Investigating Electronic, Optical, and Phononic Properties of Bulk γ-M2ON2and β-M7O8N4(M = Hf and Zr) Insulators Using Density Functional Theory

Abstract

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 (Eg) insulator, while the β-M7O8N4has a "pseudo-direct" type of Eg. β-phase has higher Egthan γ-phase, with concomitant disappearance of the conduction band tail. Optical properties in γ-M2ON2show 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-Hf2ON2and 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.