Synthesis and crystal structure of Na1.28Ni0.86Fe2(PO4)3

A new monophosphate Na_1.28Ni_0.86Fe₂(PO₄)₃ has been synthesized by a flux method and structurally characterized by single-crystal X-ray diffraction. The compound crystallizes in the orthorhombic space group Ibmm with the parameters: a = 6.438(2) Å, b = 10.515(3) Å, c = 13.166(3) Å, Z = 4. The structure is built up from Fe₂O₁₀ units of edge-sharing FeO₆ octahedra, which are connected through the corners of MO₆ (M = Ni²⁺ or a vacancy) octahedra and PO₄ tetrahedra to form a three-dimensional framework analogous to that of -CrPO₄. Of the two crystallographically distinct Na⁺ sites contained in this structure, one is totally filled whereas the other is 28% occupied.