Phonon Raman scattering and structure of Co substituted YBa2Cu3O6+δ

Raman and x-ray diffraction measurements have been performed on Co substituted YBa₂Cu_3-xCo_xO_6+δ. The x-ray diffraction data show that the structure transforms from orthorhombic to tetragonal at x∼0.13. The features in the Raman spectra change smoothly throughout the investigated concentration range without any observable abrupt changes at the phase transition. The observed changes in the Raman spectra for increasing Co concentration are reminiscent of those observed for pure YBCO samples of increasing oxygen deficiency. In particular, the frequency of the O(4) stretching vibration is observed to be similarly correlated to T_c for both Co substituted and oxygen deficient YBCO. It is suggested that the O(4) stretching vibration is sensitive to small changes of the charge balance between the CuO chains and the CuO₂ planes.