NEW COLLAPSED STRUCTURE RELATED TO THE 2212-STRUCTURE - HREM STUDY OFBI12SR18FE10O52

A shearing mechanism strategy is applied to a 2212-type ferrite. A new compound is obtained, which exhibits a monoclinic cell with the refin ed parameters a = 16.491(9) Angstrom, b = 5.481(3) Angstrom, c = 30.08 6(16) Angstrom, and beta = 91 degrees 39(2) and the space group P2(1)/ n. This structure can he described as the member m = 4 of the general family ([011])[Bi2Sr3Fe2O9](m) [Bi4-xSr6Fe2+xO16-delta]. It consists o f ''2212''-type slices, parallel to (011)(2212), four FeO6 octahedra t hick, interconnected through [Bi4-xSr6Fe2+xO16] slices that are two Fe O6 octahedra thick and that involve BiO5 bipyramids. The HREM study of the crystals allowed to understand the structural mechanisms which ge nerate this structure and govern the nonstoichiometry. Fe for Bi subst itution, intergrowth defects, mixed crystallographic shear planes, and 3212 defective members are characterized. The relationships with the homologous copper-based collapsed phase and the other layered structur es of the system are discussed. (C) 1997 Academic Press.