Fujio P. Okamura

Crystal Structure of a Ba-Y-Cu-O Superconductor as Revealed by Rietveld Analysis of X-Ray Powder Diffraction Data

The crystal structure of a Ba-Y-Cu-O superconductor was refined by the Rietveld analysis of X-ray powder diffraction data. This material is orthorhombic with lattice parameters a = 3.8845 A, b = 3.8293 A, and c = 11.6926 A. Possible space groups are P222, Pmm2, and Pmmm. Metal atoms have essentially the same configuration as that in the perovskite-type structure. A structural model is proposed in which an oxygen atom at (0, 0, 1/2) is deficient.

Studies on the Thermal Behavior of Ba2YCu3O7-x by X-Ray Powder Diffraction Method

Thermal behavior of Ba2YCu3O7-x was investigated mainly by X-ray powder diffraction method between 70 K and 1173 K. Oxygen-rich(x~0.5) specimen is orthorhombic in symmetry in the temperature range between 70 K and 943 K. It undergoes a phase transition to tetragonal form at 943 K or 783 K in air or N2 gas, respectively. On the other hand, oxygen-poor specimen(x~1.0) semiconductive down to 4.2 k is tetragonal up to 1173 K in N2 gas. Thermal expansion coefficient of a cell parameter is about 1.1 × 10-5 K-1 and that of c cell parameter is 2.5 × 10,,5 K-1, in air.

Preparation and Properties of Single Crystals of the High-Tc Oxide Superconductor in the Bi-Sr-Ca-Cu-O System

Single crystals of the superconductor in the Bi-Sr-Ca-Cu-O system were prepared by the Bridgman method. Platy crystals annealed at various physical conditions were examined by electrical resistivity measurements and single crystal X-ray diffraction studies. The superconducting transition temperature (Tc) depended on the annealing temperatures and oxygen pressure. Corresponding to annealing conditions, single crystal X-ray diffraction patterns on (hk1)* Layer precession photographs systematically changed at room temperature. This may be related to the slight differences of the cell parameters of each specimen.

X-Ray Single Crystal Structure Analysis of Tetragonal Ba2YCu3-xO7-y, a High Temperature Form of the Superconductor

Crystal data: tetragonal, space group P4/mmm, a=3.8640(4), c=11.755(3) A, final R factor=0.036 for 161 independent observed reflections. Cu1 and two O1 form linear O-Cu-O dumbell (Cu-O1 1.808 A) with additional square planer O2 group (site occupancy=0.2) around Cu1. Cu2 is in square pyramidal coordination of oxygen composed of square-planer O3 group (Cu-O3 1.944 A) and an apical O1 (Cu-O1 2.434 A). The Y and Ba are in ordered arrangement along the c axis. This tetragonal phase is a high temperature form of the orthorhombic phase and the transition is related with ordering of the oxygen vacancy. Presence of strong metal-metal interactions were presumed because there exist Ba-Ba, Ba-Y and Ba-Cu distances significantly shorter than sum of their corresponding metallic radii.

Crystallographic parameters of atoms in the single crystals of the compounds RB6 (R=Y, La, Ce, Nd, Sm, Eu, Gd)

The lattice parameters, bond length between rare-earth metals R and boron atoms, intra- and inter-bond lengths between boron atoms and temperature parameters of constituent atoms in rare-earth hexaborides RB 6 (R=Y, La, Ce, Nd, Sm, Eu, Gd) were determined by analyzing integrated X-ray intensities with the use of ultra-short wavelength at room temperature. These parameters were related to the peculiar physical properties in the compounds. The Einstein temperature of rare-earth metals in the crystal structure of RB 6 (R=La, Ce, Nd, Sm, Eu, Gd) has also been determined by analyzing the temperature dependence of integrated X-ray intensities. Except for EuB 6 , the value is monotonously decreasing with increasing atomic number.

X-ray Fluorescence Analysis of Rare-Earth Atoms in Materials by Use of Ultrashort Wavelength X-rays.

A new technique of X-ray fluorescence analysis using ultrashort-wavelength X-rays was tested to obtain the K-series of spectra for rare-earth elements in two different kinds of materials, i.e. powder samples of rare-earth oxides and single crystals of rare-earth hexaborides. This technique utilizes a solid-state detector and multichannel pulse-height analyzer, and satisfies the requirements for quantitative identification in samples containing heavy elements. This technique was applied to determine the composition of the mixed compound La1-xCexB6 (x=0.25, 0.50 and 0.75). There is good agreement, within the limits of experimental error, between the values thus obtained and the nominal values.