Novel bismuth oxophosphate halides [Bi8O8][BiO2](PO4)2X (X=Cl, Br) based on oxocentered 2D blocks and their relationships to the Aurivillius phases

Two new bismuth phosphate halides [Bi8O8][BiO2](PO4)2Cl and [Bi8O8][BiO2](PO4)2Br, high-temperature phases have been obtained by the solid-state reaction method. Oa–Bi bonds (Oa—additional oxygen atoms not bonded to P) are the shortest and therefore the strongest in the studied structures, which makes it reasonable to consider the Bi–O substructure consisting of OBi4 tetrahedra and OBi3 triangles as an independent structural unit interacting with X atoms and PO4 tetrahedra through relatively weak Bi–X and Bi–O bonds. The topology of this oxocentered Bi–O structural unit is two-dimensional and obviously related to the [O2Bi2]2+ layer typical for the Aurivillius type compounds. The formula of the oxocentered block is ([O8Bi8][O2Bi])7+ with the first part in square brackets designating the moiety formed by the OBi4 tetrahedra, whereas the second part shows 3-coordinated O atoms and the Bi atoms not participating in the OBi4 tetrahedra. The interlayer in between the oxocentered 2D blocks is occupied by the phosphate groups and halogen atoms which are organized in alternating stripes. In the case of studied bismuth oxyphosphate halides as well as in other previously reported compounds, ‘additional’ oxygen atoms can be considered as segregation centers for Bi3+ cations. This viewpoint may appear to be useful for design of new schemes and synthetic routes to the bismuth oxysalt compounds with different structural architectures and possible useful applications. The comparison with the similar stairs-like layers in bismuth oxysalt structures is also given.