Plasmon and polar optical phonons in reduced rutileTiO2−x

The infrared reflectivity of reduced rutile $\mathrm{Ti}{\mathrm{O}}_{2-x}$ at room temperature is reported at departures $x$ from stoichiometry varying from ${10}^{-3\mathrm{}}$ to 5 × ${10}^{-2\mathrm{}}$. The electron carriers resulting from the lattice defects are responsible for a plasmon mode which couples with the longitudinal-optical phonon modes. A suitable model for the dielectric function involving a factorized form for the pure-phonon modes added to a plasmon contribution allows a good fit of calculated spectra to experimental data and is of special interest to decouple observed excitations into elementary phonon and plasmon modes. The parameters entering the decoupled phonon modes are then discussed in relation to the case of stoichiometric rutile, while a simple linear relation is found between the plasma frequency squared and the sample composition at room temperature. The plasma frequency rises for a small increase of temperature and it is concluded that activation of carriers to the conduction band should constitute the essential feature of the electrical-conductivity transport mechanism.