Neutron powder diffraction study of the products from fluorinating YBa₂Cu₃O_7−s at room temperature and at 400 °C

Abstract

Fluorination of YBa₂Cu₃O_7−s at room temperature results in no change of the critical temperature but a drastic reduction of superconducting volume fraction as determined by magnetic flux exclusion measurements. Rietveld refinement of neutron powder diffraction data indicates that the bulk structure has not changed in any significant way despite the loss of superconductivity. In marked contrast, when the superconducting oxide is fluorinated at 400 °C, a structural transformation from orthorhombic to tetragonal symmetry occurs that is accompanied by essentially complete filling of eight of the nine perovskite anion sites and total loss of superconductivity above 7 K. Interpretation of neutron activation analysis and neutron diffraction results render the formula of the fluorinated product as YBa₂Cu₃F_3.5O_4.5, but fluoride ions in the unit cell were not distinguished from oxide ions.