Selective hydrothermal synthesis of ammonium vanadates(V) and (IV,V)

The reduction of vanadium pentoxide with formamide under hydrothermal–microwave treatment at 180–210 °C allows for the selective synthesis of ammonium hexavanadate(V) (NH4)2V6O16, ammonium trivanadate(IV,V) (NH4)2V3O8 or ammonium vanadate(IV,V) with the hypothesized composition of (NH4)0.22V2O5. An increase in both the temperature of the hydrothermal reaction and the molar ratio of the reagents (HC(O)NH2:V2O5) promotes the reduction of vanadium(V) to vanadium(IV). The synthesis is time-saving and provides a sufficiently high yield of single-phase reaction products (up to 60–70%). Our data call into question the existence of a recently reported crystalline ammonium tetravanadate(IV), (NH4)2V4O9.