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.