SYNTHESIS OF THE HEXAGONAL FORM OF TUNGSTEN TRIOXIDE FROM PEROXOPOLYTUNGSTATE VIA AMMONIUM PARATUNGSTATE DECAHYDRATE

Two types of crystalline (NH4)(10)[H2W12O42]. 10H(2)O were precipitate d in sequence when an ammonia solution of peroxopolytungstate was neut ralized with hydrochloric acid. It was found by X-ray diffraction that one of them is an unknown phase of ammonium paratungstate, and the ot her is the reported one. Heating the former, it turned directly to com mon triclinic WO3. On the other hand, the latter yielded the hexagonal form of WO3 (h-WO3) in the temperature range 350-370 degrees C. This is a new synthetic method of h-WO3, though the final precursor is a kn own compound. The color of h-WO3 thus formed was pale yellow and conta ined a small amount of residual ammonia (N/W = 0.017 in molar ratio). Its crystallographic c-axis (7.689 Angstrom) was shorter than than the reported value (7.798 Angstrom) for the same WO3 phase synthesized fr om 3WO(3) . H2O.