Intergrowth between the Oxynitride Perovskite SrTaON and the Ruddlesden-Popper Phase SrTaON.

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 SrTaON and the Ruddlesden-Popper phase SrTaON with increasing SrO content. X-ray diffraction analyses showed that a single-phase perovskite was obtained up to x = 1.1, after which SrTaON gradually appeared at x ≥ 1.25. High-resolution scanning transmission electron microscopy observations identified the gradual intergrowth of a Ruddlesden-Popper SrTaON 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 SrTaON. Characterization of SrTaON 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 SrTaON perovskite, especially under nitrogen. SrTaON could keep its structure in a sealed tube, and some amount of SrCO 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 SrTaON perovskite under these same conditions. Postammonolysis of the SrTaON ceramics was not required prior to studying its dielectric behavior. This is in contrast to the SrTaON perovskite, which requires postammonolysis to recover its stoichiometric composition and electrical insulating properties.