Abstract
We have investigated neutron powder diffraction of YBa2-xSrxCu3O7-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 0(5) sites to give full and empty Cu-0 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 Tc in YBa2-xSrxCu3O7-d is discussed in terms of both local oxygen disordering and associated charge-transfer effect.
