Polymorphism and variable structural dimensionality in the iron(III) phosphate oxalate system: a new polymorph of 3D [Fe2(HPO4)2(C2O4)(H2O)2]•2H2O and the layered material [Fe2(HPO4)2(C2O4)(H2O)2]

Two new iron (III) phosphate oxalates have been isolated under hydrothermal conditions as phase-pure samples and their crystal structures determined from single crystal X-ray diffraction. [Fe-2(HPO4)(2)(C2O4)(H2O)(2)]center dot 2H(2)O, I, is a polymorph of a known phase and differs in the local arrangement of hydrophosphate, oxalate and coordinated water about iron, presenting a mer orientation of three coordinated phosphates, rather than fac as previously seen. The structure of I is three-dimensionally connected with similar network connectivity to the known phase but different overall topology. [Fe-2(HPO4)(2)(C2O4)(H2O)(2)], II, has a layered structure constructed from octahedral Fe(III) centres coordinated by hydrophosphate, oxalate and water in a fac arrangement. The amount of water used in synthesis is one of the key experimental parameters in stabilising one phase over the other. Thermogravimetric analysis shows that both I and II ultimately collapse into dense tridymite type FePO4 above 600 degrees C and variable temperature powder XRD shows that this occurs via crystalline intermediate phases. Variable temperature magnetisation measurements show that both materials order antiferromagnetically at low temperatures, with similar Neel temperatures (similar to 29 K) despite their long-range structural differences.