Solid-state chemistry of high-temperature oxide superconductors: the experimental situation

High-temperature superconductivity in oxides of the type (La, Ln)2−xBax(Sr)xCuO4, Y(Ln)Ba2Cu3O7−δ, La3−xBa3+xCu6O14, and related systems is discussed with emphasis on aspects related to experimental solid-state chemistry. All of these oxides possess perovskite-related structures. Oxygen-excess and La-deficient La2CuO4 also exhibit superconductivity in the 20-40 K just as La2−xBax(Srx)CuO4; these oxides are orthorhombic in the superconductivity phase. The crucial role of oxygen stoichiometry in the superconductivity of YBa2Cu3O7−δ (Tc=95±5K) is examined; this oxide remains orthorhombic up to δ≈ 0.6 and becomes tetragonal and nonsuperconducting beyond this value of δ. Oxygen stoichiometry in this and related oxides has to be understood in terms of structure and disorder. The structure of La3−xBa3+xCu6O14 is related to that of YBa2Cu3O7, the orthorhombic structure manifesting itself when the population of O1 oxygens (along the Cu—O—Cu chains) is preponderant compared to that of O5 oxygens (along the a-axis); nearly equal populations of O1 and O5 sites give rise to the tetragonal structure. A transition from a high-Tc (95 K) superconductivity regime to a low-Tc (~60 K) regime occurs in YBa2Cu3O7−δ accompanying a change in δ. There is no evidence for Cu3+ in these nominally mixed valent copper oxides. Instead, holes are present on oxygens giving rise to O− or O2−2 species, the concentration of these species increasing with the lowering of temperature. Certain interesting aspects of the superconducting oxides such as domain or twin boundaries, Raman spectra, microwave absorption, and anomalous high-temperature resistivity drops are presented along with the important material parameters. Preparative aspects of the superconducting oxides are briefly discussed. Phase transitions seem to occur at Tc as well as at ~240 K in YBa2Cu3O7