Preparation and characterization of tungstic acid C phases containing various cations

Abstract

Several tungstic acid C phases containing small amounts of cations, x(A 2O)·WO 3· y(H 2O) (A = Li +, Na +, K + or NH + 4, x = 0.05−1.3, y = 0.5−1.0), have been prepared by acidification of W(VI) solutions with strong acid at a concentration range of 1.5–2.0 N and at a temperature of 100°C. The amounts of the cations A contained in the C phase decrease with increasing ionic radius of the cations. When the C phases were heat-treated at ≈350°C in air, all the water molecules in the C phases were lost to form x(A 2O)·WO 3, while maintaining the original crystal structure. On heating above 450°C, however, the structures of the C phases except the K + salt were converted to a monoclinic cell. The Li + sallt has an orthorhombic cell and the other salts with larger cations have a hexagonal cell. The C phases have structural elements similar to those of a hexagonal WO 3. The electric conductivities of x( A 2 O)· WO 3 were of the order of 10 −8 S/cm at 25°C and they are mixed conductors. The cyclic voltammetric measurements revealed that a reversible lithium intercalation into the structure takes place with cycling.