Ammonium Iron(II,III) Phosphate: Hydrothermal Synthesis and Characterization of NH4Fe2(PO4)2

The mixed-valence iron phosphate NH 4 Fe 2 (PO 4 ) 2, which was synthesized by the high-temperature, high-pressure hydrothermal method, crystallizes in the monoclinic space group C 2/ c with a = 20.0066(1) Å, b = 14.8319(2) Å, c = 9.9899(1) Å, β = 119.278(1)°, and Z = 16. Its compact structure consists of chains of edge-sharing Fe II O 6 octahedra and chains of corner-sharing Fe III O 6 octahedra and PO 4 tetrahedra. These chains are connected to each other and delimit tunnels in the [221], [22̄1], and [001] directions, in which NH 4 + cations are located. Mössbauer spectroscopy confirms the presence of Fe II and Fe III . The title compound is the first example of a mixed-valence ammonium iron phosphate.