Several methods of chemical analysis, for example various redox titrations and thermogravimetric approaches, have been adapted to establish the oxygen content in superconducting cuprates and to determine the higher oxidation states of copper and other mixed-valent metals. The present review discusses and complements the authors' own experimental results on the oxygen content and metal valence analyses, but literature data from about 150 other references are reviewed as well.

https://iopscience.iop.org/article/10.1088/0953-2048/9/3/001/pdf

Determination of oxygen content and metal valences in oxide superconductors by chemical methods

Defect Chemistry of La2−xSrxCuO4−δ: Oxygen Nonstoichiometry and Thermodynamic Stability

Metal oxides are widely used in devices such as sensors, fuel cells, and oxygen permeation membranes. Understanding the oxide ion migration mechanism would provide fundamental insights into the relationships between the structure and properties such as ionic conductivity. The Ruddlesden-Popper perovskite (Sr,La)n+1(Fe,Co)nO3n+1 (n = 2) has characteristic oxygen permeation and ion conduction properties, resulting from the layered perovskite structure. To elucidate the ion migration mechanism in Sr2.46La0.54Fe2O7-δ (SLF) we used a combination of experimental techniques [X-ray powder diffraction (XRPD) and enthalpy investigations of the oxygen vacancy formation reaction] and computational techniques [the bond valence sum (BVS) approach and ab initio density functional theory (DFT)]. The structural analyses of SLF by XRPD and DFT agreed well. They showed that the oxygen vacancies in SLF are located at the O1 oxygen site, which is on the vertex shared by two FeO6 octahedra in the perovskite layer. Enthalpy of the oxygen vacancy formation changed at 830 °C. This is similar to the ionic conduction behavior reported for Sr3Fe2O7. The XRPD study indicates that the host structural framework did not change with temperature, while the oxygen/vacancy arrangement in SLF did change at 830 °C. The BVS and DFT studies suggested a change in the ion migration pathway, in which the ion migration through O1 sites becomes more important at temperatures higher than 830 °C.

Oxygen vacancy formation and the ion migration mechanism in layered perovskite (Sr,La)3Fe2O7−δ

Polarization and stability of La2NiO4+δ in comparison with La0.6Sr0.4Co0.2Fe0.8O3−δ as air electrode of solid oxide electrolysis cell

Hierarchically nanoporous materials based on layered perovskite oxides La1.7Ca0.3NixCu1−xO4−δ (x = 0, 0.25, 0.50 or 0.75) have been synthesized by a facile citrate-modified evaporation-induced self-assembly (EISA) method. These La1.7Ca0.3NixCu1−xO4−δ oxides have been evaluated as potential cathodes for intermediate-temperature solid oxide fuel cells (IT-SOFCs) with Ni–YSZ cermet supported type cells. It was found that La1.7Ca0.3CuO4−δ cathode exhibits the maximum power density at high temperature (e.g., 1.5 W cm−2 at 850 °C), while La1.7Ca0.3Ni0.75Cu0.25O4−δ cathode shows the highest power density at intermediate temperature (e.g. 0.71 W cm−2 at 750 °C) using humidified H2 and air as the fuel and oxidant, respectively. The electrochemical performance of single cells with La1.7Ca0.3Ni0.75Cu0.25O4−δ cathode materials with different morphologies demonstrated better performance in the intermediate temperature range when using the cathode prepared by the citrate-modified EISA method, which has a bigger grain size, but with higher surface area and pore volumes.

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Hierarchically nanoporous La1.7Ca0.3CuO4−δ and La1.7Ca0.3NixCu1−xO4−δ (0.25 ≤ x ≤ 0.75) as potential cathode materials for IT-SOFCs

The oxygen deficit δ of (La1-xSrx)2CuO4-δ was determined as a function of oxygen partial pressure(PO2 ) at various temperatures. When the δ-log PO2 curve at a specified temperature was chosen as the reference and the curves for other temperatures were shifted along the abscissa, all the data points were found to form a single smooth curve (master curve). A theoretical equation providing δ as functions of PO2 and strontium content was derived by combining defect equilibrium, mass action law and site conservation condition. The calculated curve was found to be in good agreement with the master curve. The relationship between copper valence and strontium content at constant oxygen pressure and temperature was determined experimentally and compared with calculated values. Good agreement was found between them.

Defect thermodynamics of (La1-xSrx)2CuO4-δ superconducting oxide

High temperature electrical properties and defect chemistry of La2-xCaxCuO4-y superconductors—II. Defect structure modeling

The single crystal was grown by the TSFZ method, and the in-plane and c -axis conductivities were determined as functions of temperature and oxygen partial pressure. Arrhenius plots for in-plane and c -axis conductivity in the intrinsic region fall on straight lines. The in-plane mobility calculated by the combination of conductivity with oxygen nonstoichiometry was nearly independent of temperature and was regarded as 1.33 cm 2 V −1 s −1 . Assuming the temperature independence of the mobility, the CT gap was determined from the slope of the Arrhenius plot for conductivity. The CT gaps in the ab -plane and along the c -axis were found to be 1.21 and 1.22 eV, respectively. It was concluded that the same band contributes to conduction along the c -axis and in the ab -plane.

Kazuo Fueki

Papers

Defect thermodynamics of (La1-xSrx)2CuO4-δ superconducting oxide

High temperature electrical properties and defect chemistry of La2-xCaxCuO4-y superconductors—II. Defect structure modeling

Determination of the CT gap of a Nd2CuO4-δ single crystal by high-temperature conductivity measurement