Crystal and electronic structure of the novel nitrides MYSi4N7 (M = Sr, Ba) with peculiar NSi4 coordination

The new ternary nitrides MYSi4N7 ( M = Sr, Ba) have been investigated using first-principles techniques and experimental methods. The crystal structure data predicted are in good agreement with our measurements on prepared powder samples. Calculations also show that the electronic structure is strongly dependent on the local chemical bonding. The top of the valence bands is dominated by the 2p states of the N atoms coordinated to two Si atoms (NSi2), while the 2p states of the N atoms coordinated to four Si atoms (NSi4) lie about 1.0 eV below the top. The bottom of the conduction band is dominated by the Y 4d states. Both MYSi4N7 compounds are calculated to be wide band-gap semiconductors, with a band gap of about 2.9 eV, in reasonable agreement with the experimentally determined values (4.0 to 4.5 eV).