New insight on bismuth cuprates with incommensurate modulated structures

The incommensurate modulated crystal structure of Bi2.27Sr1.73CuO6 + δ (2201) phase [a = 5.3874 (5), b = 5.3869 (4), c = 24.579 (3) Å; β = 90.01 (1)°, q = 0.2105 (3)a(*) + 0.538 (4)c(*), Z = 4, the (3 + 1)-dimensional monoclinic A2/a(α0γ) group] has been refined with R = 0.041, wR = 0.052 from X-ray single-crystal data including up to third-order satellite reflections. The same structure has also been considered as incommensurate composite with a2 = 2.437, b2 = 5.387, c2 = 24.614, β2 = 93.06, q2 = 0.4524a2(*)-0.243c2(*) and the (3 + 1)-dimensional A2/m(α0γ)0s group for the second component. Both approaches give quite similar results. The structure possesses oxygen disorder in the oxygen-rich region of the BiO layer. An extra O atom is determined in the bridging position shifted ∼ 0.6 Å from BiO towards the SrO layer. Its presence is the cause of the tremendous increase of the bismuth U(11) atomic displacement parameter in ∼ 20% of the unit cells (t = -0.05-0.15). Vacancies are determined in the oxygen site of the SrO layer, which may result in the oxygen content variation with annealing at different oxygen pressures. The total refined oxygen content 6.18 (1) corresponds to the results of chemical analysis.