TEMPERATURE DEPENDENCE OF EXTENDED X-RAY ABSORPTION FINE STRUCTURE IN THE HIGH-T//cSUPERCONDUCTING SYSTEM Bi-Ca-Sr-Cu-O.

Measurements of extended X-ray absorption fine structure (EXAFS) have been performed at the Cu K-edge between 20 and 300 K in a high-T//c pseudo-tetragonal phase of the Bi-Ca-Sr-Cu-O system with nominal composition Bi//4Ca//3Sr//3Cu//4O//x, which shows a superconductivity onset at about 82 K. The temperature dependence of the EXAFS disorder parameter sigma **2 indicates strong in-plane Cu-O bonds and a weaker axial Cu-O bond. The usual Einstein-like behavior of sigma **2 is found for temperatures above 100 K, whereas in the superconducting region the sigma **2 of both in-plane and out-of-plane Cu-O pairs seem to indicate some structural disorder related to the Cu-O atoms. This fact points out the unique role of the copper-oxygen local structure in the mechanism of superconductivity.