Toshihiro Moriga

Crystal structure analyses of the pyrochlore and fluorite-type Zr2Gd2O7 and anti-phase domain structure

Abstract The detailed structure investigations of Zr2Gd2O7 with pyrochlore- and fluorite-type structures have been carried out by X-ray single crystal method at room temperature. The bond distances between cations and oxide ions versus the site population for cations reasonably support that the fluorite phase is made up of the structure with microdomains of the pyrochlore. Sharp fundamental and diffuse superstructure reflections of the pyrochlore are interpreted assuming that anti-phase domain boundaries lie parallel to the &{211} and that domain sizes are not uniform. It should be proposed that the layers composed of the 48f site oxide ions are anti-phase boundaries. The probability density function maps and effective one-particle potentials of the pyrochlore structure reveal that the magnitudes of anharmonic thermal motions for the 48f site oxide ions are large toward the unoccupied 8b site. Compared to the s-pyrochlore phase (with sharp superstructure reflections), the 48f site oxide ions of the d-pyrochlore (with diffuse superstructure reflections) and fluorite phases have larger temperature factors and gentler potential curves. This is due to that the anti-phase domains are coherent each other and that their electron density distributions are the averages of individual domains.

Transparent conductive ZnO film preparation by alternating sputtering of ZnO:Al and Zn or Al targets

Abstract Multilayered ZnO:Al and ZnO:O v film was prepared by alternately sputtering ZnO:Al(2% wt.) and Zn targets. The films obtained were optically transparent and had lower resistivity than those prepared by ZnO:Al sputtering. Deposition from the Zn target gave a ZnO film containing native donors (a ZnO:O v film). The carrier concentration increased in the ZnO:Al/ZnO:O v multilayered film. Our results cannot be explained by the redistribution of the carrier between ZnO:Al and ZnO:O v . This result indicates an improvement in the doping efficiency. Multilayered films of ZnO:Al and Al oxide compounds were also prepared. In this case, with increasing thickness of the Al oxide film, a decrease was observed in the carrier mobility, due to the scattering at the interface.

Transparent conducting amorphous Zn–Sn–O films deposited by simultaneous dc sputtering

The films of ZnO–SnO2 system were deposited on glass substrates by simultaneous dc magnetron sputtering apparatus, in which ZnO and SnO2:Sb (Sb2O5 3 wt % doped) targets faced each other. The substrate temperatures were maintained at 150, 250, and 350 °C, respectively. As an experimental parameter, current ratio δ=IZn/(IZn+ISn), which corresponds to ZnO target current (IZn) divided by the sum of ZnO and SnO2:Sb target currents (IZn+ISn), was adopted. Amorphous transparent films appeared for 0.50⩽δ⩽0.73, which could be correlated to compositions as [Zn]/([Sn]+[Zn])=0.33–0.67 by x-ray fluorescent analysis. At [Zn]/([Sn]+[Zn])=1/2 (δ=0.62), 2/3 (δ=0.73) and all other ratios in as-deposited films, neither crystalline ZnSnO3 nor Zn2SnO4 was obtained. Minimum resistivity of 4–6×10−2 Ω cm was found at δ=0.50, whose composition was approximately SnO2⋅ZnSnO3. Resistivity increased linearly with an increase of the current ratio, until the composition reached Zn2SnO4. The amorphous phase showed a constant Hall mobili...

Surface and bulk properties, catalytic activities and selectivities in methane oxidation on near-stoichiometric calcium hydroxyapatites

Various calcium hydroxyapatites [Ca10–z(HPO4)z(PO4)6–z(OH)2–z; z= 0, stoichiometric apatite (Ca/P = 1.67; atomic ratio); 0 Ca/P 1.50)], together with the apatites of Ca/P = 1.72, were analysed by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) spectroscopy and their catalytic properties in the oxidation of methane at 973 K were examined in the presence and absence of tetrachloromethane as a gas-phase additive. The XRD patterns of each hydroxyapatite were the same. However, the nearest-neighbour distances of the Ca–O bond of stoichiometric and non-stoichiometric hydroxyapatites as estimated by EXAFS were 2.41,2.37, 2.38 and 2.39 A for each catalyst of Ca/P = 1.72,1.68,1.64 and 1.58, respectively. In the oxidation of methane on each hydroxyapatite in the absence of CCl4, the conversion of methane was little influenced by the value of Ca/P but the selectivities to C2H6 and CO showed a maximum and minimum, respectively, at Ca/P = 1.68. On addition of CCl4 into the feedstream, the selectivity to CO2 on Ca/P = 1.68 and 1.72 decreased as the time on-stream increased while sharp decreases in conversion were observed with time on-stream on non-stoichiometric hydroxyapatites of Ca/P = 1.64 and 1.53, respectively, without suppression of the selectivity to CO2.

Enhancement of the catalytic activities in propane oxidation and H–D exchangeability of hydroxyl groups by the incorporation with cobalt into strontium hydroxyapatite

The oxidative dehydrogenation of propane to propylene on strontium and cobalt–strontium hydroxyapatites (SrHAp and Co–SrHAp, respectively) and H–D exchangeability of hydroxyl groups in those catalysts with D2O have been studied. Both the yield of C3H6 and H–D exchangeability were found to be enhanced with increasing the cobalt contents in the catalysts, the latter of which was investigated by solid state 1H MAS MNR at 25 kHz. It is suggested that oxygen species, like lattice oxygen in oxide catalysts, as formed from hydrogen desorption from OH groups in the hydroxyapatites, may contribute to the activation of C3H8. The structure of Co–SrHAp, of which the XRD pattern was identical to that of SrHAp, was also identified with extended X-ray absorption fine structure (XAFS) and solid state 31P MAS NMR.

