  Perovskite oxynitrides have recently attracted attention for their ability to
photocatalytically split water. Compared to oxides the arrangement of anions in
the material represents a further structural degree of freedom. The bulk
oxynitride LaTiO$_2$N prefers a bonding-dominated \textit{cis} nitrogen
arrangement, while we have previously shown that the (001) surface prefers a
non-polar \textit{trans} order to compensate polarity. Here we consider, using
density functional theory calculations, the polar/non-polar interface that
would necessarily be present between the two anion orders. We show that the
Ti-terminated surface will adopt up to two \textit{trans} ordered surface
layers, which has a beneficial effect on the oxygen evolution efficiency. We
then consider the hypothetical case of a polar \textit{cis} ordered surface
layer atop a non-polar \textit{trans} bulk and show that similar electronic
reconstructions as in the LaAlO$_3$/SrTiO$_3$ interface can be expected when
interfaces between different anion orders are engineered in one and the same
oxynitride material.
