Small-polaron-induced infrared opacification in rutile TiO2

The infrared emittance of single-crystal rutile TiO 2 has been measured from room temperature to 2000 K in dry air for two polarization states. A very strong thermally induced opacification phenomenon has been found, with the sample being completely opaque in the near-infrared range several hundred K before its melting point. A large contribution from small-polaron hopping conduction can explain this anomalous degree of opacification, much stronger than similar phenomena present in other octahedrally coordinated oxide materials. These quasiparticles manifest in the high-temperature optical conductivity as a thermally activated contribution, with an activation energy close to that of DC conductivity. The small-polaron nature of this phenomenon is strengthened by the observation of broad near-infrared bands.