Optical study of metal–insulator transitions in Ti4O7 single crystals

We have investigated the optical properties of Ti4O7 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.