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

High-resolution X-ray scattering experiments have been performed on si ngle crystals of Bi2Sr2CaCu2Oy grown independently by three different research groups. Detailed measurements are presented of the X-ray scat tering in the b-c* plane, paying particular attention to the weak dif fuse features arranged around Bragg reflections in addition to the sat ellites from the incommensurate modulation. The results clarify exactl y which features are fundamental to the Bi2Sr2CaCu2Oy structure and wh ich 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. T here are however, (in all crystals studied) extra diffuse peaks lying on either side of the Bragg reflections which are unaccounted for by c urrent structural models. These are shown to be characteristic of a lo ss in ordering of the incommensurate modulation from layer to layer al ong the c-axis.