OXYGEN-SUBLATTICE ORDERING AND INTERCALATION MECHANISM OF CHLORINE INYBA2CU3O6+DELTA

We have investigated the modifications occurring in the oxygen sublatt ice and in the phonon spectra of chlorinated YBa2Cu3O6+delta (YBCO) co mpounds. Susceptibility measurements suggest that the diffusion of chl orine is different in oxygen-deficient and in oxygen-rich materials. W e find occurrence of superconductivity after chlorine treatment at 200 -degrees-C in an initially nonsuperconducting reduced sample. X-ray ph otoelectron spectra and x-ray-diffraction patterns of this sample show that the oxidation number of chlorine is between -1 and 0 in the latt ice. To explain this result we assume that when an oxygen-deficient sa mple of approximate composition YBa2Cu3O6.20 is substantially chlorina ted, the chlorine enters the structure at vacancies of BaO planes and promotes the diffusion of oxygen towards empty O(1) sites of copper pl anes. This conclusion is supported by a comparison of vibrational resu lts from chlorinated YBCO with those recently obtained on YBCO pristin e thin films presenting oxygen-sublattice disorder and by the differen t magnetic behavior of chlorinated reduced and oxidized samples. A Ram an resonant feature is found at 72 meV (580 cm-1) and is ascribed to d efect-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 indicatin g phononic interaction with an electronic spectrum. The unusual broade ning of this line on going from normal to superconducting state sugges ts the presence of charge-transfer excitons traveling from CuO2 planes to CuO chains.