A study of iron–tungsten oxides and iron–chromium–tungsten oxides

Abstract

A 57Fe Moessbauer study of the Fe2O3–WO3 and Fe2O3–Cr2O3–WO3 systems has been carried out. Single phase materials could be prepared having the composition Fe2−xWO6−3x/2 (x = 0.28), the stoichiometric Fe2WO6 always contained detectable amounts of α-Fe2O3. The iron tungstates are found to be unstable to prolonged heating at 1000 °C. The Cr2−yFeyWO6 (0 < y ≤ 2) system was shown to be of the trirutile type when y ≤ 1.7 but multiphasic when y ≥ 1.7, these latter phases have structures based upon the iron and chromium tungstate phases, respectively. The Moessbauer parameters for the Cr2−xFeyWO6 compositions do not appear to behave in the same fashion as the related Fe2−xCrxTeO6 system.

Investigating the Cycling Stability of Fe2WO6 Pseudocapacitive Electrode Materials

“…Mössbauer spectra of the as prepared electrode, after being soaked in the electrolyte (blank) and after 10,000 cycles are shown in Figure 7. The as prepared electrode spectrum agrees well with the data reported by Birchall et al for Fe 2 WO 6 powder [33]. The fitting was performed with an appropriate distribution of 6 Lorentzian profiles representing quadrupole doublets by least-squares methods (Figure S2).…”

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

“…Historically, the interest in the properties of Fe 2 WO 6 was mostly focused on the magnetism and/or transport properties of its polymorphs, considered in the framework of the AB 2 O 6 structures combining a magnetic cation B with an A cation of high formal charge. − In recent years, the environmentally benign character of iron has spurred the interest for iron-based oxides. Accordingly, Fe 2 WO 6 has become the focus of much more varied investigations regarding the photoelectrochemical activity, − photocatalytic properties, − reversible capacity as a lithium-ion battery electrode, , pseudocapacitance in electrochemical capacitors, magnetodielectric properties, thermoelectric properties, , and biological activity .…”

“…Some of them concern the polymorphism of Fe 2 WO 6 . Most of the older literature and all of the recent papers have focused on one polymorph only, usually either the high-temperature γ polymorph − ,,− or the α polymorph obtained by low-temperature solution routes. ,,,, In the early 90s, Walczak et al identified a third polymorph, labeled β, whose synthesis required long heat treatments; shortly afterward, Guskos et al , reported some magnetic and electric transport data for the three polymorphs. To date, their work remains the only comparative study of the α/β/γ polymorphs, and further work is needed to clarify the influence of the synthesis conditions on the polymorphism and on structural properties such as the cationic order/disorder.…”

“…The spray-drying technique is an industrially upscalable method suitable for the reproducible production of large quantities. , The composition of the solution was optimized so that all counterions decompose during the heat treatment, and citric acid was used as a complexing agent to promote a homogeneous distribution of cations in the precursor powder. The synthesis procedure also allowed us to prepare samples with well-controlled nonstoichiometric Fe/W ratios to investigate another unsettled issue in the Fe 2 WO 6 literature, that is, the existence of a solid solution domain. ,, …”

“…The synthesis procedure also allowed us to prepare samples with well-controlled nonstoichiometric Fe/ W ratios to investigate another unsettled issue in the Fe 2 WO 6 literature, that is, the existence of a solid solution domain. 6,10,23 In the following, we first report about the crystallographic structures at room temperature for the three polymorphs, with particular emphasis on polymorph α for which synchrotron Xray powder diffraction (SXRPD) and high-resolution neutron powder diffraction (HRNPD) data were obtained for the first time. Results of the solid solution range for Fe/W ratios ≠ 2 are then used to discuss the defect distribution in Fe 2 WO 6 in the temperature range around 850 °C, while data collected during the synthesis of the single-phase samples provide information about the α → γ and γ ↔ β transitions.…”