OXYGEN ORDERING IN YBA2CU3OZ (7-GREATER-THAN-OR-EQUAL-TO-Z-GREATER-THAN-6) SUPERCONDUCTORS WITH EQUILIBRIUM DEFECT STRUCTURE AT ROOM-TEMPERATURE

Oxygen ordering has been studied in YBa2Cu3Oz (7 greater-than-or-equal-to z > 6.2) superconductors. By analysing resistive relaxation curves at constant temperature, especially room temperature, and at constant composition z, it has been shown that thermodynamic equilibrium regarding the rearrangement of oxygen in the CuO planes of the material can be obtained at room temperature by annealing over a period of ca. 1 week. Moreover, samples of the same composition but prepared by quenching or by slow cooling at constant z, exhibit after room-temperature annealing the same final state, as determined by their superconducting transition critical temperature. Oxygen ordering in such specimens obtained at equilibrium at room temperature has been studied by electron diffraction. The results of these investigations suggest that in the composition range 6.4 < z less-than-or-equal-to 7 only the two orthorhombic varieties OI and OII, whose perfect structures correspond to ideal compositions z=7 and z=6.5, respectively, exist as thermodynamically stable phases.