Binary nitrides α-M3N2 (M = Be, Mg, Ca):: a theoretical study

Detailed results of DFT band structure calculations for the alpha-M3N2 (M=Be, Mg, Ca) binary nitrides are reported. The cell parameters and atomic positions were optimized with the molecular dynamics pseudopotential approach of the Vienna ab initio simulation program (VASP). The optimized Structures were subsequently calculated with the WIEN97 program, based on the full potential linearised augmented plane waves (FP-LAPW) method, within the framework of density functional theory (DFT). As expected, the valence band is composed of the nitrogen 2p states, while the conduction band comprises the metallic states. The band gap was found to decrease when going from Be to Mg and Ca. This counterintuitive result is due to the existence of a significant anion-cation covalent interaction which decreases when going from Be to Mg and Ca. The electronic Structures of the mixed nitride CaMg2N2 and of the hypothetical alpha-M3N2 (M=Sr, Ba) phases are also discussed in the light of the above results.