Structure et propriétés magnétiques du composé 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 groupC2/c,a=16.8450(9) ?,b=9.6111(1) ?,c=17.6470(7) ?, andBETA=90.91(2)°,R=0.031, and wR=0.078. The structure consists of a two-dimensional framework made of HPO4, H2PO4groups, and FeO6octahedra sharing corners. In one layer, the NH+4cations are located inside 12 membered windows, which are generated by the corner-sharing FeO6and 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.