Synthesis of the Hexagonal Form of Tungsten Trioxide from Peroxopolytungstate via Ammonium Paratungstate Decahydrate

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Abstract
Two types of crystalline (NH4)10[H2W12O42]·10H2O were precipitated in sequence when an ammonia solution of peroxopolytungstate was neutralized with hydrochloric acid. It was found by X-ray diffraction that one of them is an unknown phase of ammonium paratungstate, and the other is the reported one. Heating the former, it turned directly to common triclinic WO3. On the other hand, the latter yielded the hexagonal form of WO3 (h-WO3) in the temperature range 350—370 °C. This is a new synthetic method of h-WO3, though the final precursor is a known compound. The color of h-WO3 thus formed was pale yellow and contained a small amount of residual ammonia (N/W = 0.017 in molar ratio). Its crystallographic c-axis (7.689 Å) was shorter than than the reported value (7.798 Å) for the same WO3 phase synthesized from 3WO3·H2O.