Browsing by Author "Tassel, C"

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    Direct synthesis of chromium perovskite oxyhydride with a high magnetic-transition temperature
    (Wiley Online Library, 2014-09-22) Tassel, C; Goto, Y; Kuno, Y; Hester, JR; Green, MA; Kobayashi, Y; Kageyama, H
    We report a novel oxyhydride SrCrO2H directly synthesized by a high-pressure high-temperature method. Powder neutron and synchrotron X-ray diffraction revealed that this compound adopts the ideal cubic perovskite structure equation image with O2−/H− disorder. Surprisingly, despite the non-bonding nature between Cr 3d t2g orbitals and the H 1s orbital, it exhibits G-type spin ordering at TN≈380 K, which is higher than that of RCrO3 (R=rare earth) and any chromium oxides. The enhanced TN in SrCrO2H with four Cr-O-Cr bonds in comparison with RCr3+O3 with six Cr-O-Cr bonds is reasonably explained by the tolerance factor. The present result offers an effective strategy to tune octahedral tilting in perovskites and to improve physical and chemical properties through mixed anion chemistry. © 2014, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
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    Impact of lanthanoid substitution on the structural and physical properties of an infinite-layer iron oxide
    (American Chemical Society, 2016-11-01) Yamamoto, T; Ohkubo, H; Tassel, C; Hayashi, N; Kawasaki, S; Okada, T; Yagi, T; Hester, JR; Avdeev, M; Kobayashi, Y; Kageyama, H
    The effect of lanthanoid (Ln = Nd, Sm, Ho) substitution on the structural and physical properties of the infinite-layer iron oxide SrFeO2 was investigated by X-ray diffraction (XRD) at ambient and high pressure, neutron diffraction, and 57Fe Mössbauer spectroscopy. Ln for Sr substituted samples up to ∼30% were synthesized by topochemical reduction using CaH2. While the introduction of the smaller Ln3+ ion reduces the a axis as expected, we found an unusual expansion of the c axis as well as the volume. Rietveld refinements along with pair distribution function analysis revealed the incorporation of oxygen atoms between FeO2 layers with a charge-compensated composition of (Sr1–xLnx)FeO2+x/2, which accounts for the failed electron doping to the FeO2 layer. The incorporated partial apical oxygen or the pyramidal coordination induces incoherent buckling of the FeO2 sheet, leading to a significant reduction of the Néel temperature. High-pressure XRD experiments for (Sr0.75Ho0.25)FeO2.125 suggest a possible stabilization of an intermediate spin state in comparison with SrFeO2, revealing a certain contribution of the in-plane Fe–O distance to the pressure-induced transition. © 2016 American Chemical Society
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    Oxyhydrides of (Ca,Sr,Ba)TiO3 perovskite solid solutions
    (American Chemical Society, 2012-11-05) Sakaguchi, T; Kobayashi, Y; Yajima, T; Ohkura, M; Tassel, C; Takeiri, F; Mitsuoka, S; Ohkubo, H; Yamamoto, T; Kim, JE; Tsuji, N; Fujihara, A; Matsushita, Y; Hester, JR; Avdeev, M; Ohoyama, K; Kageyama, H
    The oxyhydride solid solutions (Ca,Sr)TiO3-xHx and (Sr,Ba)TiO3-xHx have been prepared by reducing the corresponding ATiO(3) oxides with calcium hydride. Under the reaction conditions examined, a hydride content of x = 0.1-0.3 was obtained for all compositions. Compared to our previous result with BaTiO3-xHx, the larger particle size in this study (20-30 mu m vs 170 nm) resulted in a somewhat lower hydride amount despite prolonged reaction times. We examined changes in cell volume, octahedral tilt angle, and site occupancy of different anion sites after conversion to oxyhydrides; it appears that these oxyhydrides fit the geometrical descriptions typical for regular ABO(3) perovskites quite well. The hydrogen release temperature, previously shown to be indicative of the hydride exchange temperature, however, does not scale linearly with the A-site composition, indicating a potential effect of chemical randomness. © 2012, American Chemical Society.