STRUCTURAL FEATURES OF THE INCOMMENSURATE MODULATION IN THE PB-DOPED BI-2223 HIGH-TC PHASE REVEALED BY DIRECT-METHOD ELECTRON-DIFFRACTION ANALYSIS

Structural modulation waves in the Pb-doped Bi-2223 (Bi2Sr2Ca2Cu3Ox) phase are observed for the first time on the potential distribution function projected along the a axis. The newly developed multidimensional direct methods were used. Unlike the previous techniques, there is no need to assume any modulation model before the modulation waves can be seen directly from the resulting Fourier map. The symmetry of the title compound belongs to the four-dimensional space group P111BAR(Bbmb) with three-dimensional unit cell parameters a = 5.49 angstrom, b = 5.41 angstrom, c = 37.1 angstrom, alpha = beta = gamma = 90-degrees and the modulation wavevector q = 0.117b*. Occupational and positional modulations are obvious for most atoms. Disordered oxygen atoms arranged perpendicular to the b axis bridging the Cu(2)-Ca-Cu(1)-Ca-Cu(2) layers were observed.