CHANGES IN THE APICAL OXYGEN VIBRATIONAL FREQUENCY AND TC DUE TO SR SUBSTITUTION FOR BA IN YBA2CU3O7-Y, TL2BA2CACU2O8-Y AND TL2BA2CUO6-Y SUPERCONDUCTORS

Variations of T(c) and Raman spectra due to partial Sr substitution for Ba have been investigated for YBa2-xSrxCu3O7-y (Y123), x = 0-1.0, Tl2Ba2-xSrxCaCu2O8-y (Tl2212), x = 0-0.6, and Tl2Ba2-xSrxCuO6-y (Tl2201), x = 0-0.6. T(c) decreases rapidly with increasing Sr content for the Tl2201 and Tl2212 systems, while only a slight decrease is observed for Y123. The vibrational frequency of the apical oxygen shows a strong dependence on Sr content. The frequency changes are consistent with changes in bond distances, induced by the increased compression upon Sr doping, for the Y123 system only. For the Tl systems, however, we have to assume additional effects from charge redistributions, wich we suggest are connected to the strong deacrease in T(c) for these systems.