Crystal Chemistry and Magnetism in the Polymorphs of Iron(III) Tungsten Oxide Fe2WO6

Compounds with the general formula A2BO6, where A is a transition metal cation and B is a cation with a high nominal charge, have attracted interest due to the variety of their structures and the complexity of their physical properties. Within this family, Fe2WO6 is an interesting compound because of its three polymorphic structures (α, γ, and more recently β) and its magnetic and photoelectrochemical properties. However, the studies reported so far presented only a partial view of Fe2WO6, since they mostly focused on one polymorph (γ) and did not investigate the influence of the structures on the functional properties. In this manuscript, we present the study of the crystal chemistry of the three polymorphs of Fe2WO6 and report about their optical and magnetic properties. The developments of repeatable spray-drying synthesis procedures from aqueous Fe/W solutions and quantification by ICP-OES allowed us to prepare homogeneous powders with well-controlled Fe/W ratios. A solid solution domain was identified for Fe/W ratios in the [1.91-2.00] range and the general formula Fe2-2xW1+xO6 was proposed to describe the off-stoichiometry. The unknown nuclear and magnetic structures of polymorph β were solved by combining synchrotron X–ray diffraction, neutron diffraction and physical properties measurements. The structure of polymorph α was revised and the impact of the synthesis-dependent cationic ordering in polymorph γ is discussed. The polymorphic transitions and the structural and magnetic relationships with other related compounds (FeWO4 and Fe2TeO6) are also presented.