Optical study of metal–insulator transitions in Ti4O7 single crystals

Abstract We have investigated the optical properties of Ti 4 O 7 single crystals. It is found that the reflection spectrum changes discontinuously at about 150 and 130 K. These temperatures correspond to two-phase transitions, i.e., metal–semiconductor and semiconductor–semiconductor transition. The spectra are analyzed with the use of a classical dielectric function. It is inferred that the changes in the spectrum at the two-phase transitions originate from formation of bipolarons, which induce a splitting of the Ti 3d band into bonding and antibonding states.