Rietveld refinement of whitlockite-related K(0.8)Ca(9.8)Fe(0.2)(PO(4))(7)

The title compound, K(0.8)Ca(9.8)Fe(0.2)(PO(4))(7) (potassium deca­calcium iron hepta­phosphate), belongs to the whitlockite family. The structure is built up from several types of metal–oxygen polyhedra: two [CaO(8)], one [CaO(7)] and one [(Ca/Fe)O(6)] polyhedron with a mixed Ca/Fe occupancy in a 0.8:0.2 ratio, as well as three tetra­hedral [PO(4)] units. Of the 18 sites in the asymmetric unit, the site with the mixed Ca/Fe occupation, the K site, one P and one O site are on special positions 6a with 3 symmetry, whereas all other sites are on general positions 18b. The linkage of metal–oxygen polyhedra and [PO(4)] tetra­hedra via edges and corners results in formation of a three-dimensional framework with composition [Ca(9.8)Fe(0.2)(PO(4))(7)](0.8−). The remaining K atoms (site-occupation factor = 0.8) are located in large closed cavities and are nine-coordinated by oxygen.