Crystal growth and physical properties of oxide superconductor YBa2Cu3Ox are reviewed. Flake crystals, which are obtained by a solution growth technique where excess CuO is used as a solvent, display a low temperature superconducting transition below 60K. On the other hand, block crystals, which are prepared using a solid-liquid reaction at peritectic temperature Tp, show a sharp transition above 90K in the as-grown state. A clear anisotropy in the upper critical field is observed in the block crystals. Twinning structures are also discussed in terms of a strain releasing model in the tetragonal-orthorhombic transformation near 600°C.
Preparation and properties of superconducting oxide thin films are described. (La1-xSrx) 2 CuO4 (LSCO) and Ba2YCu3O7-σ (BYCO) can be epitaxially grown on (100) and (110) SrTiO3 single crystal substrates by magnetron sputtering method. Anisotropic properties of BYCO are measured by using thin films deposited on the (110) plates. The critical current density along the CuO layers has reached 1.8×106A/cm2 at 77.3k. This current density is adequate for device applications.
The structures of high-Tc superconducting oxides are classified into two groups: K2NiF4 and oxygen-deficient triperovskite types. The crystal chemistry of these two types of superconductors and related compounds is explained on the basis of coordination polyhedra and oxidation states of Cu, metal-oxygen bond lengths, and coordination numbers of metals.
High Tc superconducting oxide, Ba2YCu3Ov, and some related compounds are examined by means of high resolution transmission electron microscopy. Twin structure, planar defects, microdomain structure, out-of-phase domain structures etc are observed in these compounds. Structural changes, including orthorhombic-to-tetragonal phase transition, are observed when the specimen is heated by electron beam or by the heating-stage. Super-lattice reflections or diffuse scatterings are sometimes observed in the electron diffraction patterns of these materials.
The role of oxygen vacancies in superconducting properties of Ba2YCu3Oy (6.22<y<6.96) has been investigated by EXAFS and XANES on the Cu K-edge. Fourier transform analysis of EXAFS shows that oxygen vacancies influence the local structure through (1) the disappearance of O1-Cu1-O1 chain and displacement of atomic positions along the c-axis, (2) the Cu-O vibrational properties and (3) introducing structural disorder between the CuO2 planes. Cu XANES spectra show that Cu valency in Ba2YCu3Oy is essentially 2+ with some 3+ contribution. Oxygen vacancies create Cu1+for y<6.7 at which high Tc (90K) degrades to low Tc (50-60K) superconducting states.
The results of the neutron scattering experiments of three kinds of superconducting oxides, BaPb1-xBixO3, (La1-xSrx) 2CuO4, and YBa2Cu3Ox, are reviewed. The phonon structures of superconducting compositions are compared with those of nonsuperconducting ones. In each oxide system, the phonons in the high-energy region are softened and become broad in the superconducting materials in contrast with the nonsuperconducting materials. In particular, in BaPb1-xBixO3 case, the obtained spectrum is in good agreement with our previous FIR experimental results.
This paper describes the new techniques of measuring the Pendellosung beats using white radiation with the energy-dispersive diffraction method. Two techniques have been developed in the author's laboratory on the basis of the dynamical diffraction theory, in which the intensity of a Laue spot diffracted from a parallel-sided single crystal is successively measured at different Bragg angles, i.e., with different wavelengths. In the fast method, the integrated intensity diffracted from the whole exit area is measured and in the second case, the measurement is made only at the center of the Borrmann fan. A discussion is made on the advantage points as well as on the accuracy of these methods in the structure factor determination.
This article reviewes the section method which is convenient to understand all the quasiperiodic structures including the modulated and quasicrystal structures on the same basis. A model construction of an icosahedral Al - Mn quasicrystal is examplified by use of this method. This model explains the X -ray and neutorn powder diffraction patterns of an icosahedral A14Mn.