Investigating Electronic, Optical, and Phononic Properties of Bulk γ-M₂ON₂ and 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-M 2 ON 2 (M = Hf and Zr) is an indirect band-gap ( E g ) insulator, while the β-M 7 O 8 N 4 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-M 2 ON 2 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-Hf 2 ON 2 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.

Band-gap engineering of zirconia by nitrogen doping in reactive HiPIMS: a step forward in developing innovative technologies for photocatalysts synthesis

“…This phase transition may be connected to the presence of Zr 2 ON 2 and Zr 3 N 4 phases, both of which have much lower band gap energies compared to ZrO 2 or ZrO 2-x N x . Previous reports have cited band gap values between 1.5 and 2.6 eV for Zr 2 ON 2 (Thapa et al, 2022) and between 0.6 and 1.1 eV for Zr 3 N 4 (Yablonskikh et al, 2014). Moreover, the optical properties of the thin films could also be influenced by the presence of internal stress.…”