CRYSTAL-STRUCTURE OF DOUBLE-COLLAPSED-PHASE BI6-XFE5O26 AND ITS RELATION TO MODULATED-PHASE BI2SR2CACU2O8+DELTA(XSR9)

A structural analysis of the double-collapsed-phase Bi6+xSr9-xFe5O26 i s presented in light of the modulated-phase Bi2.4Sr2.6Fe2O9+delta prev iously investigated [Y. Lepage, W. R. McKinnon, J. M. Tarascon, and P. Bar boux, Phys. Rev. B 40, 6810 (1989); O. Perez, W. LeIigny, D. Greb ille, J. M. Greneche, Ph. Labbe, D. Groult, and B. Raveau, Phys. Rev. B 55, 1236 (1997)]. The study was carried out on a single crystal, usi ng synchrotron radiation (lambda = 0.326 Angstrom). The crystal is mon oclinic with cell parameters a = 16.491(9) Angstrom, b = 5.481(3) Angs trom, c = 30.086(16) Angstrom, beta = 91.39(2)degrees. The real struct ure of symmetry P2(1)/n appears as a small perturbation of the ideal s tructure with higher symmetry B2/m. The undulating (001)(m) layers of the modulated (m) phase built from one kind of cation (Bi, Sr, or Fe) are replaced by mixed layers. The waving of these layers is less regul ar than in the modulated phase because interrupted layers are connecte d. One of the most interesting results relates to the interrupted Bi l ayers: the isolated blocks composed of two adjacent infinite [010] rib bons, six Bi atoms wide, are similar to the ordered part (condensed zo nes) of the Bi-2212 modulated phase. The Bi disordered regions (dilute d zones) are missing in the double-collapsed phase. An original octahe dral coordination is implied for the Bi atoms located near the Fe atom s in the same undulated mixed layer. Disorder phenomena have been obse rved in a complementary HREM study and appear as localized faults modi fying mainly the length of the Bi ribbons.