Synthesis of tungsten trioxide hydrates and their structural properties

Tungsten trioxide hydrates were synthesized by (a) cation exchange reaction from sodium tungstate solution and (b) precipitation from sodium tungstate solution by the addition of HCl solution. The samples were analyzed by XRD, DTA/TGA, Raman and FT-IR spectroscopy. XRD showed formation of WO3·H2O by cation exchange reaction, whereas WO3·0.33H2O was identified by XRD as a product of the acidification of sodium tungstate solution with HCl solution. After heating at 320°C, WO3·H2O transformed into WO3, whereas the WO3·0.33H2O crystal structure remained and these results were in agreement with DTA/TGA measurements. The WO3·H2O sample synthesized by cation exchange reaction showed a weight loss corresponding to one molecule of water in the crystal structure. However, samples WO3·0.33H2O showed a much greater weight loss upon heating than could be expected on the basis of the WO3 0.33H2O formula. The phase transition WO3·H2O→WO3 was also monitored by Raman and FT-IR spectroscopy. In the case of WO3·0.33H2O samples, the basic features of Raman and FT-IR spectra did not change on heating to 320°C, thus indicating that the heating of WO3·0.33H2O up to this temperature did not destroy the original crystal structure. Contrary to this, after heating the WO3·H2O sample to 320°C, the Raman and FT-IR spectra showed a series of new bands caused by the phase transition WO3·H2O→WO3.