Lithium intercalation into ${\mathrm{WO}}_3$ and the phase diagram of ${\mathrm{Li}}_{\mathit{x}}$${\mathrm{WO}}_3$

We report studies of the electrochemical intercalation of lithium in ${\mathrm{WO}}_3$ powder and in polycrystalline evaporated ${\mathrm{WO}}_3$ films. Electrochemical cells with beryllium x-ray windows were used to study the structure changes in the ${\mathrm{WO}}_3$ host in situ. Rietveld profile refinement was used to analyze the x-ray-diffraction patterns of ${\mathrm{Li}}_{\mathit{x}}$ ${\mathrm{WO}}_3$ after correcting for the absorption of the beryllium window. Upon the intercalation of Li, the monoclinic structure of ${\mathrm{WO}}_3$ proceeds to higher and higher symmetry. First a first-order phase transition to a tetragonal phase with x∼0.1 in ${\mathrm{Li}}_{\mathit{x}}$ ${\mathrm{WO}}_3$ occurs. Then another first-order phase transition to a cubic phase with x∼0.36 occurs. The cubic phase can accommodate further Li, up to a maximum of about x=0.5. All the phase transitions can be reversed, although some hysteresis is observed; upon complete deintercalation, monoclinic ${\mathrm{WO}}_3$ is recovered. We report the structural changes, prove that the transitions are first order, and give a phase diagram for ${\mathrm{Li}}_{\mathit{x}}$ ${\mathrm{WO}}_3$ as a function of x. Our results suggest that the formation of the tetragonal phase upon intercalation of Li in ${\mathrm{WO}}_3$ is responsible for the electrochromatic properties of crystalline ${\mathrm{Li}}_{\mathit{x}}$ ${\mathrm{WO}}_3$, and that conventional interpretations that ignore the existence of this phase are most likely in error.