Colossal positive Seebeck coefficient and low thermal conductivity in reduced TiO₂

Reduced (oxygen deficient) single crystal TiO₂ exhibits a very large positive Seebeck coefficient S at low temperature. S as large as 60 000νVK^-1 was observed near 10K for the least reduced sample, which gives a thermoelectric power factor of 170νWK^-2cm^-1. This value is about four times higher than the power factor of Bi ₂Te₃-based materials near room temperature. As the temperature increases the Seebeck coefficient becomes negative. The magnitude of the room temperature Seebeck coefficient, thermal conductivity and electrical resistivity decrease with the reduction of the samples. The thermal conductivity is as low as 0.83WK^-1m^-1 for the heavily reduced sample at 390K due to phonon scattering by defect planes. The colossal Seebeck coefficient found in the materials is discussed in terms of the phonon drag of the holes.