Katsuyoshi Oh-ishi

X-Ray and electron microscopic study of a high temperature superconductor Y0.4Ba0.6CuO2.22

A superconducting oxide Y0.4Ba0.6CuO2.22 with Tc higher than 88 K was synthesized. The crystal structure was found to be a derivative of the perovskite structure with orthorhombic unit cell dimensions of a=3.818 (1) A, b=3.888 (1) A, c=11.667 (4) A, and V=173.19 (8) A3. Basic unit cell of the cubic perovskite structure was tripled along the c axis, probably due to the ordering of Y3+ and Ba2+ ions and/or of oxygen vacancy. Apparent short bond length for Cu-O may partly be explained by oxygen vacancy amounting to a quarter, but the observed high Tc suggests actual contraction of Cu-O bonds.

Electrochemical Mg2+ intercalation into a bimetallic CuFe Prussian blue analog in aqueous electrolytes

Mg2+ intercalation/deintercalation is achieved by using aqueous electrolytes and Prussian blue analog electrodes. Ex situ X-ray diffraction evidenced the solid solution process of Mg2+ intercalation/deintercalation, while the 57Fe Mossbauer spectroscopy and X-ray absorption near edge structure revealed redox of both Cu and Fe.

Neutron diffraction study on orthorhombic YBa2Cu3O6•74 and tetragonal YBa2Cu3O6•05

The crystal structures of high Tc (~90 K) orthorhombic YBa2Cu3O6.74 and moderate Tc (~50 K tetragonal YBa2Cu3O6.05 have been studied by the conventional neutron powder diffraction technique. Single phase powder samples were used. In the orthorhombic phase, the occupancies of the O1(0 1/2 0) and the O4 sites (1/2 0 z), z=0.380(2) are found to be respectively 0.91(3) and 0.92(2). In the tetragonal phase, the occupancies of the O1(0 1/2 0) and the O3 sites (0 1/2 z), z=0.383(2) are 0.20(4) and 0.91(2). Some disordering of Ba and Y atoms is suggested in the tetragonal phase.

Valence-band and Cu 2p core-level electronic structures of Nd2−xCexCuO4 studied by X-ray photoemission

Abstract We have measured photoemission spectra for Nd 2−x Ce x CuO 4 with and without reduction in order to elucidate the valence-band and Cu 2p core-level electronic structures. We find a density of states at the Fermi level formed by the electrons produced by doping the system with Ce ions. The character of the states is discussed in terms of the resonances and the photon energy dependence. It is suggested that Cu ions doped by electrons have a 3 d 10 configuration since the intensity ratio of the Cu 2p satellites to the Cu 2p 3/2 line decreases as a function of x.

Dependence of lattice parameters and Tc on the hole concentration determined by precise measurement of the oxygen content of MxLa2-xCuO4-y (M = Ba, Sr, Ca)

Abstract Systematic precise measurements of oxygen content have been performed on sintered samples of MxLa2−xCuO4−y (M = Ba, Sr, Ca) superconducting oxides by both thermogravimetry and iodometry. The hole concentration, P, has been estimated by using the obtained oxygen content on the basis of charge neutrality. Lattice parameters and superconductivity of the samples have been measured by X-ray powder diffraction analysis and electrical resistivity or ac magnetic susceptibility measurement, respectively. The lattice parameter, a, depended on both P and the average ionic radius of the La3+ site and c depended on the average ionic radius of the La3+ site, volume compensation, and oxygen defect. Orthorhombicity (defined by 2(b − a) (a + b) ) depended on P. Superconductivity appeared at the same hole concentration, P = 0.10, regardless of the kind of alkaline-earth metal used. A linear relation between Tc and CuOI bond length within the basal plane in the structure was recognized only in the case where the hole quantity, P, was constant.

