Intergrowth between the Oxynitride Perovskite SrTaO2N and the Ruddlesden–Popper Phase Sr2TaO3N

Strontium tantalum oxynitrides were prepared within the nominal composition range of 1.0 ≤ x ≤ 2.0, where x = Sr/Ta atomic ratio. A gradual structural transition was observed between the perovskite SrTaO2N and the Ruddlesden–Popper phase Sr2TaO3N with increasing SrO content. X-ray diffraction analyses showed that a single-phase perovskite was obtained up to x = 1.1, after which Sr2TaO3N gradually appeared at x ≥ 1.25. High-resolution scanning transmission electron microscopy observations identified the gradual intergrowth of a Ruddlesden–Popper Sr2TaO3N type planar structure interwoven with the perovskite crystal lattice upon increasing x. The crystal lattice at x = 1.4 was highly defective and consisted primarily of perovskite intergrown with a large amount of the Ruddlesden–Popper phase structure. This Ruddlesden–Popper phase layer intergrowth is a characteristic of an oxynitride perovskite rather than the Ruddlesden–Popper defects previously reported in oxide perovskites. Partial substitution of Ta wit...