Phase transformations of ammonium tungsten bronzes

This article discusses the formation and structure of ammonium tungsten bronzes, (NH 4 ) x WO 3− y . As analytical tools, TG/DTA-MS, XRD, SEM, Raman, XPS, and 1 H-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-) WO 3 . 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-WO 3 , as the packing of the hexagonal channels. Between 600 and 650 °C reduced h-WO 3 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, (NH 4 ) 0.001 WO 2.79 —was hexagonal.