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 60 000 mu V K-1 was observed near 10 K for the least reduced sample, which gives a thermoelectric power factor of 170 mu WK-2 cm(-1). This value is about four times higher than the power factor of Bi2Te3-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 0.83 W K-1 m(-1) for the heavily reduced sample at 390 K 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.