EFFECTS OF FINE STRUCTURE CHANGES OF STRONTIUM HYDROXYAPATITES ON ION-EXCHANGE PROPERTIES WITH DIVALENT CATIONS

The ion exchange of various strontium hydroxyapatites [SrHAp; stoichiometric form, Sr10(PO4)6(OH)2] of different Sr/P ratios with divalent cations has been investigated and compared with those between the corresponding cations and calcium hydroxyapatites (CaHAp). SrHAp has superior exchange properties for divalent cations in comparison with those of CaHAp and the former solid with Sr/P equal to 1.73 has the highest capacity for ion exchange of the compositions examined. The addition of HCl to the exchanging solution enhanced the exchange of SrHAp with Pb2+, apparently influenced by the formation of strontium and lead chlorapatite during the exchange process. The extended X-ray absorption fine structure (EXAFS) analyses revealed that the ion-exchange characteristics are strongly influenced by the nearest-neighbour distances of the Sr—O bond of SrHAp or the Ca—O bond of CaHAp.

EXAFS study of the fluorite-type compounds in the systems ( 1 − x ) ZrO2−xYbO1.5 ( x = 0.18 ≦ x ≦ 0.5 ) and Zr2Ln2O7 (Ln = Tb, Dy, Ho, Er, and Yb)

Abstract The local structures of the fluorite-type compounds with a composition of Zr 2 Ln 2 O 7 ( Ln = Gd, Dy, Ho, Er and Yb) have been studied by means of EXAFS spectroscopy. The EXAFS results indicate that Ln 3+ ions are essentially 8-coordinated by oxide ions but Zr 4+ ions are 6-coordinated. The distinct difference in coordination number between Zr 4+ and Ln 3+ ions means that the local structures of the fluorite-type Zr 2 Ln 2 O 7 are similar to the pyrochlore-type structure, even though both Zr 4+ and Ln 3+3 ions are located statistically at the cation sites (4a site) in the fluorite-type lattice. The local structure of the fluorite-type solid solution in the system ( 1 − x )ZrO 2 − x YbO 1.5 (0.18≦ x ≦0.50) has been also studied by EXAFS spectroscopy. The result indicates that the coordination numbers of Zr 4+ and Yb 3+ ions are 6 and 8, respectively, in the solid solution in the compositional range 0.18≦ x ≦0.50, and that the oxygen vacancies are located adjacent to Zr 4+ ions.

X-ray and Raman study on coordination states of fluorite- and pyrochlore-type compounds in the system ZrO2-Gd2O3

Raman spectra were measured on the pyrochlore- and fluorite-type Zr2Gd7 (x=0.50) in the system (1−x)ZrO2−xGdO 1.5 and structure refinements were carried out for both structure types with a composition of x=0.43 by means of single crystal X-ray diffraction. Bond distances in the fluorite-type structure with x=0.43 were in good agreement with those calculated from the effective ionic radii, assuming that Gd3+ ion was coordinated by eight oxygens and oxygen vacancies were adjacent to Zr4+ ion. A Raman spectrum of the fluorite-type Zr2Gd2O7 was very similar to that of the pyrochlore-type, and had the line near 220 cm−1 attributed to the pyrochlore-type Gd4O tetrahedra. Coordination states and stability of the pyrochlore- and fluorite-type structures are discussed based on the present results.

Characterization of oxygen-deficient phases appearing in reduction of the perovskite-type LaNiO3 to La2Ni2O5

Abstract Topotactic vacuum reduction of the perovskite-type LaNiO 3 was performed with aluminum metal powder as a reducing agent. An orthorhombic single phase of oxygen-deficient LaNiO 2.5 + x (0.1 ≤ x ≤ 0.15) was successfully prepared. LaNiO 3 − z ( z ≤ 0.25) with the rhombohedral perovskite-type structure showed metallic conductivity and Pauli paramagnetism, whereas the oxygen-deficient phase LaNiO 2.60 was semiconductive and was considered to be ferromagnetic below 230 K. The effective magnetic moment of LaNiO 2.60 was 1.7 μB, which suggests that LaNiO 2.60 is composed of octahedrally-coordinated Ni 3+ ions and square-planarly-coordinated Ni + ions, as seen in La 2 Ni 2 O 5 .

Properties of ZnO:In film prepared by sputtering of facing ZnO:In and Zn targets

ZnO:In films were prepared by a sputtering method with targets facing. The lowest resistivity film is obtained at a substrate temperature of 150 °C using a ZnO target doped with 3 wt % In2O3. At substrate temperatures above 300 °C the resistivity of the film increases as the carrier concentration decreases. This implies a significant decrease in the donor impurity, ascribed to evaporation of the indium during film growth. From experiments with an additional Zn supply during deposition, we found that all of the doped indium is ionized as donor in ZnO:In, and that evaporation of depositing indium atoms is enhanced above 250 °C.