Correlations between crystallographic and physical properties in (Bi,Pb)2Sr2(Ca,Y)Cu2O8+Δ superconductors

Efforts were made to synthesize pure and single-phased samples of the (Bi, Pb)(2)Sr-2(Ca, Y)Cu2O8 (+ j) system. Systematic X-ray incommensurate Rietveld refinements along with AC susceptibility measurements wen performed. The basic yttrium substitution reveals strong internal structural changes on the modulation parameters. In the mean time, diamagnetic shielding in the samples vanishes before T-c, does, suggesting a 3D-2D crossover. It is proposed that Y3+ creates correlated domains in the CuO2 planes and localizes charges, thus diminishing the diamagnetic screening. Such a localization could be accompanied by the heavy structural changes observed. Adding lead in the structure would then restore the mobility of the charges. Indeed, without affecting T-c, (i.e. without affecting the carrier concentration), Pb substitution enhances diamagnetic shielding and even restores superconductivity in the otherwise insulating Bi2Sr2(Ca-0.4,Y-0.6)Cu2O8 (+ Delta) sample. We then conclude that carrier concentration is very little affected by cationic substitution (small T-c, variations) and that Y and Pb rather affect the mobility of the charges, destroying and restoring 3D correlations, respectively. (C) 1999 Elsevier Science B.V. All rights reserved.