Bi-2201 phases Synthesis, structures and superconducting properties

Abstract

The Bi 2+ x Sr 2− x CuO 6+° (Bi-2201) phases and La substituted Bi-2201 compounds with general formulas Bi 2Sr 2− x La x CuO 6+δ and Bi 2.3− x Sr 1.7La x CuO 6+δ were prepared and characterized by X-ray powder diffraction, electron diffraction, iodometric titration and resistance measurements. The monophasic samples of Bi 2+ x Sr 2− x CuO 6+δ were obtained in air at 850°C for 0.15 ≤ x ≤ 0.4, while synthesis at 740°C in oxygen flow extended this range to the stoichiometric cation composition (0 ≤ x ≤ 0.4). The superconducting transitions were not detected for these monophasic samples in contrast to the La substituted phases which exhibited superconductivity, and the highest T c,onset = 33 K was found for the Bi 2Sr 1.6La 0.4Cu 6.33 compound. Superconductivity in the Bi 2Sr 2− xLa x CuO 6+δ series exists in overdoped and underdoped regions. Incommensurately modulated structures of nonsuperconducting Bi 2.3Sr 1.7CuO 6.23 phase and superconducting Bi 2Sr 1.7La 0.3CuO 6.28 phase ( T c,onset = 26 K) were refined from X-ray powder data by a Rietveld technique using the four-dimensional space group P:A2/a:−11. The maximal Cu displacements from the average position in the first structure was found to be sufficiently larger than in the latter one. The distorted structural arrangement of the (CuO 2) layers in the Bi 2.3Sr 1.7CuO 6.23 structure can be a reason for the suppression of superconductivity in this phase, while their less corrugated configuration in the La containing Bi-2201 structure leads to the existence of superconductivity.