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

Samples of Li_xWO₃ 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 Li_xWO₃ 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 Li_xWO₃. X-ray and IR data show a gradual transformation into the lower symmetric phases (PTB_cubic ⇒ PTB_tetragonal ⇒ PTB_orthorhombic ⇒ PTB_monoclinic). 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.