Structural analysis of the charge transfer mechanism in the superconducting compounds Pb 2Sr 2Y 1−xCa xCu 3O 8+δ

Abstract

Pb 2Sr 2Ln 1−xCa xCu 3O 8+δ is one of the copper-based superconductors with high critical temperature. Undoped Pb 2Sr 2YCu 3O 8 is not a superconductor and becomes so when some of the trivalent Y cations are replaced by bivalent Ca cations. Microprobe analysis and magnetic measurements have been carried out on several single crystals. The x Ca values are found to be between 0 and 0.42, while T c varies between 0 and 80 K. Structural determinations by X-ray diffraction (6 crystals) show that the introduction of calcium reduces the stress between the CuO 2 and SrO layers. The Y substitution with Ca increases the valence of Cu(2) cations with no appreciable variation of the apical distance Cu(2)-O(1). The oxidation of substituted compounds is accompanied by the suppression of the superconducting state because of the movement of the O(1) atoms towards the Pb 4+ cations and away from the Cu(2) ones. This mechanism of charge transfer is very different from that of YBa 2Cu 3O 7−x.