Cationic Ordering, Solid Solution Domain, and Diffuse Reflectance in Fe₂WO₆ Polymorphs

Single phases of the α, β, and γ polymorphs of the Fe2WO6 iron tungsten oxide were obtained through an aqueous solution route based on the combustion and heat treatment of a spray-dried precursor powder. Syntheses with Fe/W ratios ≠ 2 identified a domain of solid solutions consistent with a Fe2–2xW1+x□xO6 scenario (x up to ∼0.025) for the defect chemistry in the temperature range around 850 °C. The crystallographic characterizations revealed a random cationic distribution in an α-PbO2-type cell for the low-temperature polymorph (α) and pointed to a reconstructive mechanism for the formation of polymorph β. A comparison of diffuse reflectance spectra confirmed the visual observation of minor color differences between the polymorphs by revealing small shifts of the absorption threshold; the Kubelka–Munk function and Tauc plots were used for comparison of the polymorphs and discussion of the results with respect to relevant literature.