The perovskite oxynitride SrTaO 2 N as a promising candidate for lead-free dielectric materials has been studied with a special emphasis on the structural characterization using neutron powder diffraction. SrTaO 2 N powders were prepared by means of ammonia nitridation via precursors obtained from a soft chemistry method and a solid state reaction route. All the products crystallized in tetragonal I4/mcm space group with only slight variation in the lattice constant, e.g., a = 5.7023(1) ¡ and c = 8.0786(1) ¡ for the soft-chemistry derived sample. Neutron diffraction analysis suggested the short-range O/N ordering involving a cis configuration in TaO 4 N 2 octahedra together with large anionic displacement within the ab plane on their axial position in the average crystal lattice. These facts are associated with local tilting of the TaO 4 N 2 octahedra which is likely to play a key role in the unusual dielectric behavior of SrTaO 2 N.

Oxynitride perovskite SrTaO2N has been attracting attention as a possible new dielectric material owing to the cis-type anion ordering in its crystal structure. It is currently difficult to obtain it in bulk form because it easily loses a part of its nitrogen to become semiconducting during densification at high temperature. We found that its surface layer recovered its original orange color and dielectric properties after postannealing in ammonia. Piezoresponse force microscopy measurements clearly revealed a ferroelectric behavior on the entire orange surface layer peeled off from the black sintered body inside. The present sample was free-standing at about 8 μm thick and clearly distinct from the compressively strained SrTaO2N thin films for which ferroelectricity was recently reported in small domains (10–102 nm) in the locally assumed trans-type anion ordering. The present results represent the first experimental observation of ferroelectric response in a bulk oxynitride perovskite with cis-type anion ordering.

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Abstract:The catalytic activity of manganese oxynitrides in the oxygen reduction reaction (ORR) was investigated in alkaline solutions to clarify the effect of nitrogen on ORR activity. These oxynitrides, having rocksalt-based structures with different nitrogen content, were synthesized by reactions of MnO, Mn2O3, and MnO2 with molten NaNH2 at 240-280 C. Anion contents and Mn valence states were determined by combustion analysis, powder X-ray diffraction, and X-ray absorption near edge structure analysis. An increase in nitrogen content in rocksaltbased manganese oxynitrides enhanced the valence of manganese and reinforced the catalytic activity for the ORR in 1 M KOH solution. Nearly single-electron occupancy in antibonding eg states and high covalency in Mn-N bonding would enhance the ORR activity of nitrogen-rich manganese oxynitrides.Catalysts for the oxygen reduction reaction (ORR) in alkaline solutions (O2 + 2H2O + 4e -→ 4OH -) are key materials for nextgeneration energy conversion and storage systems, including fuel cells and metal-air batteries, and highly efficient ORR catalysts are highly desirable. [1] A group of well-studied catalysts includes perovskite oxides, such as LaCoO3 and LaMnO3, whose structure-activity correlation has been extensively explored. [1][2] Although the ORR is mechanistically complicated and is influenced by crystal structure, composition, electronic conductivity, surface absorption behavior, and the incorporation of conductive additives, [1a] the structure-activity correlation gives the rational guidelines for designing and understanding ORR catalysts.Nitrides and related compounds have been studied as ORR catalysts. Binary and ternary nitrides, such as AlN, TiN, Cu3N, and Cu3PdN, show catalytic activity for the ORR. [3] Oxynitrides, including ZrOxNy, MoOxNy, TaOxNy and CoMoOxNy, also show the catalytic activity. [4] Nonetheless, the correlation between structure and ORR activity has not been extensively studied, and the role of nitrogen in altering catalytic activity is not completely understood. The challenge would be the difficulty of tuning the amount of incorporated nitrogen, mainly due to the thermodynamic stability of the triple bond in molecular N2. Additionally, the different nitrogen amount often has a significant effect on the crystal structure of synthesized products, [5] thus complicating the interpretation of results.Pervoskite oxides contain metal-oxygen octahedron, whose electronic structure can be schematically depicted as tg and eg states. Suntivich et al. have reported that nearly single-electron occupancy in eg states composed by antibonding metal-oxygen interaction is beneficial for an electron transfer during the ORR cycle. [2d] ; a typical example of a single electron in eg states is LaMnO3 with trivalent manganese. Additionally, more covalency of metal-oxygen bonding in perovskite oxides is favorable for the ORR . [2c, 2d] Since nitrogen has less electron than oxygen and metal-nitrogen bonding is more covalent than metal-oxygen one, the tuning of nit...

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