title: Stacking faults and superstructures in a layered brownmillerite.
abstract: Single crystals of Ca(4)Fe(2)Mn(0.5)Ti(0.5)O(9) have been synthesized using a flux method. The structural characterization using single-crystal X-ray diffraction revealed the space group Amma and unit-cell dimensions of a = 5.3510 (6), b = 26.669 (3), c = 5.4914 (6) Å. The structure is isotypic with Sr(3)NdFe(3)O(9) [Barrier et al. (2005). Chem. Mater. 17, 6619-6623] and exhibits separated brownmillerite-type layers. One-dimensional diffuse scattering shows that the unit cell is doubled along c by alternating the intra-layer order of tetrahedral chains, causing stacking faults along the b direction. A computer simulation was performed, proving that the observed intensity variations along the diffuse scattering rods originates from two different local structures depending on the configuration of the tetrahedral chains. Selected-area electron diffraction experiments exhibit well ordered regions characterized by satellite reflections corresponding to two different superstructures. Both superstructures can be described using the superspace group A2(1)/m(0βγ)0s, with γ = 0.5 and β ≃ 0.27 or β = 0.
authors.affiliation: Institute of Mineralogy and Petrography, University of Innsbruck, Innsbruck, Austria. hannes.krueger@uibk.ac.at
authors.forename: H
authors.initials: H
authors.lastname: Krüger
authors.forename: S
authors.initials: S
authors.lastname: Stöber
authors.forename: T R
authors.initials: TR
authors.lastname: Welberry
authors.forename: R L
authors.initials: RL
authors.lastname: Withers
authors.forename: J D
authors.initials: JD
authors.lastname: Fitz Gerald
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