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Journal of Thermal Analysis and Calorimetry
Authors: Imre Szilágyi, István Sajó, Péter Király, Gábor Tárkányi, Attila Tóth, András Szabó, Katalin Varga-Josepovits, János Madarász, and György Pokol

Abstract  

This article discusses the formation and structure of ammonium tungsten bronzes, (NH4)xWO3−y. As analytical tools, TG/DTA-MS, XRD, SEM, Raman, XPS, and 1H-MAS NMR were used. The well-known α-hexagonal ammonium tungsten bronze (α-HATB, ICDD 42-0452) was thermally reduced and around 550 °C a hexagonal ammonium tungsten bronze formed, whose structure was similar to α-HATB, but the hexagonal channels were almost completely empty; thus, this phase was called reduced hexagonal (h-) WO3. In contrast with earlier considerations, it was found that the oxidation state of W atoms influenced at least as much the cell parameters of α-HATB and h-WO3, as the packing of the hexagonal channels. Between 600 and 650 °C reduced h-WO3 transformed into another ammonium tungsten bronze, whose structure was disputed in the literature. It was found that the structure of this phase—called β-HATB, (NH4)0.001WO2.79—was hexagonal.

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