INDUCING SUPERCONDUCTIVITY AND STRUCTURAL TRANSFORMATIONS BY FLUORINATION OF REDUCED YBCO

Bulk superconductivity with T-c up to 94 K has been induced by fluorin ation of non-superconducting YBa2Cu3O6.11 using XeF2 as a fluorination agent. Strong changes on X-ray patterns were found after fluorination of reduced YBCO. High resolution electron microscopy of superconducti ng samples showed the presence of a new phase with c approximate to 13 Angstrom which exists as noticeable areas included within a matric of the 123 structure or which occurs as isolated defects with a limited extension. All fluorinated compounds exhibited a strong disorder along the c-direction resulting in a 'loss' of c-parameter on X-ray pattern s, The present results demonstrate that fluorine indeed enters the YBC O structure with a significant structural rearrangement for the high l evel of fluorination. The structure of the fully fluorinated YBa2Cu3O6 F2 phase, possibly responsible for superconductivity, has been deduced from high resolution electron microscopy. (C) 1997 Elsevier Science B .V.