NA7FE4(PO4)(6) - A MIXED-VALENCE IRON PHOSPHATE-CONTAINING A TETRAMEROF EDGE-SHARING FEO6 OCTAHEDRA

A novel mixed-valence iron phosphate, Na7Fe4(PO4)(6), has been synthes ized by a high-temperature, high-pressure hydrothermal method and char acterized by single-crystal X-ray diffraction and Mossbauer spectrosco py. The compound crystallizes in the trigonal space group R(3) over ba r c with a=13.392(2), c=17.858(3) Angstrom, U=2773.7(8) Angstrom(3), Z =6 and R=0.0179. The structure consists of tetrameric units of edge-sh aring FeO6 octahedra, which are connected by corner-sharing PO4 tetrah edra to form a three-dimensional framework structure, enclosing a netw ork of cavities where the sodium cations reside. Mossbauer spectroscop y supports the presence of one Fe-II and three Fe-III in a tetrameric unit.