Oxygen disordering and charge transfer effect in YBa_2-xSr_xCu₃O_7-d

We have investigated neutron powder diffraction of YBa_2-xSr_xCu₃O_7-d with x=0, 0.4, 0.8 and 1. Their Rietveld analyses were carried out in space group Pmmm assuming two different disordering models: Model A where the O(1) is displaced from (0,1/2,0) to (x,1/2,0) so that each alternate chain has some kind of zig-zag chain and Model B where the available oxygen atoms are located at both the O(1) and O(5) sites to give full and empty Cu-O chains along the a-axis. The refinements based on model B have revealed that substitution of Sr for Ba causes an increase of isotropic temperature factor for the O(1), indicating an increasing in local oxygen disordering. This disordering in the model B is consistent with the Raman results that the defect induced mode gains intensity with increasing Sr content. A .possible picture of the slight decrease of T_c in YBa_2-xSr_xCu₃O_7-d is discussed in terms of both local oxygen disordering and associated charge-transfer effect.