Synthesis of 3La₂O₃·2WO₃ by the TW Method

Abstract

The gel of 3La₂O₃·2WO₃ composition was prepared by the TW method from the starting mixture containing La(NO₃)₃ aqueous solution and WO₃ powder with each small amount of tetraethyl orthosilicate (TEOS) and 2M-HCl as a catalyst, which was heated at a fixed temperature in the range of 800°C to 1200°C for 30 min. In the obtained material (the sample No.1), the formation of 3La₂O₃·2WO₃ began at about 800°C and was most remarkable at 1000°C. However, in the case of the sample No.1, the diffraction lines for 7La₂O₃·8WO₃, α-La₂W₂O₉, La₂O₃, β-La₂W₂O₉ and γ-La₂WO₆ were observed in addition to those for 3La₂O₃·2WO₃. Therefore, pure 3La₂O₃·2WO₃ in a single phase was not obtained at any heating temperature. Whereas in the case of the sample No.4, which was obtained by heating the gel with 8.99×10^-3 mol KCl at 1000°C for 30 min, almost pure 3La₂O₃·2WO₃ was obtained in a single phase, though slight γ-La₂WO₆ and β-La₂W₂O₉ were present. It was thought that 3La₂O₃·2WO₃ was obtained as a degradation product of 7La₂O₃·8WO₃, simultaneously as a product of the reaction of La₂O₃ and γ-La₂WO₆ in the progress of its decomposition when a part of β-La₂W₂O₉ decomposed to γ-La₂WO₆. Furthermore, it was thought that α-La₂W₂O₉ was transformed into β-La₂W₂O₉, because the minimum in intensity of α-La₂W₂O₉ was corresponding well to the maximum in that of β-La₂W₂O₉.