Oxygen disorder phenomena in the modulated Bi2SrCoO-type structure

The incommensurate modulated structure of Bi1.91Sr2CoO5.93 (a 5.4614(6) A, b = 5.4471(5) A, c = 23.415(2) A) with q-vector* = 0.240(2) b-vector* + c-vector* has been refined in the superspace group F2mm(0β1)00s using the four-dimensional formalism for modulated structures and single-crystal x-ray diffraction data. Two different regions with different sites for Bi 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. (author)