Tetrahedral Chain Order in the Sr2Fe2O5 Brownmillerite

The crystal structure of the Sr2Fe2O5 browninillerite has been investigated using electron diffraction and high resolution electron microscopy. The Sr2Fe2O5 structure demonstrates two-dimensional order: the tetrahedral chains with two mirror-related configurations (L and k) are arranged within the tetrahedral layers according to the -L-R-L-R- sequence, and the layers themselves are displaced with respect to each other over 1/2[111] or 1/2[11 (1) over bar] vectors of the brownmillerite unit cell, resulting in different ordered stacking variants. A unified superspace model is constructed for ordered stacking sequences in brownmillerites based on the averacge brownmillerite structure with a = 5.5298(4)angstrom, b = 15.5875(12)angstrom, c = 5.6687(4)angstrom, and (3 + 1)-dimensional superspace group 12/m(0 beta gamma)0s, q = beta b* + gamma c*, 0 <= beta <= 1/2, 0 <= gamma <= 1.