  Perovskite oxynitrides are a promising class of material for photocatalytic
water-splitting due to their small band gaps and suitably aligned band edges.
Recently, epitaxially strained oxynitrides started to attract interest due to
the possibility to engineer the anion order and ferroelectric distortions.
However, contrary to oxides there have been no studies on the band-gap
evolution in oxynitrides with epitaxial strain. Here, we investigate, using
density functional theory calculations, the influence of epitaxial strain on
the band gap of two different oxynitrides and compare our results with oxides.
As opposed to cubic oxides, where both compressive and tensile strain narrow
the band-gap, we find that the anion order leads to non-degenerate N
\textit{2p} bands already at zero strain, which leads to opposite and anion
order-dependent evolutions of the band gap under compressive and tensile
strain. Further, we find that in oxynitrides polar distortions increase the
band gap by up to 1 eV for 4% strain whereas for oxides the increase is only
about 0.1 eV for the same amount of strain. The reason for this difference is
the larger ionic radius of nitrogen leading to larger ferroelectric distortions
and a stronger dependence of the band gap on the distortions amplitude. These
results imply that ferroelectric distortions are strongly detrimental to light
absorption and that their suppression, for example above a critical
temperature, should result in a marked decrease in band gap and enhanced
absorption of visible light.
