Structure and magnetic properties of NH4Fe3(H2PO4)6(HPO4)2,4H2O

The title compound is prepared by adding aqueous ammonia to a solution of phosphoric acid containing a small amount of iron (III) ions (3%). It crystallizes in the monoclinic system, ;space group C2/c, a = 16.8450(9) Angstrom, b = 9.6111(1) Angstrom, c = 17.6470(7) Angstrom, and beta = 90.91(2)degrees, R = 0.031, and wR = 0.078. The structure consists of a two-dimensional framework made of HPO4, H2PO4 groups, and FeO6 octahedra sharing corners. In one layer, the NH4+ cations are located inside 12 membered windows, which are generated by the corner-sharing FeO6 and H2PO4. Water molecules lay in the interlayer space. With regard to magnetic properties, carried out in the range 1.7-300 K, the polycrystalline sample follows a Curie-Weiss law down to 30 K. At lower temperatures is a magnetic phase transition, where a spontaneous magnetization appears. This behavior suggests either a ferrimagnetic or a weak ferromagnetic order. (C) 1999 Academic Press.