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

Abstract

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.

Tailoring of an unusual oxidation state in a lanthanum tantalum(IV) oxynitride via precursor microstructure design

“…Typically, the bandgap decreases, making the formed perovskite-type oxynitrides AB(O,N) 3 interesting for visible light-driven applications 1,4 . An important task in order to identify perovskite-type oxynitrides is the exact determination of the O:N ratio which often causes difficulties 11,12 .…”

Defect Engineering for Photocatalysis: From Ternary to Perovskite Oxynitrides

“…The same intergrowth defect has been reported in BaTaO 2 N, for which extended annealing leads to nitrogen loss and the growth of Ba 2 TaO 3 N and Ta 2 O 5 phase impurities . Suemoto et al . investigated the compositional range between SrTaO 2 N and Sr 2 TaO 3 N via variation of the Sr/Ta atomic ratio.…”

“…While there are examples of oxide photocatalysts reported for each of these layered phases, only the RP phase for perovskite oxynitrides has been investigated as a photocatalyst. The RP perovskite phase has recently attracted attention in relation to their AB(O,N) 3 counterparts due to their presence as intergrowth phase impurities in SrTaO 2 N and BaTaO 2 N, as well as the computational predictions of Castelli et al., that several RP phase oxynitride perovskite materials may be viable photoanodes for water splitting.…”

Perovskite‐Type Oxynitride Nanofibers Performing Photocatalytic Oxygen and Hydrogen Generation

“…At careful observation, the XRD reflections of BaNbO 2 N exhibit asymmetric tailing toward lower diffraction angles, which has been ascribed to possible defect formation within the perovskite lattice. [ 26 ] However, a single‐phase Rietveld refinement of BaNbO 2 N (with asymmetry assumption) is not satisfactory, as can be seen in Figure S3 of the Supporting Information. Additionally, Suemoto et al.…”