CAUSES OF MODULATION AND HOLE CONDUCTIVITY OF THE HIGH-T(C) SUPERCONDUCTOR BI2SR2CACU2O8+X ACCORDING TO X-RAY SINGLE-CRYSTAL DATA

The one-dimensionally modulated structure of the high-T(c) superconductor Bi2Sr2CaCu2O8+x with a0 = 5.407(2), b0 = 5.412(3), c0 = 30.771(8) angstrom, q = 0.210(2)b0* has been analysed in the commensurate approximation using the conventional three-dimensional space group Pnnn and utilizing single-crystal x-ray diffraction data. A reliable model (R = 0.07, R(w) = 0.079 for 1011 unique reflections) has been obtained. The extra oxygen atoms have been found inside and between the Bi-O-Bi chains of the BiO layer. The modulation displacement functions of the atoms have been obtained by means of harmonic analysis of the modulation displacements. It has been shown that the topology of Bi-O-Bi chains, the modulation of the structure and the form of the modulation displacement functions can be explained by the shifts of the oxygen atoms in the BiO layer correlated according to the antiferroelectrical law. The cationic vacancies in the Sr sites provide the hole conductivity in the copper-oxygen plane and lead to the formation of the percolation picture in this plane.