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

A new iron(III) phosphate Na3Fe3(PO4)4 has been synthesized and characterized. It decomposes before melting at 860°C into FePO4 and Na3Fe2(PO4)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) Å, b=6.387(1) Å, c=10.575(6) Å and β=91.81(4)°; Z=4; space group: C2/c. Na3Fe3(PO4)4 exhibits a layered structure involving corner-linkage between FeO6 octahedra, and corner- and edge-sharing between FeO6 octahedra and PO4 tetrahedra. The Na+ cations occupying the interlayer space are six- and seven-fold coordinated by oxygen atoms. The relationship between the structure of Na3Fe3(PO4)4 and the previous reported hydrate K3Fe3(PO4)4·H2O will be discussed.