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

The mixed-valence iron phosphate NH4Fe2(PO4)2, which was synthesized by the high-temperature, high-pressure hydrothermal method, crystallizes in the monoclinic space group C2/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 FeIIO6 octahedra and chains of corner-sharing FeIIIO6 octahedra and PO4 tetrahedra. These chains are connected to each other and delimit tunnels in the [221], [22̄1], and [001] directions, in which NH4+ cations are located. Mössbauer spectroscopy confirms the presence of FeII and FeIII. The title compound is the first example of a mixed-valence ammonium iron phosphate.