Superconducting ceramics: preparation, XRD, SEM and EPR measurements to study the influence of compositional variations on superconductivity of La-(Ba/Sr/Ca)-CuO ceramic systems

A systematic substitution of strontium ions by barium and/or calcium ions in the 21-structure ternary oxide La_1.85Sr_0.15CuO_4-y, considered as the host system in this study, was carried out to produce four- and five-component metal oxides of the type La_1.85(Sr_0.15-xR_x) CuO_4-y, where R=Ba and/or Ca and x<0.15. A series of samples was prepared using an oxalate coprecipitation procedure where high-purity starting materials, usually required in solid-state pyrolysis reactions, were not required. The experimental conditions for co-precipitation, annealing and pulverization processes were optimized. The X-ray powder diffraction patterns were used to indicate when the materials would become superconducting after annealing. The transition temperatures, T_c, were measured from resistance versus temperature data which confirm that strontium is the best alkaline-earth metal among calcium, barium and strontium. Substitution or partial doping of strontium by calcium and/or barium in the host system decreased the T_c of these ceramics. SEM measurements were carried out to determine the grain size of these materials and characteristic electron paramagnetic resonance spectra for these materials at different temperatures are reported.