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

The title compound is prepared by adding aqueous ammonia to a solution of p hosphoric acid containing a small amount of iron (III) ions (3%). It crysta llizes 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)degre es, 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. I n 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 t he range 1.7-300 K, the polycrystalline sample follows a Curie-Weiss law do wn to 30 K. At lower temperatures is a magnetic phase transition, where a s pontaneous magnetization appears. This behavior suggests either a ferrimagn etic or a weak ferromagnetic order. (C) 1999 Academic Press.