Structural variations in the brownmillerite series Ca ₂ (Fe _{2− <i>x</i>} Al _<i>x</i> )O ₅ : Single-crystal X-ray diffraction at 25 °C and high-temperature X-ray powder diffraction (25 °C ≤ <i>T</i> ≤ 1000 °C)

Abstract

A total of 30 synthetic samples of the Ca2Fe2-xAlxO5, 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.