In order to elucidate the correlations between composition, structure and electrical properties of a high-Tc superconductor Ba-Y-Cu-O system, thermogravimetric measurement and chemical analysis of oxygen nonstoichiometry were made at a temperature range of 350 to 1000°C and under the oxygen partial pressure of 10-4 to 1 atm. Within a stability range of the composition Ba2YCu3O7-δ, the oxygen deficiency, δ, was found to vary approximately from zero to 0.9, with associated mean valence of copper ions varying from 2.33 to 1.73. At a possible phase boundary region of tetragonal to orthorhombic crystal structures, no sign of discontinuity in δ was observed, suggesting its phase transition was of higher than a first order.

https://iopscience.iop.org/article/10.1143/JJAP.26.L1228/pdf

Determination of Oxygen Nonstoichiometry in a High-Tc Superconductor Ba2YCu3O7-δ

Ba2YCu3O7-y (BYCO) thin films have been epitaxially grown on SrTiO3 substrates by magnetron sputtering. The films exhibit zero resistance at 84 K, with a transition width of 6 K. The resistivity along the basal plane is one order of magnitude smaller than that along the c-axis, and the transport property is more metallic in the former direction. Furthermore, the critical current density is two order of magnitudes larger in the former direction than in the latter direction. These results strongly suggest a quasi-two-dimensional electronic structure in BYCO. The maximum critical current at the liquid nitrogen temperature has reached 1.8×106 A/cm2.

Largely Anisotropic Superconducting Critical Current in Epitaxially Grown Ba2YCu3O7-y Thin Film

This paper reports on the dependence of the nonstoichiometry of (YBa{sub 2}Cu{sub 3}O{sub 7 {minus} x}) (solid), studied over 5 orders of magnitude in oxygen pressure and from 573 to 1173 K. Hydrogen-reduction methods for determining the absolute oxygen-to-metal ratio were developed. The resulting data were used to derive a chemical thermodynamic representation of the experimental variables. The data were also compared with several other investigations to identify the self-consistent sets of data. The present data and thermodynamic data from the literature were correlated on an Ellingham diagram.

Experimental and Thermodynamic Study of Nonstoichiometry in 〈YBa2Cu3O7‐x〉

We have established the low‐temperature process for the preparation of high Tc superconducting films by rf magnetron sputtering. The films were deposited at a temperature (650 °C) lower than the tetragonal‐orthorhombic transition point with sufficient crystallizing and oxidizing conditions. The as‐deposited Er‐Ba‐Cu‐O film on MgO exhibited a sharp superconductive transition with zero resistance at 86 K. This process prevented diffusion at the film and substrate interface and reduced the porous structure in the films.

Low‐temperature process for the preparation of high Tc superconducting thin films

InGaAs/GaAs strained layers superlattices have been grown by molecular beam epitaxy on GaAs. The best growth temperature was found to be 520–540 °C from photoluminescence measurements. Double x‐ray diffraction was performed. It shows very good agreement with the kinematical theory. This technique proves to be of particular interest for such structures: a single profile leads to reliable and complete information on layer thickness and InGaAs composition.

Growth conditions and characterization of InGaAs/GaAs strained layers superlattices

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.

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

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.

https://iopscience.iop.org/article/10.1143/JJAP.26.L869/pdf

Studies on the thermal behavior of Ba2YCu3O7−x by X-ray powder diffraction method

An X-ray study with CuKα was carried out on a fine-powder sample of GaAs. Relative intensities of 11 lines from the 111 to the 620 were measured and the scale factor was determined from absolute intensities of the 111 and the 220. Form the three-dimensional difference Fourier synthesis on the (110) plane, two sorts of peaks were found in Δ ρ ( x , y , z ). One lies near (1/8, 1/8, 1/8), which is the midpoint of chemical bond between Ga and As. This peak seems to correspond to a covalent bond. The other lies near (1/2, 1/2, 1/2), which is the position of anions in the NaCl structure with a cation at the origin. It may be possible that this latter peak corresponds to the ionicity of the bond.

Electron Distribution in GaAs as Revealed by the X-Ray Diffraction

When a specimen is rocked about the θ axis of an X-ray powder diffractometer with the detector fixed at a given Bragg peak 2θB, the resulting diffraction pattern reflects both the dimensional and the orientational distribution features of the crystallites in the specimen. For instance, reproducible fluctuations in the pattern can be attributed to coarse crystallites in the specimen, and the magnitude of the fluctuations corresponds to that of the peak intensity obtained by the θ-2θ scanning method, whereas a systematic deviation of the averaged pattern from that estimated from the absorption factor gives the angular dependence of the preferred orientation of the specimen directly. This method of evaluating the specimen using the X-ray powder diffractometer is reliable and convenient because of its applicability under the same experimental conditions as in the usual θ-2θ scanning method.

“ε-Scanning”–A Method of Evaluating the Dimensional and Orientational Distribution of Crystallites by X-Ray Powder Diffractometer

A new powder diffractometer for the Photon Factory was designed and constructed in 1986. Its performance has been confirmed as follows: (1) it is capable of determining the wavelength of the Cu K edge as 1.380619±0.000012 A using NBS standard Si powder 640b, while the reported one is 1.38059 A; (2) the accuracy of monitoring the incident beam intensity was ±0.2%, which was estimated by the normalized integrated intensity; (3) the angle resolution of diffraction lines was comparable with that of SSRL; and (4) the sum of three Gaussians was the best profile function fitted for lines from NBS standard Si powder 640b.

Ken Yukino

Papers

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

Studies on the thermal behavior of Ba2YCu3O7−x by X-ray powder diffraction method

Electron Distribution in GaAs as Revealed by the X-Ray Diffraction

“ε-Scanning”–A Method of Evaluating the Dimensional and Orientational Distribution of Crystallites by X-Ray Powder Diffractometer

New powder diffractometer for the Photon Factory