Dopant-dependent reflectivity and refractive index of microcrystalline HxWO3 and LixWO3 bronze thin films

Reflectivity spectra of HxWO3 and LiWO3 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 < or = x < or = 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 epsilon(hf) of MxWO3 (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 Ea and dispersion energy Ed 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.