Effect of Variation in Composition on Electrical and Chemical Properties of YBa2Cu3O7-δ

In this study, several samples with slight deviations from the metal ion stoichiometry of YBa2Cu3O7-δ were synthesized at temperatures from 935° to 980°C. The extent of the single-phase region of the superconducting phase YBa2Cu3O7-δ, and the effects of variation in composition on the electrical and the chemical properties were investigated. X-ray diffraction studies of samples with up to 10 at% excess or 1 at% deficit of Cu, and 1.5 at% excess or 5 at% deficit of Ba showed only the YBa2Cu3O7-δ phase. The results of DTA measurements and resistivity measurements, however, demonstrated that solid solubility in YBa2Cu3O7-δ was very limited: less than 0.5 at% excess or deficit of Cu or Ba could be incorporated. Samples deviating from YBa2Cu3O7-δ stoichiometry differed sharply from stoichiometric material in their electrical behavior. Tailing or two-step superconductive transitions were found, as well as decreased Tcend. Composition variations affected sintering and grain growth. Sintered Cu-poor pellets were porous, having densities 60% or less of theoretical. For Cu-rich and Y-rich-Ba-poor pellets, densities about 90% of theoretical were obtained, and extensive grain growth was observed.