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Structural variations in the brownmillerite series Ca₂(Fe_2–xAl_x)O₅: Single-crystal X-ray diffraction at 25 °C and high-temperature X-ray powder diffraction (25 °C T 1000 °C)

¹ Institute of Crystallography, Rheinisch-Westfalische Technische Hochschule Aachen, Jagerstrasse 17/19, D-52056 Aachen, Germany
² Institute of Mineralogy, University of Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg, Austria

Correspondence: ^* E-mail: guenther.redhammer{at}aon.at

A total of 30 synthetic samples of the Ca₂Fe_2–xAl_xO₅, 0.00 x 1.34 solid solution series have been investigated by single crystal X-ray diffraction at 25 °C. Pure Ca₂Fe₂O₅ and samples up to x = 0.56 have space group Pnma, Z = 4, whereas samples with x >0.56 show I2mb symmetry, Z = 4. The substitution of Fe³⁺ by the smaller Al³⁺ cation decreases unit-cell parameters and average octahedral and tetrahedral bond lengths and induces distinct changes in the O-atom coordination of the interstitial Ca atom. Discontinuities in the structural parameters vs. the Al³⁺_tot content and changes in slope of these quantities are associated with the phase transition. The essential difference between the two modifications is the cation-O atom-cation angle within the planes of corner sharing octahedra, which is close to 180° in I2mb, but 184° in the Pnma phase, and the existence of two different orientations of the tetrahedral chains in Pnma as opposed to one in I2mb. At low overall Al³⁺ concentrations Al³⁺ preferentially enters the tetrahedral site until 2/3 of it is filled. Additional Al³⁺ cations, substituted for Fe³⁺, are equally distributed over octahedral and tetrahedral sites. At high temperature pure Ca₂Fe₂O₅ transforms to a body-centered structure at 724(4)°C. Substituting Al³⁺ for Fe³⁺ linearly decreases the transition temperature by 15 °C per 0.1 Al³⁺ down to 623(5)°C for x = 0.65.