INCOMMENSURATE MODULATIONS IN BI-2212 HIGH-T-C, SUPERCONDUCTOR REVEALED BY SINGLE-CRYSTAL X-RAY-ANALYSIS USING DIRECT-METHODS

The incommensurate modulated structure of the Bi2Sr2Ca1Cu2Oy high-T-c superconductor has been studied by direct methods with X-ray diffracti on data From a single crystal. The super-space group is N-111(Bbmb) wi th three-dimensional unit cell parameters a=0.54222 nm, b=0.5437 nm, c =3.0537 nm, alpha=beta=gamma=90 degrees and the modulation wave vector p=(0, 0.22, 1). 543 main reflections, 867 first-order and 469 second- order satellite reflections haw: been observed. Phases of main reflect ions are derived by a conventional direct-method program SAPI91. The p hases of satellite reflections are derived by the program DIMS running with default control based on the known phases of main reflections. T he atomic parameters of the basic structure and of the modulation so o btained consist with the results from other authors. For the first tim e the structure details of the incommensurate modulation in the Bi-221 2 phase are observed directly without relying on any assumed model of modulation.