STRUCTURAL DISORDER IN THE PB2SR2Y1-XCAXCU3O8+DELTA CUPRATES

High-resolution powder x-ray-diffraction results on Pb2Sr2Y1-xCaxCu3O8 +delta are presented. Assuming an orthorhombic cell with Cmmm symmetry , the reflections that index with h and l simultaneous nonzero are obs erved to be distinctly broader than the other reflections. A model is presented that is able to explain quantitatively and qualitatively the relative widths of the different hkl reflections. The widths of the r eflections are found to be independent of the doping level. The model employed is based on anisotropic distributions of the d spacings withi n the (a,c) plane, corresponding to a fluctuation of the beta angle ar ound 90 degrees that occurs as a result of oxygen nonstoichiometry. Th e distortions may be related to the incommensurable modulation along t he b direction in the structurally similar (Bi/Pb)(2)Sr-2(Ca/Y)Cu2O8+d elta compounds. At elevated temperatures, a disorder-order phase trans ition (T-do=240 degrees C) is observed. At temperatures above T-do, th e structure relaxes to the undistorted average orthorhombic structure. This order-disorder transition results from oxygen diffusion, which s ets in at the transition temperature.