Actual fluorination of YBa 2 Cu 3 O x superconductors: Enhancement of superconducting properties and neutron diffraction studies

Samples of YBa 2 Cu 3 O x with x ∼ 6.7 and 7 were treated under diluted NF 3 . Two different evolutions in the superconducting behaviour are observed: at short time reaction, Tc increases progressively with fluorine concentration up to the limit of 90 K (samples with x ∼ 6.7) or stay at this limit (samples with x ∼ 7), while Meissner effect is complete. At longer time reaction, Tc varies no more and the amplitude of the Meissner effect decreases suddenly with the fluorine content. The different sites occupied by fluorine are determined by neutron diffraction, performed on two YBa 2 Cu 3 O 6.7 samples situated at both sides of this apparent limit. Fluorine localization is discussed in relation with the two different evolutions of the superconducting behaviour.