X-RAY-SCATTERING STUDIES OF A VARIETY OF BI2SR2CACU2OY SINGLE-CRYSTALS

High-resolution X-ray scattering experiments have been performed on single crystals of Bi2Sr2CaCu2Oy grown independently by three different research groups. Detailed measurements are presented of the X-ray scattering in the b*-c* plane, paying particular attention to the weak diffuse features arranged around Bragg reflections in addition to the satellites from the incommensurate modulation. The results clarify exactly which features are fundamental to the Bi2Sr2CaCu2Oy structure and which are due to sample-dependent effects. Features previously suggested as indicating a primitive monoclinic symmetry are shown to be caused by defects, thus confirming the generally accepted Bbmb space group. There are however, (in all crystals studied) extra diffuse peaks lying on either side of the Bragg reflections which are unaccounted for by current structural models. These are shown to be characteristic of a loss in ordering of the incommensurate modulation from layer to layer along the c-axis.