When the difference between incommensurate and commensurate structures becomes elusive: the case of Bi₄BO₇X (X = Cl, Br) oxyhalides

Data are reported on the incommensurately modulated structures of bismuth oxyhalide borates, Bi4BO7X (X = Cl and Br). Their crystal structure is similar to that of the previously reported oxyiodide and can be described as alternating litharge-related [Bi4O4](4+) layers of edge-sharing OBi4 tetrahedra. The X- and BO33- anions fill the interlayer space; they alternate in the following sequence: center dot center dot center dot -X-X-X--BO3-BO3-BO3-center dot center dot center dot, wherein the part of the sequence is maintained for 152 cells for X = Cl and 56 cells for X = Br. The incommensurate structure is deceptively close to the commensurate fourfold a x b x 4c superstructure due to the extreme closeness of the q vector to (0, 0, 1/4). The incommensurate nature of the structure could be revealed from accurate analysis of single-crystal and Rietveld diffraction data. Fine details of the diffraction pattern are discussed, indicating the incommensurate deviation of the crystal structures. The compound Bi4BO7Br was additionally characterized by thermal analysis and vibrational and UV-Vis-NIR spectroscopy, as well as by density functional theory (DFT) calculations. On the basis of the DFT calculations, it is possible to predict that the a x b x 4c superstructure of Bi4BO7Br is unstable under ambient conditions.