From dihydrated iron(III) phosphate to monohydrated ammonium–iron(II) phosphate: Solvothermal reaction mediated by acetone–urea mixtures

By reaction between synthetic phosphosiderite FePO4·2H2O, urea (NH2)2CO, and acetone (CH3)2CO, we report a novel solvothermal synthesis of polycrystalline NH4FePO4·H2O. The preparation of other two individual phases, NH4Fe2(OH)(PO4)2·2H2O and NH4Fe2(PO4)2, is also described. The obtained product is a function of the reaction time and the N/P molar ratio in the reagent mixture, and the existence of structural memory in the dissolution–precipitation processes is discussed. Below 25 K, NH4FePO4·H2O behaves magnetically in a complex way, because both ferromagnetic and antiferromagnetic signals are superimposed, suggesting the existence of a canting of iron(II) magnetic moments.