Investigating Electronic, Optical, and Phononic Properties of Bulk γ-M₂ON₂ and β-M₇O₈N₄ (M = Hf and Zr) Insulators...

Hafnium and zirconium oxynitrides have similar properties, yet a consolidated investigation of their intrinsic properties has not been carried out. In this paper, we perform first-principles density functional theory calculations of γ- and β-phase hafnium and zirconium oxynitrides, which show that the γ-M₂ON₂ (M = Hf and Zr) is an indirect band-gap (E _g) insulator, while the β-M₇O₈N₄ has a "pseudo-direct" type of E _g. β-phase has higher E _g than γ-phase, with concomitant disappearance of the conduction band tail. Optical properties in γ-M₂ON₂ show that the anisotropy is negligible, and the optical constant values are in the range of other superhard materials. Phonon calculations present peculiar characteristics such as a small phonon band gap in γ-Hf₂ON₂ and imaginary phonon frequencies in β-phases relating to lattice instability. The phononic properties are unfavorable for their potential use as an absorber material of the hot carrier solar cell-an emerging photovoltaic concept.