Reflectivity spectra of HxWO3 and LixWO3 thin films were measured over the photon energy range from 0.4 to 4.2 eV. It was found that microcrystalline tungsten bronzes have reflectances of 8%–30% over the dopant concentration range x (0 ≤ x ≤ 0.25). Values for the real part of refractive index n were also determined from the refined reflectivity data. The optical data are interpreted by use of a modified Drude-Zener model together with a single-oscillator model to differentiate between bound and free electronic states. The values of high-frequency dielectric constant εhf of M xWO3 (M = H +, Li+) bronzes were determined from the refractive-index data for estimation of the effective electronic masses involved in optical and polaronic transitions. A single-oscillator model showed that oscillator energy E a and dispersion energy E d increased and decreased, respectively, with increasing x values, opposite what occurs in crystalline tungsten bronzes. These findings support the fact that Bloch electrons are almost absent; instead, the polaronic species (W5+ and W4+) are assumed to control the reflectivity modifications (or variations in the refractive index) that are associated with the microcrystalline tungsten bronzes.
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