Crystal chemistry and polymorphism of an iron-tungsten mixed oxide

Iron tungstate Fe2WO6 attracts increasing interest in two areas of applications due to its photoelectrochemical behaviour. Fe2WO6 is studied either as a photocatalyst for degradation of environmental pollutants or as a photoanode for water oxidation in tandem devices. Recently, some of its magneto-dielectric properties have been reported but its magnetic and electric properties remain nebulous. Indeed, Fe2WO6 iron tungstate exists as two or three crystal structures with different organizations of the FeO6 and WO6 octahedra. The conditions of polymorphism are still unclear: structures were reported in the literature for only two of the three polymorphs (α and γ) and very little is known about the third polymorph (β).
In the present work, we could synthesize for the first time a complete set of the three polymorphs as single-phase products by spray-drying, thereby confirming the existence of the β-polymorph. We revised the structural model for the α-polymorph and solved the structure of the β-polymorph. The new procedure of synthesis also allowed us to control and tune the Fe/W ratio and investigate the possibility for a solid solution domain. The final products were characterized by means of X-Ray Diffraction (synchrotron data), Rietveld refinement, Scanning Electron Microscopy and 57Fe Mossbauer spectroscopy.