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

Abstract

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.