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 l ess than or equal to 1.46. Analysis of the reflectivity spectra by the four parameter semiquantum model shows the occurrence of the insulato r/metallic transition for y = 0.76, in agreement with the de conductiv ity. The omega(LO)-omega(TO) frequency splittings related to the intra layer modes involving the atomic vibrations of the BaO and CuO2 planes decrease, whereas the omega(LO)-omega(TO) splittings related to the i nterlayer modes, involving the atomic vibrations of the Cu2-O4-Cu1 bon ding, increase. We discuss three possible reasons for the change of th e LO-TO splitting with increased fluorine doping: (i) charge transfer between ions which changes effective ionic charges; (ii) effect of lat tice disorder induced by the fluorine, and (iii) the effect of the pla sma in ceramics. We show that the charge transfer assumption is more t han likely and is consistent with both FIR and RS investigations. Rama n spectra exhibit frequency hardening of the O4-Cu2 modes and frequenc y softening of the O2-O3 modes with increased fluorine doping. The for mer points out the increase of the Coulomb interactions between ions w ithin the interlayer bonds and the latter, their decrease and screenin g inside the CuO2 planes. The phonon line shape of the B-1g mode relat ed to the CuO2 planes changes with the fluorine doping. The phonon lin e shape depends on the electronic polarization operator (EPO) which ha s been related to the free-carrier density of the CuO2 planes. The inc rease of the EPO points out the enhancement of the free-carrier densit y in the CuO2 planes. The charge transfer is revealed by the increase of the ionic Coulomb interactions within the Cu1-O4-Cu2 interlayer bon ds, and their decrease and screening by the free carriers enhancement inside the CuO2 planes as fluorine doping increases.