The incommensurate modulated structure of Bi1.91Sr2CoO5.93 (a = 5.4614(6) A
ngstrom, b = 5.4471(5) Angstrom, c = 23.415(2) Angstrom) with (p) over righ
t arrow* = 0.240(2)(b) over right arrow* (c) over right arrow* has been ref
ined in the superspace group F2mm(0 beta 1)00s using the four-dimensional f
ormalism 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 disord
er 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 wit
h related compounds (BiCu or BiFe) is performed.