123, 247, and 124 cuprate superconductors: Investigations of thermodynamic stabilities, defect structures, and intergrowths

Careful investigations employing nonresonant microwave or rf absorption, Cu2+ EPR spectra, X-ray diffraction, and electron microscopy show that orthorhombic YBa2Cu3O7−δ is thermodynamically stable and monophasic when δ is 0.0–0.2 (T c = 90 K), 0.25 (T c = 80 K), and 0.5 (T c = 45 K). The last two compositions are associated with ordered oxygen-vacancy structures; in the δ = 0.0–0.2 regime, 3b = c. Compositions in the range δ = 0.3–0.4 (T c = 60 K) do not appear to be thermodynamically stable and decompose on annealing, suggesting thereby that the so-called 60 K superconducting phase in the YBa2Cu3O7−δ system may not be genuine. 123 cuprates prepared with excess CuO often show fringes of 124 in lattice images. Both 124 and 247 cuprates prepared by the ceramic method in a flowing oxygen atmosphere frequently show intergrowths of each other or with 123. Such epitaxial relationships between 123, 124, and 247 cuprates could be of significance with respect to their superconducting properties. Both 124 and 247 cuprates undergo thermal decomposition to give 123 and CuO.