Oxynitrides have been explored extensively in the past decade because of their interesting properties, such as visible-light absorption, photocatalytic activity and high dielectric permittivity. Their synthesis typically requires high-temperature NH3 treatment (800-1,300 °C) of precursors, such as oxides, but the highly reducing conditions and the low mobility of N(3-) species in the lattice place significant constraints on the composition and structure-and hence the properties-of the resulting oxynitrides. Here we show a topochemical route that enables the preparation of an oxynitride at low temperatures (<500 °C), using a perovskite oxyhydride as a host. The lability of H(-) in BaTiO3-xHx (x ≤ 0.6) allows H(-)/N(3-) exchange to occur, and yields a room-temperature ferroelectric BaTiO3-xN2x/3. This anion exchange is accompanied by a metal-to-insulator crossover via mixed O-H-N intermediates. These findings suggest that this 'labile hydride' strategy can be used to explore various oxynitrides, and perhaps other mixed anionic compounds.

We investigated topochemical anion exchange reactions for a Sc-substituted SrTiO perovskite, Sr(TiSc)O□ (y ≤ 0.1), using CaH. It was found that the initial introduction of a small amount of anion vacancies (y/2) is crucial to enhance the anion (H/O) exchangeability. For example, hydride reduction of Sr(TiSc)O yielded the oxyhydride SrTiScOH in which the hydride concentration is increased by 33% with respect to pristine SrTiO (leading to SrTiOH). This observation highlights the importance of anion vacancies to improve anion (H/O) diffusion, which is a well-known strategy for improving oxide anion conductivity, and suggests that such a vacancy-assisted reaction could be applied to other anion exchange reactions (e.g., F/O and N/O) to extend the solubility range.