Rietveld refinement of whitlockiterelated K0.8Ca 9.8Fe0.2(PO4)7

Abstract

The title compound, K0.8Ca9.8Fe0.2(PO 4)7 (potassium decacalcium iron heptaphosphate), belongs to the whitlockite family. The structure is built up from several types of metaloxygen polyhedra: two [CaO8], one [C aO7] and one [(Ca/Fe)O6] polyhedron with a mixed Ca/Fe occupancy in a 0.8:0.2 ratio, as well as three tetrahedral [PO4] 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 metaloxygen polyhedra and [PO4] tetrahedra via edges and corners results in formation of a three-dimensional framework with composition [Ca9.8Fe0.2(PO4)7] 0.8-. The remaining K atoms (siteoccupation factor = 0.8) are located in large closed cavities and are ninecoordinated by oxygen. © 2010.