Acta Crystallographica Section B

Structural Science

Volume 61, Part 6 (December 2005)

research papers

Acta Cryst. (2005). B61, 656-662 [ doi:10.1107/S0108768105026480 ]

Incommensurately modulated ordering of tetrahedral chains in Ca₂Fe₂O₅ at elevated temperatures

H. Krüger and V. Kahlenberg

Abstract: The basic building units of brownmillerite-type A₂B₂O₅ structures are perovskite-like layers of corner-sharing BO₆ octahedra and zweier single chains of BO₄ tetrahedra. A three-dimensional framework is formed by alternate stacking of octahedral layers and sheets of tetrahedral chains. The compound Ca₂Fe₂O₅ is known to have symmetry at ambient conditions. The space group was reported to be evident above 963 K. New high-temperature single-crystal X-ray diffraction experiments at 1100 K revealed that Ca₂Fe₂O₅ forms an incommensurately modulated structure adopting the superspace group Imma(00 [gamma] )s00, with [gamma] = 0.588 (2). The modulation affects the sequence of the enantiomorphic (right- and left-handed) oriented tetrahedral chains within the layer, breaking the lattice periodicity along c. This ordering can be modelled with crenel occupation modulation functions for the tetrahedrally coordinated Fe, as well as for the O atom interconnecting the tetrahedra.

Keywords: brownmillerite type; incommensurately modulated ordering; tetrahedral chains.


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