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

► Solvothermal synthesis of NH4FePO4·H2O from an Fe(III) phosphate: reduction process. ► Formation of two intermediate metastable phases: phase diagram. ► Thermal decomposition in two steps: mass loss of both water and ammonia. ► Magnetic behaviour: AF+constant spontaneous magnetization.