Crystal Structure, Oxygen Content and Superconductivity of MxLa2-xCuO4-y (M=Ba, Sr and Ca)

Unit cell dimensions, oxygen vacancy y, and superconducting properties of MxLa2-xCuO4-y (M=Ba, Sr and Ca) with K2NiF4 structure have been determined by means of X-ray powder diffraction, thermogravimetry, and electrical resistivity measurements. The existence percentage of Cu as Cu3+, P=(Cu3+/Cu)%, was estimated from the measured y values on the basis of charge neutrality. The superconducting critical temperature, Tc, is discussed in terms of the P value and Cu-O bond length within the basal plane. Composition dependence of the unit cell dimensions is discussed by taking the P value and the ionic radii into consideration.

139La Pure Quadrupole Resonance of High Tc Superconducting Materials (La1-xMx)2CuO4 (M=Ba, Sr)

Pure Quadrupole Resonance (PQR) of 139La in high Tc superconductor, (La1-xMx)2CuO4 with M=Ba, Sr, has been observed at zero external field between 1.3 K and 4.2 K. In undoped La2CuO4, the PQR spectrum is very complicated, reflecting the orthorhombic structure, while in the superconducting compounds, the spectrum exhibits a drastic change and becomes more simple due to the tetragonal structure. The alloying is suggested to be responsible not only for the change of crystal structure but also for the change of microscopic electronic state.

Thermal and x-ray analyses of high temperature superconductor YBa2Cu3O6.74

In situ observation of oxygen absorption and desorption during heating and cooling cycle with TG and DTA was done to determine the optimum preparation conditions of high Tc Y-Ba-Cu-O compounds. We suggest the best recipe in air is to fire the raw mixture of 1/2Y2O3, 2BaCO3 and 3CuO at 930°C for more than 10 h, followed by slow cooling in the furnace to take as much oxygen as possible within the specimen. With this method single phase material of an orthorhombic superconducting oxide YBa2Cu3O6.74 with Tc=91.3 K was synthesized, with orthorhombic unit cell dimensions of a=3.8180(6) A, b=3.8888(7) A, and c=11.668(2) A.

Synthesis of the B1-type tantalum nitride by shock compression

B1-type tantalum nitride (polycrystalline bulk state) was synthesized by shock compression. Shock-compression recovery experiments of the porous sample of tantalum nitride with a hexagonal structure (CoSn-type) were performed in the impact-velocity range up to 1.5kms−1. The instrumental chemical analysis, X-ray and electron diffraction experiments were carried out on the recovered tantalum nitride sample and the combustion-synthesized B1-type sample for comparison. The recovery rate of the B1 -type phase increased with increasing porosity, impact velocity and impactor thickness; among these above all the effect of porosity was most remarkable. Almost 100% recovery rate was achieved for the powder of 70% porosity impacted by a 2 mm thick tungsten plate with velocities above 1.4 kms−1. It was confirmed by chemical analysis that the nitrogen content did not change by shock compression. The chemical formulae of the shock-synthesized and combustioned-synthesized B1-type phases were estimated to be TaN0.96–0.99 and TaN1.13–1.16, 1.27–1.30, respectively. The lattice parameter of the shock-synthesized B1 -type (Fm3m) phase was ao=0.43363- + (1) nm. The results of X-ray and electron diffraction experiments also supported that the shock-synthesized B1-type tantalum nitride has a good stoichiometry compared with the combustion-synthesized ones.

Direct Observation of Atomic Arrangement of High-Tc Superconductor YBa2Cu3O6.74 by High-Resolution Electron Microscopy

The crystal structure of YBa2Cu3O6.74 superconducting oxide with a transition temperature higher than liquid nitrogen temperature has been studied by high-resolution electron microscopy. The observed high-resolution micrograph is consistent with the oxygen-deficient perovskite structure model proposed in previous work with neutron and X-ray diffraction. In the image, Y, Ba and Cu atoms are represented as dark spots with different contrast depending on their atomic numbers. Unoccupied oxygen atom positions are distinguished from occupied oxygen atom positions as brighter regions.