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.