Incommensurately modulated cadmium apatites

Two cadmium apatites, Cd-5(PO4)(3)Br and Cd-5(VO4)(3)I, earlier reported to be halogenide deficient, mere prime suspects of being modulated, In this study, incommensurate ordering was found in satellites occurring in planes perpendicular to c*, The structure of Cd,(PO,),Br was refined from single-crystal X-ray diffraction data in the four-dimensional super space group R:P (3) over bar:(00 gamma): a = 16.932(2) Angstrom,= 6.451(1) Angstrom, Z = 6, R = 0.043. The modulation of the structure is due to a misfit between the large halogenide ions and the surrounding rigid Ca-PO4 substructure. From the refined model of the Cd,(PO,),Br structure a "chain-packing" model was confirmed with a Br-Br distance of 3.466 Angstrom. (C) 2000 Academic Press.