Optical absorption and metal-insulator transitions in Ti₄O₇

Abstract

The optical transmission of Ti₄O₇ single crystals has been measured in the infrared at wavelengths between 3·8 and 6·8 μ in the temperature range of the metal-insulator transitions. In the low temperature phase (T>140K), the data are characteristic of a semiconductor with an optical gap of ∼ 0·25 eV. In both the intermediate (140 K< T < 150 K) and the metallic phases (T >150K), no optical transmission is observed. These results are discussed in relation to the bipolaron model and provide an upper estimation of the bipolaron binding energy.