The observation of cubic tungsten trioxide at high-temperature dehydrationof tungstic acid hydrate

The dehydration of tungstic acid hydrate, H2WO4.H2O samples prepared by the methods of Freedman [J. Am. Chem. Soc. 81 (1959) 3834.] ([Na](residual) le ss than or equal to 10 ppm) and Zocher and Jacobson [Kolloidchem. Beih. 28- 6 (193,9) 167.] ([Na](residual) = 140 ppm) has been studied by in situ high -temperature XRD, thermogravimetry and FTIR at room temperature. The format ion of cubic tungsten oxide, c-WO3 has been observed in the course of the d ehydration at the loss of the second water molecule: H2WO4 --> c-WO3. Keepi ng the temperature constant at 300 degreesC under air, the metastable c-WO3 transforms into the stable orthorhombic, o-WO3 phase. FTIR and TG results have shown that the 300 degreesC dehydration products contained hydrogen in the form of OH groups and bronze-like W-O-H even after the loss of the sec ond water molecule. A H2WO4.H2O sample embedded in silicone grease has also been studied by high-temperature XRD, TG and FTIR. The c-WO3 --> o-WO3 con version was blocked in this sample, and FTIR has indicated the presence of CO vibrations in the transmittance spectrum. Lattice parameters of c-WO3 sa mples produced from H2WO4.H2O (this work), and results from the literature (from other precursors) have been compared with the conclusion, that the fo rmation of closely monophase c-WO3 and the relative stability of this phase are in correlation with the presence of some kind of impurity (originating from silicone grease in this experiment and from various organic or inorga nic precursors applied at other laboratories). (C) 2001 Elsevier Science B. V. All rights reserved.