Oxygen-sublattice ordering and intercalation mechanism of chlorine in YBa2Cu3O6+ delta.

We have investigated the modifications occurring in the oxygen sublattice and in the phonon spectra of chlorinated ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathrm{\ensuremath{\delta}}}$ (YBCO) compounds. Susceptibility measurements suggest that the diffusion of chlorine is different in oxygen-deficient and in oxygen-rich materials. We find occurrence of superconductivity after chlorine treatment at 200 \ifmmode^\circ\else\textdegree\fi{}C in an initially nonsuperconducting reduced sample. X-ray photoelectron spectra and x-ray-diffraction patterns of this sample show that the oxidation number of chlorine is between -1 and 0 in the lattice. To explain this result we assume that when an oxygen-deficient sample of approximate composition ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.20}$ is substantially chlorinated, the chlorine enters the structure at vacancies of BaO planes and promotes the diffusion of oxygen towards empty O(1) sites of copper planes. This conclusion is supported by a comparison of vibrational results from chlorinated YBCO with those recently obtained on YBCO pristine thin films presenting oxygen-sublattice disorder and by the different magnetic behavior of chlorinated reduced and oxidized samples. A Raman resonant feature is found at 72 meV (580 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) and is ascribed to defect-induced modes of the oxygen sublattice rather than to electronic excitations. Indeed at 10 K, the line shapes of this band are clearly asymmetric and can be fitted using different Fano functions indicating phononic interaction with an electronic spectrum. The unusual broadening of this line on going from normal to superconducting state suggests the presence of charge-transfer excitons traveling from ${\mathrm{CuO}}_{2}$ planes to CuO chains.