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, crystalli zes in the monoclinic space group C2/c with a = 20.0066(1) Angstrom, b = 14.8319(2) Angstrom, c = 9.9899(1) Angstrom, beta = 119.278(1)degre es, and Z = 16. Its compact structure consists of chains of edge-shari ng (FeO6)-O-II octahedra and chains of corner-sharing (FeO6)-O-III oct ahedra and PO4 tetrahedra. These chains an connected to each other and delimit tunnels in the [221], [2(2) over bar1$], and [001] directions , in which NH4+ cations are located. Mossbauer spectroscopy confirms t he presence of Fe-II and Fe-III. The title compound is the first examp le of a mixed-valence ammonium iron phosphate.