Oxygen disorder phenomena in the modulated Bi2Sr2CoO6-type structure

The incommensurate modulated structure of Bi1.91Sr2CoO5.93 (a = 5.4614(6) Angstrom, b = 5.4471(5) Angstrom, c = 23.415(2) Angstrom) with (p) over right arrow* = 0.240(2)(b) over right arrow* (c) over right arrow* has been refined in the superspace group F2mm(0 beta 1)00s using the four-dimensional formalism for modulated structures and single-crystal x-ray diffraction data. Two different regions with different sites for Ri and O atoms are evident along the modulation direction (called M and I regions). The oxygen disorder observed in the M region leads to different possible configurations for [BiO] layers. In particular, a double-chain configuration comparable to the one observed in related compounds is outlined. Therefore, a comparison with related compounds (BiCu or BiFe) is performed.