Partial thermal reduction of ammonium paratungstate tetrahydrate

In situ HT‐XRD Study on the Formation of Hexagonal Ammonium Tungsten Bronze by Partial Reduction of Ammonium Paratungstate Tetrahydrate

“…Our present study, as well as a comparison of earlier results, [1,2,7,[37][38][39][40][41][42][43][44][45][46] shows that the heating conditions of APT in a reducing gas influence the formation of the bronzes, i.e. formation of the hexagonal bronze is favoured when APT is heated rapidly whereas for the formation of the tetragonal bronze, slow heating is favourable.…”

“…[46] In an attempt to explain the effect of the heating conditions on the partial reduction of APT, we report here our results obtained by in situ high-temperature powder X-ray diffraction (HT-XRD) in flowing 10 % H 2 /He on the formation and thermal stability of hexagonal ammonium tungsten bronze. Supported by the results of our HT-XRD measurements and also by earlier studies, [1,2,7,[37][38][39][40][41][42][43][44][45][46] we have proposed a model for the formation of the title compound through the partial reduction of APT. In harmony with this model, HATB has been prepared in a monophasic, highly crystalline form and its composition and structure have been studied by high-precision powder XRD, X-ray photoelectron spectroscopy (XPS), chemical analysis and solidstate 1 H NMR spectroscopy ( 1 H-MAS-NMR).…”

“…This is much higher than in the case of heating APT slowly (5°C min -1 ) to 450°C, whereby the proportion of HATB was 34 %. [47] During consecutive heating at 400°C, the sample became more crystalline. This is indicated by the growth in intensity of reflections from both the hexagonal and tetragonal bronzes (Figure 1).…”

“…In previous studies, when heating APT at a rate of 5-10°C min -1 , hexagonal ammonium tungsten bronze readily appeared between 350 and 600°C but was accompanied by some other phases (monoclinic, orthorhombic or hexagonal WO 3 in air, [1,2,[33][34][35][36][37] tetragonal hydrogen tungsten bronze, WO 2.9 , WO 2.72 and WO 2 in reducing gases [1,2,7,[37][38][39][40] and X-ray amorphous phase in all cases). On the other hand, a monophasic, crystalline and/or partly amorphous hexagonal ammonium tungsten bronze could be produced when APT was decomposed in a furnace between 300 and 500°C in reducing [41][42][43][44][45] or inert gases.…”

Structure and thermal decomposition of ammonium metatungstate

“…Previously AMT [1,33,38,[47][48][49][50][51][52] or a structure close to AMT [38] was said to be a decomposition product of APT at 240 °C. Based on these, it was unknown whether the decomposition of AMT would be the same as the decomposition of APT above 240 °C, or it would differ significantly.…”