Stacking faults and superstructures in a layered brownmillerite

• Published in: Acta crystallographica. Section B, Structural science Structural science, 2011-12, Vol.67 (6), p.476-485
• Main Authors: Krüger, H., Stöber, S., Welberry, T. R., Withers, R. L., Fitz Gerald, J. D.
• Format: Article
• Language: English
• Subjects: Chemical synthesis; Computer simulation; Crystallography; Diffraction; diffuse scattering; Diffusion; layered brownmillerite; modulated structure; Research Papers; Rods; Scattering; Simulation; Single crystals; Stacking faults; Superstructures; Unit cell
• ISSN: 0108-7681; 2052-5192; 1600-5740; 2052-5206
• DOI: 10.1107/S0108768111042005

Single crystals of Ca4Fe2Mn0.5Ti0.5O9 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 Sr3NdFe3O9 [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 A21/m(0βγ)0s, with γ = 0.5 and β≃ 0.27 or β = 0.