Electrochromic thin films have been deposited from peroxopolytungstic acid aqueous solutions. These solutions are obtained via the dissolution of tungstic acid in hydrogen peroxide. The nature and properties of the films depend on the drying procedure. Amorphous films are formed upon fast drying. They exhibit good electrochromic properties but have to be heat treated in order to become insoluble. Crystalline films are formed upon slow drying. Their electrochromic properties are not very good but better amorphous or crystalline films are then obtained after a thermal treatment above 200°C. In both cases, the reversibility of lithium insertion is not good during the first cycles but improve significantly upon cycling. This should be due to the electrochemical reduction of peroxo groups.

Stable solutions of WO3precursors have been prepared via the dissolution of tungstic acid, H2WO4, in hydrogen peroxide. A crystalline peroxopolytungstic acid WO3·H2O2·nH2O (n≈0.1) is obtained upon drying. Peroxo groups decompose at 200°C, giving an amorphous tungsten oxide that crystallizes into the stable monoclinic WO3around 400°C. Completely different results are obtained when Ti(OPri)4is added to the precursor solution. The orthorhombic phase WO3·13H2O is first obtained. As is well known, this hydrated oxide leads to h-WO3and m-WO3upon heating. However, in the presence of TiIV, a new metastable orthorhombic tungsten oxide is formed around 400°C. It then transforms irreversibly upon further heating into the stable monoclinic WO3. The presence of TiIVseems to stabilize this new orthorhombic phase.