Investigating Electronic, Optical, and Phononic Properties of Bulk gamma-M2ON2 and beta-M7O8N4 (M = Hf and Zr) Insulators Using Density Functional Theory

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 gamma- and beta-phase hafnium and zirconium oxynitrides, which show that the gamma-M2ON2 (M = Hf and Zr) is an indirect band-gap (E-g) insulator, while the beta-M7O8N4 has a "pseudo-direct" type of E-g. beta-phase has higher E-g than gamma-phase, with concomitant disappearance of the conduction band tail. Optical properties in gamma-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 gamma-Hf2ON2 and imaginary phonon frequencies in beta-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.