Fluorinated RBa2Cu3O6+δ (R = Dy, Ho, Tm) phases:: Synthesis and structure

Strongly reduced RBa2Cu3O6.12 (R = Dy, Ho, Tm) phases are fluorinated using XeF2 as the fluorinating agent. The fluorinated compounds are structurally studied by X-ray powder diffraction, electron diffraction, and high-resolution electron microscopy. Fluorine is incorporated into anion-deficient layers Cu(1), forming an octahedral surrounding around some copper atoms. The consequences are an appreciable lengthening of the apical distances Cu-O and an increase in the unit cell parameter c. Because of the inhomogeneous incorporation of fluorine, regions with completely occupied anionic positions in the Cu(1) layer exist only as local defects for which c reaches a value of 12.9 Angstrom. The fluorination basically occurs by the anion-exchange mechanism, which does not allow the copper to acquire the oxidation number necessary for superconductivity to appear.