CHARGE-TRANSFER IN YBA2CU3O6 DOPED WITH FLUORINE - INFRARED REFLECTANCE AND RAMAN-SCATTERING STUDIES

Charge transfer between the Cu-O chains and the Cu-O-2 planes has been studied by far infrared reflectivity (FIR) and Raman scattering (RS) measurements on YBa2Cu3O6Fy compounds with 0 less than or equal to y less than or equal to 1.46. Analysis of the reflectivity spectra by the four parameter semiquantum model shows the occurrence of the insulator/metallic transition for y = 0.76, in agreement with the de conductivity. The omega(LO)-omega(TO) frequency splittings related to the intralayer modes involving the atomic vibrations of the BaO and CuO2 planes decrease, whereas the omega(LO)-omega(TO) splittings related to the interlayer modes, involving the atomic vibrations of the Cu2-O4-Cu1 bonding, increase. We discuss three possible reasons for the change of the LO-TO splitting with increased fluorine doping: (i) charge transfer between ions which changes effective ionic charges; (ii) effect of lattice disorder induced by the fluorine, and (iii) the effect of the plasma in ceramics. We show that the charge transfer assumption is more than likely and is consistent with both FIR and RS investigations. Raman spectra exhibit frequency hardening of the O4-Cu2 modes and frequency softening of the O2-O3 modes with increased fluorine doping. The former points out the increase of the Coulomb interactions between ions within the interlayer bonds and the latter, their decrease and screening inside the CuO2 planes. The phonon line shape of the B-1g mode related to the CuO2 planes changes with the fluorine doping. The phonon line shape depends on the electronic polarization operator (EPO) which has been related to the free-carrier density of the CuO2 planes. The increase of the EPO points out the enhancement of the free-carrier density in the CuO2 planes. The charge transfer is revealed by the increase of the ionic Coulomb interactions within the Cu1-O4-Cu2 interlayer bonds, and their decrease and screening by the free carriers enhancement inside the CuO2 planes as fluorine doping increases.