Difference in electronic structure between tetragonal and cubic $SrNbO2N$

We report here a comparative study of the electronic structure of earlier known cubic phase and the recently, reported correct tetragonal phase of $SrNbO2N$⁠. We performed the calculations within the framework of density-functional theory (DFT) by using the WIEN97 code. The valence band consists of N $2p$ while the conduction band consists of Nb $4d$ in cubic system; on the contrary, these Nb $4d$ states show a strong interaction with anion $p$ states in tetragonal system and indicate theoretically, a metallic behavior. The direct forbidden gap of tetragonal system was $0eV$⁠, whereas for cubic system was $0.2eV$⁠; the relative difference in the band gap of two systems is possibly attributed to the high covalency and is possibly acquired due to the presence of the shorter bond length of Nb–N than that of Nb–O in the tetragonal system. The results of electronic structures have been discussed in the light of optical applications of this oxynitride in solar energy conversion. The calculated optical-absorption coefficient $I(w)$ from real and imaginary parts of dielectric function, further clarifies the suitability of the application.