PHONON CHARACTERIZATION OF YBA2CU3O7-DELTA DOPED WITH AL AND GA BY RAMAN-SPECTROSCOPY

We have tried to partly substitute Al and Ga for Cu in YBa2Cu3O7-d (YBCO). Raman scattering measurements show that, on the one hand, Al is completely introduced in YBCO structure up to the maximum concentration of Al (10 %) studied, on the other hand, only minor contents of Ga (<1.7 %) enter the YBCO phase. The main impurity, which is present in the Ga-substituted YBCO, is identified to be Y4Ba10 CuGa5Oy by Raman spectroscopy. The impurity is very hard to detect with x-ray diffraction probably due to its amorphous character and this can explain why the phase could not be detected in previous reports. The observed changes in the Raman spectra for increasing Al concentration are primarily reminiscent of those observed for Co-substituted and O deficient YBCO samples. Specifically the frequency of the 0(4) axial stretching vibration is found to be similarly correlated to Tc for Al-doping, Co-doping and 0 deficiency. It is argued that the decrease of Tc for increasing Al concentration is caused by a charge transfer mechanism between the CuO chains and the CuO2 planes similar to those in case of Co substitution on the Cu(1) site and 0 depletion from the chains in pure YBCO. © 1990, Elsevier Science Publishers B.V. (North-Holland). All rights reserved.