Browsing by Author "Omoto, K"
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- ItemCrystal structure, optical properties, and electronic structure of calcium strontium tungsten oxynitrides CaxSr1-xWO2N(American Chemical Society, 2013-09-12) Yashima, M; Fumi, U; Nakano, H; Omoto, K; Hester, JRNovel calcium strontium tungsten oxynitrides CaxSr(1-x)WO(2)N (x = 0.25 and 0.5) have been synthesized. The crystal and electronic structures, electron-density distribution, and optical properties of CaxSr(1-x)WO(2)N and CaxSr(1-x)WO(2)N (x = 0, 0.25, and 0.5) have been investigated by neutron, synchrotron, and X-ray powder diffraction; transmission electron microscopy energy-dispersive spectroscopy (TEMEDS); scanning electron microscopy; UV visible reflectance measurements; and ab initio density functional theory (DFT)based calculations. Precursor materials CaxSr(1-x)WO(2)N (x = 0, 0.25, 0.5, and 1) with a scheelite-type structure were prepared by solid-state reactions, and heated at 900 degrees C for 5 h under an ammonia flow. The main phase in the product for the composition x = 1 was metallic tungsten W, whereas cubic Pm3m perovskite-type oxynitrides CaxSr(1-x)WO(2)N were obtained for the compositions x = 0, 0.25, and 0.5. The unit-cell parameter a of the cubic perovskite-type CaxSr(1-x)WO(2)N obtained from the Rietveld analysis of synchrotron X-ray and neutron powder diffraction data decreases with an increase of Ca concentration x (0 < x < 0.5), which indicates the substitution of Ca for Sr. The existence of nitrogen in CaxSr(1-x)WO(2)N was confirmed by (I) the refined occupancy factor in the Rietveld analysis of neutron data and (2) EDS. The maximum-entropy-method electron-density analysis combined with the DFT calculations indicates W N and W-O covalent bonds in CaxSr1_xWO2N, which are formed by the overlapping of W 5d and anion 2p orbitals. The minimum electron density at the W N bond is higher than that at the W-0 one, which indicates that the W N bond is more covalent due to the smaller difference in the electronegativity between W and N atoms compared to the W and O ones: The oxidation number of W in CaxSr(1-x)WO(2)N was estimated to be 5.2 by bond valence sum, which indicates the W5+ ion with the 5di electron configuration. Precursor oxides Ca Sr,,WO, with W6* having the 5cl electron configuration are white and insulating, whereas the CaxSr(1-x)WO(2)N oxynitrides with the W5* ion having the 5di configuration are black and exhibit metallic character. These results indicate the insulator metal transition from the d oxide CaxSr(1-x)WO(2)N to the di oxynitride CaxSr(1-x)WO(2)N. © 2013, American Chemical Society.
- ItemNew perovskite-related structure family of oxide-ion conducting materials NdBaInO4(ACS Publications, 2014-03-21) Fujii, K; Esaki, Y; Omoto, K; Yashima, M; Hoshikawa, A; Ishigaki, T; Hester, JROxide-ion conducting ceramic materials, which include pure oxide-ion conductors and mixed oxide-ion and electronic or hole conductors, have received considerable attention because of their potential application for oxygen separation membranes, oxygen sensors, and solid oxide fuel cells (SOFCs) electrolytes and cathodes.1 Perovskite-type and perovskite-related materials have been widely investigated as oxide-ion conductors.2 For example, the K2NiF4-type compounds are known to exhibit high oxide-ion conductivity.3 Because the oxide-ion conductivity is strongly dependent on crystal structure, it is necessary to design and synthesize novel materials belonging to a new structure family for further innovative developments of the oxide-ion conductors. Herein, we report a new perovskite-related structure family with AA′BO4 composition, which exhibits oxide-ion conduction. Here, A and A′ are relatively larger cations and B is a smaller cation. In this work we have succeeded in solving the crystal structure of NdBaInO4 and show oxide-ion conduction in NdBaInO4. © 2014, American Chemical Society.
- ItemStructural origin of the anisotropic and isotropic thermal expansion of K2NiF4-type oxides(American Chemical Society, 2015-04-02) Kawamura, K; Yashima, M; Fujii, K; Omoto, K; Hibino, K; Yamada, S; Hester, JR; Avdeev, M; Miao, P; Torii, S; Kamiyama, TK2NiF4-type LaSrAlO4 and Sr2TiO4 exhibit anisotropic and isotropic thermal expansion, respectively; however, their structural origin is unknown. To address this unresolved issue, the crystal structure and thermal expansion of LaSrAlO4 and Sr2TiO4 have been investigated through high-temperature neutron and synchrotron X-ray powder diffraction experiments and ab initio electronic calculations. The thermal expansion coefficient (TEC) along the c-axis (αc) being higher than that along the a-axis (αa) of LaSrAlO4 [αc = 1.882(4)αa] is mainly ascribed to the TEC of the interatomic distance between Al and apical oxygen O2 α(Al–O2) being higher than that between Al and equatorial oxygen O1 α(Al–O1) [α(Al–O2) = 2.41(18)α(Al–O1)]. The higher α(Al–O2) is attributed to the Al–O2 bond being longer and weaker than the Al–O1 bond. Thus, the minimum electron density and bond valence of the Al–O2 bond are lower than those of the Al–O1 bond. For Sr2TiO4, the Ti–O2 interatomic distance, d(Ti–O2), is equal to that of Ti–O1, d(Ti–O1) [d(Ti–O2) = 1.0194(15)d(Ti–O1)], relative to LaSrAlO4 [d(Al–O2) = 1.0932(9)d(Al–O1)]. Therefore, the bond valence and minimum electron density of the Ti–O2 bond are nearly equal to those of the Ti–O1 bond, leading to isotropic thermal expansion of Sr2TiO4 than LaSrAlO4. These results indicate that the anisotropic thermal expansion of K2NiF4-type oxides, A2BO4, is strongly influenced by the anisotropy of B–O chemical bonds. The present study suggests that due to the higher ratio of interatomic distance d(B–O2)/d(B–O1) of A22.5+B3+O4 compared with A22+B4+O4, A22.5+B3+O4 compounds have higher α(B–O2), and A22+B4+O4 materials exhibit smaller α(B–O2), leading to the anisotropic thermal expansion of A22.5+B3+O4 and isotropic thermal expansion of A22+B4+O4. The “true” thermal expansion without the chemical expansion of A2BO4 is higher than that of ABO3 with a similar composition. © 2015 American Chemical Society
- ItemStructural origin of the anisotropic thermal expansion of a K2NiF4-type oxide CaErAlO4 through interatomic distances(Chemistry Letters, 2013-12-11) Omoto, K; Yashima, M; Hester, JRThe anisotropic thermal expansion and crystal structure of K2NiF4-type CaErAlO4 have been investigated by neutron diffraction from 298 to 1473 K. The average thermal expansion coefficient (TEC) along the c axis αc is larger than that along the a axis, mainly due to the larger Al–(apical oxygen O2) TEC compared to the Al–(equatorial O1) TEC. The larger Al–O2 TEC is attributable to the weaker Al–O2 bond. Contrary to the literature, the mean TEC of K2NiF4-type CaErAlO4 is larger than that of perovskite-type ErAlO3.