RAMAN-SCATTERING AND X-RAY-DIFFRACTION STUDY IN LAYERED CUPRATES

Raman scattering and powder X-ray diffraction in the parent compounds of T, T-phase and (Ca0.86Sr0.14)CuO2 were measured to examine the two magnon (omega2m) and the highest phonon energy (omega2p) dependences on Cu-O bond length (d), and to determine the structural parameters of the T'-phase. We found that both omega2m and omega2p exhibit strong power-law dependence on d, omega2m is-proportional-to d-4.6, and omega2p is-proportional-to d-6.3, in the T'-phase except for Gd2CuO4, and both energies for Gd2CuO4 deviate from the power-law relation. The strong d dependence of omega2p suggests that this phonon is strongly coupled with charge-transfer electronic state. The observed phonon is assigned to two-phonon scattering of the bond-stretching mode of Cu and O atoms in the basal plane. The depression Of omega2p for Gd2CuO4 implies a structural change, such as oxygen distortion in the CuO2 plane, which influences the charge-transfer energy gap J and omega2m. The DELTA values were estimated from the Madelung site potentials V(i), which were calculated based on the crystal structural parameters determined by Rietveld analysis. Cu 3d-O 2p transfer energy t(pd), and on-site Coulomb repulsive potentials, U(d) and U(p), were determined using a one-dimensional Cu2O cluster model by fitting omega2m as a function of d. Finally, we discuss the relationship between crystal structure and carrier doping. The result of DELTAV:(V(R) - V(Cu)) dependence on d suggests that the carrier concentration is not constant in T'-phase and change in the carrier concentration influences T(c) in these materials.