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7. Crystal structure and superconducting properties of hexagonal lithium-niobium oxynitride

Author

Motohashi, Teruki, Ito, Masahiko, Masubuchi, Yuji, Wakeshima, Makoto, and Kikkawa, Shinichi

Subjects
Condensed Matter - Superconductivity, Condensed Matter - Strongly Correlated Electrons
Abstract

A hexagonal oxynitride (Li0.88_0.12)Nb3.0(O0.13N0.87)4 was synthesized through ammonia nitridation of LiNb3O8. The structural analysis revealed that this oxynitride consists of alternate stacking of octahedral and prismatic layers with different Li/Nb ratios: significant amounts of Li and Nb atoms (Li/Nb = 43/57) coexist in the octahedral layer, while the prismatic site is preferentially occupied by Nb (Li/Nb = 3/97). A metallic behavior was accompanied by an abrupt drop of electrical resistivity at about 3 K. Furthermore, large diamagnetism and specific-heat anomaly were observed below this temperature, suggesting the appearance of superconductivity in the Li-Nb oxynitride., Comment: 28 pages, 7 figures, 3 tables, to appear in Inorganic Chemistry

Published
2012
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23. Ferroelectric BaTaO2N Crystals Grown in a BaCN2 Flux

Author

Hosono, Akira, Masubuchi, Yuji, Yasui, Shintaro, Takesada, Masaki, Endo, Takashi, Higuchi, Mikio, Itoh, Mitsuru, and Kikkawa, Shinichi

Subjects
Oxynitride, Ferroelectricity, Crystal growth, Metal carbodiimide, Perovskite
Abstract

Perovskite-type oxynitride BaTaO2N has been attracting attention for its large dielectric constant, which is almost independent of the temperature by measurements on its ceramics. Its dielectric characteristics are attributed to polar nanoregions (PNRs) in the average cubic crystal structure. Polarization saturation to produce a butterfly-like piezoresponse force microscopy (PFM) signal was observed on BaTaO2N crystals in the present study. Reddish crystallites of BaTaO2N of up to 3.1 mu m in size were grown using a BaCN2 flux. Grain growth proceeded through the formation of a Ruddlesden-Popper-type oxynitride from the reaction between BaTaO2N powder and molten BaCN2. Their electrical property was studied using PFM with special care because of the small size of the crystals. They were found to be much more highly insulating than its ceramics. Ferroelectricity with complete phase inversion was observed on an oxynitride perovskite crystal for the first time. A large coercivity of 50-60 V was observed in the measurement. Such ferroelectricity is ascribed to the PNRs induced by the polar linkages between cis-type TaO4N2 octahedra.

Published
2019

37. Preparation and thermal stability of oxynitride perovskite solid solution Sr1-xLaxTa1-xTixO2N

Author

1000080466440, Masubuchi, Yuji, Ohtaki, Sota, Fujii, Kotaro, Yashima, Masatomo, 1000040198990, Higuchi, Mikio, Kikkawa, Shinichi, 1000080466440, Masubuchi, Yuji, Ohtaki, Sota, Fujii, Kotaro, Yashima, Masatomo, 1000040198990, Higuchi, Mikio, and Kikkawa, Shinichi

Abstract

The thermal stability of oxynitride perovskites is very important for the fabrication of their dense ceramics. In this study, a solid solution of the oxynitride perovskite Sr1-xLaxTa1-xTixO2N was prepared via the ammonia nitridation of oxide precursors. The oxynitrides products obtained at x <= 0.2 and x >= 0.7 had tetragonal and triclinic perovskite-type structures, respectively. A neutron diffraction study on the oxynitride product obtained at x = 0.2 indicated that this tetragonal perovskite had a cis-type anion distribution similarly to SrTaO2N. The tetragonal perovskite released a part of nitrogen at approximately 1000 degrees C in a nitrogen atmosphere while maintaining the perovskite structure at least to 1450 degrees C. The oxynitride perovskites at x >= 0.5 gradually decomposed to a mixture of LaTiO2N, LaTiO3, La(OH)(3), La2O3, TiN and TaO above 1100 degrees C.

Published
2020

38. Formation and morphological change of BaTaO2N perovskite from BaCN2/Ta2O5 mixture

Author

Hosono, Akira, 1000080466440, Masubuchi, Yuji, Koyama, Kyohei, 1000040198990, Higuchi, Mikio, Kikkawa, Shinichi, Hosono, Akira, 1000080466440, Masubuchi, Yuji, Koyama, Kyohei, 1000040198990, Higuchi, Mikio, and Kikkawa, Shinichi

Abstract

Metal oxynitrides are generally synthesized by heating their component oxides in a flow of ammonia for longer than 20 h. In the present study, pure BaTaO2N oxynitride perovskite powder was obtained by heating a mixture of tetragonal BaCN2 and Ta2O5 in a flow of nitrogen. The BaTaO2N powder synthesized at slightly below the melting point of tetragonal BaCN2 (910 degrees C) was found to consist of coarse aggregates of 300 nm-sized particles based on scanning electron microscopy observations. In contrast, BaTaO2N powder obtained above the melting point of tetragonal BaCN2 contained highly-dispersed particles after washing with acid. This difference is attributed to the dissolution and precipitation of the surfaces of the BaTaO2N grains in the BaCN2 melt to generate grain boundary layers soluble in acid. This work represents the first report of the synthesis of a metal oxynitride using a metal carbodiimide as both the metal and nitrogen source. (C) 2020 Elsevier B.V. All rights reserved.

Published
2020

39. Red-emission over a wide range of wavelengths at various temperatures from tetragonal BaCN2:Eu2+

Author

Masubuchi, Yuji, Nishitani, Sayaka, Hosono, Akira, Kitagawa, Yuuki, Ueda, Jumpei, Tanabe, Setsuhisa, Yamane, Hisanori, Higuchi, Mikio, and Kikkawa, Shinichi

Abstract

A new polymorph of BaCN2 was obtained via a simple nitridation reaction of BaCO3 under an NH3 flow at 900 1C. The product was analyzed via single crystal X-ray diffraction and infrared spectroscopy, and it was found to have a tetragonal I4/mcm crystal structure (space group no. 140) with a = 0.60249(4) nm and c = 0.71924(5) nm. In this structure, each Ba2+ cation is situated in the square antiprism of N atoms of NCN(2-)anionic groups. Eu2+ doped BaCN2 can be excited by irradiation of blue and green light (from 400 to 550 nm), and it generates an intense red emission peak at 660 nm with a quantum efficiency of 42% in response to 465 nm excitation at room temperature. The peak emission wavelength varies over an extremely wide range of temperature, from 640 nm at 500 K to 680 nm at 80 K, and this red-shift with decreasing temperature is attributed to a unit cell shrinkage that results in significant crystal field splitting of the 5d energy levels of the Eu2+ ions.

Published
2018

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