Reinvestigation of the binary diagram Na3PO4–FePO4 and crystal structure of a new iron phosphate Na3Fe3(PO4)4

A new iron(III) phosphate Na{sub 3}Fe{sub 3}(PO{sub 4}){sub 4} has been synthesized and characterized. It decomposes before melting at 860 deg. C into FePO{sub 4} and Na{sub 3}Fe{sub 2}(PO{sub 4}){sub 3}. The structure of the compound was determined by single-crystal X-ray diffraction. The unit cell is monoclinic with the following parameters: a=19.601(8) A, b=6.387(1) A, c=10.575(6) A and {beta}=91.81(4) deg.; Z=4; space group: C2/c. Na{sub 3}Fe{sub 3}(PO{sub 4}){sub 4} exhibits a layered structure involving corner-linkage between FeO{sub 6} octahedra, and corner- and edge-sharing between FeO{sub 6} octahedra and PO{sub 4} tetrahedra. The Na{sup +} cations occupying the interlayer space are six- and seven-fold coordinated by oxygen atoms. The relationship between the structure of Na{sub 3}Fe{sub 3}(PO{sub 4}){sub 4} and the previous reported hydrate K{sub 3}Fe{sub 3}(PO{sub 4}){sub 4}{center_dot}H{sub 2}O will be discussed.