Incommensurately Modulated Cadmium Apatites

Abstract Two cadmium apatites, Cd 5 (PO 4 ) 3 Br and Cd 5 (VO 4 ) 3 I, earlier reported to be halogenide deficient, were prime suspects of being modulated. In this study, incommensurate ordering was found in satellites occurring in planes perpendicular to c * . The structure of Cd 5 (PO 4 ) 3 Br was refined from single-crystal X-ray diffraction data in the four-dimensional super space group R = P 3 :(00 γ ): a =16.932(2) A, c =6.451(1) A, 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–PO 4 substructure. From the refined model of the Cd 5 (PO 4 ) 3 Br structure a “chain-packing” model was confirmed with a Br–Br distance of 3.466 A.