Optical properties of Nb-doped SrTiO3 single crystal

Far-infrared reflectivity spectra of (1 0 0) SrTi1−xNbxO3 single crystal have been measured under different temperatures in the frequency region between 100 and 6000 cm−1. The obtained spectra are analyzed based on the double damping extended Drude model. Extra absorption near the highest frequency longitudinal phonon mode shows that the polaron coupling must be taken into account. The splitting of the last high reflectivity band, the temperature dependence of the 615 cm−1 structure and the appearance of a new weak peak-like defect mode provide some evidence for the existence of small polarons.