Infrared Spectroscopy of (In,Nb) co-doped TiO₂

The infrared reflectance of ceramic samples of TiO₂ and 5 and 10 percent (In,Nb) co-doped TiO2 has been measured between 50 and 8000 cm^-1 for various temperatures between 4 and 300 K. Kramers Kronig derived optical conductivity spectra reveal four defect related modes at frequencies that are close to the Raman frequencies of rutile. Co-doping produces a significant loss in the partial spectral weight between 75 and 350 cm^-1 which is the spectral range associated with the lowest frequency transverse optical mode (the mode that softens significantly with decreasing temperature). The loss in spectral weight is of the same order of magnitude as the mid-infrared spectral weight observed in transmission measurements of reduced rutile and which has previously been identified with small polaron absorption.