Thermal Decomposition of Ammonium Hexafluorovanadate (III)

Abstract

In order to establish the conditions for the preparation of pure vanadium trifluoride, VF₃, from ammonium hexafluorovanadate (III), (NH₄)₃VF₆, the preparation of (NH₄)₃VF₆ and its thermal decomposition have been studied by means of thermogravimetric analysis, differential thermal analysis, and X-ray diffraction analysis. Ammonium hexafluorovanadate can be prepared by heating a mixture of V₂O₃ and NH₄HF₂ in the molar ratio of 1:8–1:12 at 185–250°C in an inert atmosphere. Vanadium trifluoride can be prepared by heating (NH₄)₃VF₆ at 600–700°C in an inert atmosphere. The mechanism of thermal decomposition of (NH₄)₃VF₆ is as follows:
(NH₄)₃VF₆ → α → β → γ → VF₃,
in which α is believed to be ammonium pentafluorovanadate (III), (NH₄)₂VF₅, and β has been found to be a nonstoichiometric compound with the chemical formula of (NH₄F)_1−x·VF₃, 0≤x≤∼0.3, which belongs to a tetragonal system. The lattice parameters of ammonium tetrafluorovanadate (III), NH₄VF₄(NH₄F·VF₃), oare a=7.58, c=6.36, c⁄a=0.839. The composition of γ is very near to that of VF₃ and γ contains a small amount of NH₄F (about 0.1–1.5 wt% NH₄).