Yoshishige Uchida

Rietveld Refinement of the Structure of Ba2YCu3O7-x with Neutron Powder Diffraction Data

The crystal structure of a high Tc superconductor, Ba2YCu3O7-x, has been refined by the Rietveld analysis of TOF neutron powder diffraction data. Two polymorphs of Ba2YCu3O7-x are contained in the sample: orthorhombic (74%) and tetragonal (26%) forms. The two forms have nearly the same structure closely related to that of perovskite-type compounds. In the orthorhombic form, the occupation factor of an oxygen atom on the a axis differed greatly from that on the b axis, which is the main reason why Ba2YCu3O7-x crystallizes in the orthorhombic form.

High Tc Superconductor YBa2Cu3Oy–Oxygen Content vs Tc Relation–

High Tc superconductor YBa2Cu3Oy, having various oxygen contents have been examined by a.c. susceptibility, d.c. conductivity and Raman spectrum measurements. The Tc is greatly affected by the oxygen content, being lowered by decreasing oxygen content. Particularly, the Tc drops markedly when the compound takes a tetragonal form. To confirm the y vs temperature relation, the compound was reexamined by thermogravimetry. The new results cast doubt on our previous y value (6.5>y>5.8).

Superconductivity of YBa2Cu3-xMxOy (M = Co, Fe, Ni, Zn)

The superconductivity of YBa2Cu3-xMxOy, (M = Co, Fe, Ni, Zn; x = 0.1-0.4) was examined by means of ac susceptibility measurements. The results indicated that Cu in the Cu2 site (between the Y and the BaO layer) plays a more substantial role in superconductivity than that in the Cu1 site (between the BaO layers). Combining this with our earlier results for the La1+xBa2-xCu3Oy system, we suggest that holes doped mainly in the O 2p orbitals of the rock-salt-type BaO layers form Cooper pairs through some medium (spin fluctuations or possibly other collective modes) excited in the Cu2 layer.

Thermogravimetric and High-Temperature X-Ray Studies on the Orthorhombic-to-Tetragonal Transition of YBa2Cu3Oy

Thermogravimetric experiment was performed to determine oxygen content in the high Tc superconductor YBa2Cu3Oy. The y varies from 5.8 to 6.5 depending on temperature and oxygen fugacity, indicating that the average valence of copper ions is between 2 and 1. The oxygen content is strongly affected by quenching process of a sample. High-temperature X-ray diffraction study showed orthorhombic-to-tetragonal transition (second or possibly higher order) at around 900 K. Combining this fact with the thermogravimetric data, we concluded that the compound having y less than ~6.0 has a tetragonal symmetry.

Identification of the Superconducting Phase in the Nd–Ce–Sr–Cu–O System

Phase search experiments were performed to identify the superconducting phase in the Nd-Ce-Sr-Cu-O system. A single phase superconductor was obtained form the nominal composition (Nd0.66Ce0.135Sr0.205)2CuOy. It has a tetragonal cell with a=3.8558(1) and c=12.487(1) A. From powder X-ray diffraction and electron microscopic observations, it was confirmed that the metal positions in this phase are identical to those in the K2NiF4 structure but with cationic order of Nd(Ce) and Sr. Three structure models including oxygen positions are proposed and discussed. Although we could not be absolutely conclusive, the structure which consists of alternative stacking of K2NiF4- and Nd2CuO4-type slabs seems to be most likely.

On the 110 K Superconductor in the Bi-Ca-Sr-Cu-O System

We found a new compound in the Bi-Ca-Sr-Cu-O system which has a different structure from the 80 K superconductor Bi2(Ca, Sr)3Cu2Oy. The c-dimension of this compound is ~37 A and is ~6 A longer than that of Bi2(Ca, Sr)3Cu2Oy. Although we have not obtained a single-phased sample yet, the clear correlation between the diamagnetic susceptibility just below 110 K and the content of this compound included in the sample strongly suggests that the compound with c≈37 A is responsible for the superconductivity of Tc~110 K.

Crystal Structure of the Orthorhombic Form of Ba2YCu3O7-x at 42 K

The crystal structure of the low-temperature form of Ba2YCu3O7-x at 42 K was refined by the Rietveld analysis of TOF neutron powder diffraction data. This material is orthorhombic with space group Pmmm and lattice constants a=3.8800 A, b=3.8122 A, and c=11.6264 A. It has an oxygen-deficient triperovskite structure in which oxygen atomson a z=1/2 plane are lacking completely, two oxygen sites on a z=0 plane are partially occupied, and Ba2+ and Y3+ ions are ordered in a [Ba-Y-Ba]∞ sequence along the c axis. The occupation factors of oxygen at (1/2, 0, 0) and (0, 1/2, 0) are 0.84 and 0.05, respectively.

Preparation of a New Superconductor Sr2Y0.7Ca0.3Cu2.35Pb0.65Oy

The superconducting oxide in the Sr-Ca-Y-Pb-Cu-O system was identified and prepared as a single phase. The compound examined has a tetragonal cell with a=3.815 A and c=11.85 A. The Tc was 17 K for the sample quenched from 860°C to 77 K, and below 4.2 K for the sample annealed in air at 400°C.

Crystal Structure of the Tetragonal Form of Ba2YCu3O7-x

The crystal structure of the high-temperature form of Ba2YCu3O7-x was refined by the Rietveld analysis of TOF neutron powder diffraction data taken at 49 K. This compound is tetragonal with lattice constants a = 3.8533 A and c = 11.7631 A. It has an oxygen-deficient triperovskite structure, in which oxygen atoms on a z = 1/2 plane are missing, an oxygen site at (0, 1/2, 0) is about 9% occupied, and Ba2+ and Y3+ ions are completely ordered in a [Ba-Y-Ba]∞, sequence along the c axis.

Effects of Mn ions on the piezoelectric property of (Pb, La)(Zr, Ti)O3

The effects of sintering temperature and manganese concentration on microstructure and electromechanical property were studied in [Pb0.9La0.1][Zr0.5Ti0.5]O3 piezoelectric ceramics with the substitution of the B-site by manganese by means of scanning electron microscope (SEM) and electron spin resonance (ESR). Grain growth is enhanced by increasing temperature but suppressed by manganese substitution. There was no relationship between Qm (inverse of mechanical vibration loss) and the grain size. Some part of manganese is considered to be reduced to Mn2+ ions during sintering. Qm is considered to be affected by the reduction of manganese ions because it was proportional to Mn2+ concentration, not to total manganese content.