Abstract
The system Y 1− x Ca xBa 2Cu 3O y has been investigated by X-ray diffraction, magnetic susceptibility and conductivity measurements. Solid solutions of the YBa 2Cu 3O 7 type exist in the range of x=0−0.25, while for x > 0.25, further phases CuO, Ca 2CuO 3 and BaCuO 2 are formed. The single-phase samples x=0, 0.1 and 0.2 with oxygen stoichiometry close to y=7 exhibit a sharp superconducting transition at temperatures 94, 88 and 76 K, respectively. The density of the samples increases with the calcium content and long thermal treatment is required for a proper oxidation of samples with x > 0. For x=0.2 a two-step superconducting transition was observed on an oxygen-deficient, tetragonal sample with y∼6.2. The first step, leading to zero resistance, occurred 8 K above the one-step transition pertinent to the oxidized sample x=0.2.
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