Phase transformation of lithium tungsten bronzes, LixWO3, at room temperature ambient conditions

Samples of LixWO3 with x = 0.05–0.7 were synthesized at 700 °C for 7 days using appropriate amounts of Li2WO4, WO3 and WO2 in evacuated sealed silica tubes. The products reveal different phases of perovskite tungsten bronze (PTB). An interesting phenomenon observed for the PTB phases is the gradual change in colours when they are exposed at room temperature ambient conditions (in air). This effect has been investigated using X-ray powder diffraction, infrared absorption and optical reflectivity methods for the powdered samples before and after 30 and 90 days in air. The spectra of the samples with x = 0.25–0.5 are dominated by a peak with maximum around 16,000 cm−1 in the Kubelka Munk spectra which is related to the cubic LixWO3 phase. The peak intensity increases with increasing x. After 30 days of exposure in air this peak disappeared for x < 0.5 samples due to a diffusion of Li from LixWO3. X-ray and IR data show a gradual transformation into the lower symmetric phases (PTBcubic ⇒ PTBtetragonal ⇒ PTBorthorhombic ⇒ PTBmonoclinic). The results suggest that Li is attracted by O2 to the surface forming Li2O which further reacts with H2O and CO2 in air. The in air altered samples regain their original colour when reheated at 500 °C in vacuum.