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The thermal decomposition of ammonium metavanadate(V) has been studied at atmospheric pressure, at hydrothermal, and high-pressure conditions, respectively. In a closed high-pressure system a redox reaction between vanadium(V) and ammonia takes place, to yield with increasing temperature the compounds (NH 4 ) 2 V 6 O 12 (OH) 2 , VO 2-δ (OH) δ , and V 2n O 2n-1  the composition of the product eventually approaching V 2 O 3  at 1100 °C. All these compounds can easily be converted into V 2 O 5  by heating in air. The mid-infrared spectra of the decomposition products obtained by slow heating of NH 4 VO 3  in air, are virtually identical and show that ammonium hexavanadate(V), (NH 4 ) 2 V 6 O 16 , is the most important, and perhaps only, intermediate in the “non-reduction” case. On further heating, (NH 4 ) 2 V 6 O 16  may not only loose ammonia and water to form V 2 O 5  but could as well partially be reduced by ammonia. This could explain the diversity of intermediate “compounds” described in the literature. Hydrothermal reaction of NH 4 VO 3  yielded (NH 4 ) 0 . 5 V 2 O 5 , (NH 4 )2V 3 O 8 , and V 2 O 3  at rather low temperatures.

The Thermal Decomposition of Ammonium Metavanadate(V) in Open and Closed Systems