Ammonium hexafluorovanadate

The double fluoride, ammonium hexafluorovanadate ((NH4)3VF6), forms from the oxide at 210 to 250 °C, but decomposes at 600 to 700 °C to yield pure vanadium fluoride. Examples of metal fluorides obtainable through the double fluoride route include uranium tetrafluoride, beryllium difluoride and the rare earth fluorides ... 

Pure anhydrous vanadium(III) fluoride is more conveniently prepared by the thermal decomposition, in an inert atmosphere, of ammonium hexafluorovanadate(III), the latter being formed by the fusion of ammonium hydrogen fluoride with vanadium (III) oxide. Long and Wilhelm were unsuccessful in their efforts to prepare vanadium(III) fluoride by these reactions their product became oxidized since it was not protected by an inert atmosphere. Also, there is evidence that these investigators started with impure vanadium (III) oxide. The procedure described below employs an inert atmosphere for the decomposition of the hexafluorovanadate. 

Previously, ammonium hexafluorovanadate (III) has been obtained only from aqueous solution. Its preparation in molten ammonium hydrogen fluoride has the advantage that it yields a product which is anhydrous. Aqueous preparations of ammonium hexafluorovanadate (III) tend to have sorbed moisture, which, during the decomposition, could through hydrolysis contaminate the vanadium(III) fluoride with oxide. 

Optical Crystallographic Properties. As prepared by the thermal decomposition of ammonium hexafluorovanadate-(III), vanadium(III) fluoride has too fine a crystal size for the determination of optical properties with a polarizing microscope. Heating to 1350° in a platinum tube forms twinned crystals that are easily studied. According to Weaver, these crystals are biaxial (y=-) with optic angle (2V) of 5 to 10°. The a- and y-refractive indexes are, respectively, 1.536 and 1.544. 

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