Correlations between crystallographic and physical properties in (Bi,Pb)(2)Sr-2(Ca,Y)Cu2O8+Delta 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 Rietve ld refinements along with AC susceptibility measurements wen performed. The basic yttrium substitution reveals strong internal structural changes on t he modulation parameters. In the mean time, diamagnetic shielding in the sa mples vanishes before T-c, does, suggesting a 3D-2D crossover. It is propos ed that Y3+ creates correlated domains in the CuO2 planes and localizes cha rges, thus diminishing the diamagnetic screening. Such a localization could be accompanied by the heavy structural changes observed. Adding lead in th e structure would then restore the mobility of the charges. Indeed, without affecting T-c, (i.e. without affecting the carrier concentration), Pb subs titution 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 cation ic substitution (small T-c, variations) and that Y and Pb rather affect the mobility of the charges, destroying and restoring 3D correlations, respect ively. (C) 1999 Elsevier Science B.V. All rights reserved.