicon nitrides M 2 Si 5 N 8 (M = Ca and Sr) are presented. In the structures of M 2 Si 5 N 8 (M = Ca, Sr and Ba), the N atoms show connections to two (N [2] ) and three (N [3] ) neighbouring silicon tetrahedral centres. Calculations show that the local electronic structure is strongly dependent on the local chemical bonding. The valence band is dominated by N 2p hybridized 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 bottom 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. Sr 2 Si 5 N 8 is a semiconductor with a direct energy gap at , while Ca 2 Si 5 N 8 is an indirect semiconductor. Optical diffuse reflectance spectra show an energy gap of 4.9 eV for Ca 2 Si 5 N 8 , 4.5 eV for Sr 2 Si 5 N 8 , as well as 4.1 eV for Ba 2 Si 5 N 8 , in fair agreement with the calculated values.