Effect of fluorination on the structure and superconducting properties of Y2Ba4Cu7O14+δ phases

Strongly reduced Y2Ba4Cu7O14.09 has been successfully fluorinated using XeF2 as a fluorination agent. The structure of the fluorinated material has been studied by a combination of X-ray diffraction, electron diffraction, high resolution electron microscopy (HREM) and X-ray microanalysis (EDX). For a high level of fluorination, the intercalation of fluorine is accompanied by a significant structural rearrangement and leads to an increase of T-c from 30 to 62 K. HREM reveals the presence of a partially fluorinated, well ordered Y2Ba4Cu7O14Fdelta phase with c = 51.96 Angstrom. Fluorine enters into anion-defi cient Cul layers, resulting in the formation of an octahedral arrangement of part of the Cul atoms with a significant elongation of the apical Cu-O distances and the c-parameter. Local ordering of fluorine atoms leads to the formation of defects with a limited extension where all Cul atoms have an octahedral coordination; the c-parameter in these areas is as high as 53.5 Angstrom. (C) 1998 Elsevier Science B.V. All rights reserved.