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 with Sr was also evident in this perovskite lattice. Just below x = 2, a perovskite-type structure was intergrown as defects in the Ruddlesden-Popper Sr₂TaO₃N. Characterization of Sr₂TaO₃N in ambient air was challenging due to its moisture sensitivity. Thermal analysis demonstrated that this material was relatively stable up to approximately 1400 °C in comparison with SrTaO₂N perovskite, especially under nitrogen. Sr₂TaO₃N could keep its structure in a sealed tube, and some amount of SrCO₃ was observed in XRD after 10 days of exposure to 75% relative humidity under prior ambient conditions. A compact of this material had a relative density of 96% after sintering at 1400 °C under 0.2 MPa of nitrogen, even though a drastic loss of nitrogen was previously reported for a SrTaO₂N perovskite under these same conditions. Postammonolysis of the Sr₂TaO₃N ceramics was not required prior to studying its dielectric behavior. This is in contrast to the SrTaO₂N perovskite, which requires postammonolysis to recover its stoichiometric composition and electrical insulating properties.