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Perovskite oxynitrides are, due to their reduced band gap compared to oxides,promising materials for photocatalytic applications. They are most commonlysynthesized from {110} layered Carpy-Galy (A$_2$B$_2$O$_7$}) perovskites viathermal ammonolysis, i.e. the exposure to a flow of ammonia at elevatedtemperature. The conversion of the layered oxide to the non-layered oxynitridemust involve a complex combination of nitrogen incorporation, oxygen removaland ultimately structural transition by elimination of the interlayer shearplane. Despite the process being commonly used, little is known about themicroscopic mechanisms and hence factors that could ease the conversion. Herewe aim to derive such insights via density functional theory calculations ofthe defect chemistry of the oxide and the oxynitride as well as the oxide'ssurface chemistry. Our results point to the crucial role of surface oxygenvacancies in forming clusters of NH$_3$ decomposition products and inincorporating N, most favorably substitutionally at the anion site. N thenspontaneously diffuses away from the surface, more easily parallel to thesurface and in interlayer regions, while diffusion perpendicular to theinterlayer plane is somewhat slower. Once incorporation and diffusion lead to alocal N concentration of about 70% of the stoichiometric oxynitridecomposition, the nitridated oxide spontaneously transforms to anitrogen-deficient oxynitride.

Conversion of La$_2$Ti$_2$O$_7$ to LaTiO$_2$N via Ammonolysis: An ab-initio Investigation