Optical constants and electrochromic characteristics of M_xWO₃ bronzes

Concentration (x) dependent ellipsometric data on three tungsten bronzes (H_xWO₃, Li_xWO₃, Na_xWO₃) are investigated to examine the optical constants (n, k) and their electrochromic characteristics. Maximum decrease in the refractive index (n) and maximum increase in the extinction coefficient (k) are found as the limits of electrochromic effects. Differences in the optical constants of these three tungsten bronzes are perhaps due to different impurity scattering mechanisms associated with their individual unit cell configurations. Optical densities, coloration efficiencies, and the complex dielectric loss functions (-1/ϵ) of tungsten bronzes have been calculated as the electrochromic parameters to be used in the electrochromic display systems. Oscillator strengths, dipole strengths, and the degrees of delocalization of the polaronic wave functions are calculated and found to be very low, giving the convincing argument that there is no indication of Drude free electron behavior in these bronzes, at least in the investigated concentration range.