New Iron(III) Phosphate Phases:  Crystal Structure and Electrochemical and Magnetic Properties

Two new iron(III) phosphates, FePO₄, have been synthesized from the dehydration of hydrothermally prepared monoclinic and orthorhombic hydrated phosphates FePO₄·2H₂O. The structures of both hydrates were redetermined from single crystal data. On dehydration, a topotactic reaction takes place with only those bonds associated with the water molecules being broken, so that both FePO₄ phases have essentially the same Fe−P backbone frameworks as the corresponding hydrates. They are, respectively, monoclinic FePO₄, space group P2₁/n, a= 5.480(1) Å, b = 7.480(1) Å, c= 8.054(1) Å, β = 95.71(1)°, and Z = 4; and orthorhombic FePO₄, space group Pbca, a = 9.171(1) Å, 9.456(1) Å, c = 8.675(1) Å, and Z = 8. Both of these phases are thermally unstable relative to the trigonal quartz-like FePO₄. The electrochemical studies find that the orthorhombic iron phosphate is more active than the monoclinic phase, while both are more active than trigonal FePO₄. Both phases approach Curie−Weiss behavior at room temperature, with the monoclinic phase exhibiting stronger antiferromagnetic interactions due to Fe−O−Fe interactions. The electrochemical and magnetic data are consistent with the structures of these two compounds. The properties of these new iron phosphate structures are compared with other iron phosphate phases.