Kichiro Koto

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.

The crystal structure of orthoenstatite

The crystal structure of a natural orthorhombic enstatite (orthoenstatite) from the Bishopville meteorite, has been refined by the least-squares method using 680 three-dimensional reflections. The space group and the cell dimensions of this orthoenstatite are Pbca and a = 18.210, b = 8.812 and c = 5.178 Ä. The crystal structure of orthoenstatite is compared with tha t of clinoenstatite and the geometric relationship between the two polymorphs is described. The orthoenstatite structure is shown to be an exact twin of clinoenstatite on a unitcell scale. The orthorhombic cell is composed of two monoclinic cells of clinoenstatite joined on (100) through a b-glide plane. The orthoenstatite structure is also compared with the structures of hyperethene and orthoferrosilite. The corresponding interatomic distances show the following characteristics. With increasing Fe replacement of Mg the interatomic distances Μ—Ο in the metal octahedra increase. The mean interatomic distances, Μ—O, of the octahedra can thus be used to determine the Mg:Fe Z. Kristallogr. Bd. 129, 1-4 5 6 6 N O B U O M O R I M O T O a n d K I C H I K O K O T O ratio in the octahedral sites. More surprisingly, an increase of Fe in Mg positions is accompanied by a decrease of the mean Si—Ο distances in the silicate tetra-

Stability and antiphase domain structure of the pyrochlore solid solution in the ZrO2Gd2O3 system

The stability of the pyrochlore-type solid solution in the system (1−x)ZrO2xGdO1.5 has been studied by means of X-ray diffraction. The powder samples with the composition of 0.20≤x≤0.62 were synthesized by wet-chemical method. The X-ray diffraction study results in that the pyrochlore-type solid solution is stable within the compositional range of 0.33≤x≤0.60 at 1500°C, which is consistent with composition dependence of lattice constant and physical parameters in conductivity. Broad superstructure peaks of the pyrochlore-type have been observed in the X-ray powder diffraction for the samples within the compositional ranges 0.33≤x≤0.43 and 0.57≤x≤0.62 sintered at 1500°C, and 0.46≤x≤0.54 annealed at 1600°C. This is explained by forming domains with antiphase boundaries in the pyrochlore-type solid solution.

The Mean-Square Relative Displacement and Displacement Correlation Functions in Tetrahedrally and Octahedrally Coordinated ANB8-N Crystals

The Debye-Waller factors of atoms in Ge, GaN, ZnO, CdS, CdSe, CuBr, CuCl, AgI, CdO, AgBr, KBr and RbCl in order of Phillips ionicity (f i) are studied by both EXAFS and the diffraction method. The displacement correlation functions (DCF) are derived from the mean-square relative displacement (MSRD) and the mean-square displacement (MSD) at room temperature. The magnitudes of the MSRD and MSD correlate well with the coordination number and ionicity. The MSRDs of tetrahedrally coordinated compounds show a gradual approach to those of octahedrally coordinated ones as the ionicity increases. A divergent-like curve toward f i=0.785 is observed for the MSD in the tetrahedrally coordinated compounds, which indicates the behavior of lattice instability. In the tetrahedrally coordinated covalent materials, the MSRD is nearly half the MSD cation or MSD anion and the ratio of the DCF to MSD given by 2DCF/(MSD cation+MSD anion) is about 80%. In the octahedrally coordinated ionic materials, the MSRD is comparable to the MSD and 2DCF/(MSD cation+MSD anion)=50%. An anharmonic contribution to the Debye-Waller factor determined by EXAFS appears pronouncedly when the magnitude of σ(2) is greater than 0.01 A2. The MSRD for the tetrahedrally coordinated compounds of silver and copper halides obeys the systematic ionicity dependence and pronounced specificity of anharmonicity was not observed: AgI, CuBr and CuCl of which the high-temperature forms are typical superionic conductors have a broad interatomic potential which is similar to that of the octahedrally coordinated compounds and strongly correlated displacement in thermal vibration between cation and anion.

Modified spinel, Beta-manganous orthogermanate: stability and crystal structure.

A new high-pressure polymorph with a modified spinel structure, β-Mn2GeO4, is stable in a pressure range intermediate between the field of the polymorph with the olivine structure and that of another high-pressure polymorph. Oxygen atoms are located approximately in cubic close packing with manganese and germanium atoms in octahedral and tetrahedral interstices, respectively, as in the spinel structure; however, germanium atoms form Ge2O7 groups instead of isolated GeO4 groups.

Exafs study of the fluorite and pyrochlore compounds in the system ZrO2-Gd2O3

Abstract The local structures of the fluorite and pyrochlore compounds in the system (1-x)ZrO2-xGdO1.5 (0.20≤×0.62) have been studied by means of EXAFS spectroscopy. The EXAFS results indicate that trivalent Gd ions are 8-coordinated in both the fluorite-type and the pyrochlore-type solid solutions in the compositional range 0.2≤×0.5, and that the mean coordination number of tetravalent Zr ions decreases from eight to six with increasing Gd content. This indicates that the oxygen vacancies are located adjacent to tetravalent Zr ions. Cluster models which are a part of the pyrochlore structure, are proposed in the region of the fluorite phase.

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.

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.

Crystal Structure of Umangite, Cu3Se2

Characteristic spectral information can be obtained from a surface composed of fine particles either if the spectrum is observed at sufficiently high signal-to-noise ratio or if the particles are well compacted. Recent publications (1) concerning spectral reflectance or emittance by particulate surfaces have indicated that the surface tends to appear black when the particle size is small compared with the wavelength. Thus it is generally concluded that little or no characteristic spectral information can be gained by remote sensing of lunar or planetary surfaces if the surfaces are composed of fine particles. We now show both theoretically and experimentally that most of the spectral information is not really lost under these circumstances if one observes the spectrum at sufficiently high signal-to-noise ratio or if the surface is well compacted. We consider first the case of a semiinfinite medium having a smooth surface and composed of uniformly distributed particles of diameter and spacing much less than a wavelength. Under these conditions the reflection by the medium of a beam of radiation is predominantly a coherent surface effect caused by the discontinuity at the surface of the average values of the optical 345 zo fl*l I 25 0 OAAIP

Superstructures and phase transitions of Bi2O3

Abstract Polymorphs of bismuth sesquioxide were studied by TG-DTA and X-ray diffraction methods and by electron microscopy. Phase transitions of the polymorphs are influenced not only by the temperature before cooling and the cooling rate but also by the duration of melting. New commensurate and incommensurate superstructure phases, observed under the electron microscope, are described as the Na x a types, where N is various value between 2 and 2.48. Here a stands for the cell dimension of the δ form, the fluorite-type structure. The superstructures can be derived by a scheme for ordering oxygen vacancies in the defective fluorite-type structure.