Thermal reactions of the mixed-valence iron fluorides, Fe2F5·nH2O

Abstract

The thermal reactions of Fe₂F₅·7H₂O and Fe₂F₅·2H₂O have been studied using a combination of thermogravimetric and differential thermogravimetric analysis and by differential scanning calorimetry. The initial dehydration of Fe₂F₅·7H₂O near 100 °C is shown to be a complex process which leads to a fluoride-deficient phase of nominal composition Fe₂F₅·2H₂O. Further heating leads to a product at the hemihydrate level. Isothermal dehydrations lead to two distinct high-temperature phases, but the formation of these materials is accompanied by both oxidation and fluoride loss. Vacuum dehydration of Fe₂F₅·7H₂O leads to a fully mixed-valence monohydrate phase, which was previously prepared by dehydration in an HF atmosphere. Dehydration of the authentic crystalline Fe₂F₅·2H₂O is seen to the distinguishable from the second-stage dehydration of Fe₂F₅·7H₂O. Thermogravimetric analyses, thermodynamic measurements, and the kinetic parameters extracted from the several thermal techniques demonstrate that these two forms of ‘Fe₂F₅·2H₂O’ differ significantly in their thermal reactions.