Theoretical study on the structural, electronic and physical properties of layered alkaline-earth-group-4 transition-metal nitrides AEMN₂

Thermodynamic, structural, and electronic properties of the layered ternary nitrides AEMN₂ (AE = alkaline-earth; M = group 4 transition metal) both with the KCoO₂ and α-NaFeO₂ structure-types are examined within density-functional theory. The AE:M atomic (or ionic) radius ratio seems to be the most important criterion in determining one structural arrangement over the other. We find that the majority of compounds are more stable with the KCoO₂ structure-type where M is coordinated to five nitrogen atoms in a distorted square-based pyramidal geometry. Strong interactions occur in both arrangements not only between nitrogen and transition metal atoms, but also between nitrogen and alkaline-earth metal atoms within and between the layers. Calculations show that all the AEMN₂ compounds with the tetragonal structure-type KCoO₂ are semiconducting with band gaps of approximately 1 eV. However, small band gap conductor and even semi-metallic behavior are computed for compounds with the alternative hexagonal α-NaFeO₂ structure-type.