C-Axis Dimer and Its Electronic Breakup: The Insulator-to-Metal Transition in Ti2O3

We report on our investigation of the electronic structure of Ti2O3 using (hard) X-ray photoelectron and soft X-ray absorption spectroscopy. From the distinct satellite structures in the spectra, we have been able to establish unambiguously that the Ti-Ti c-axis dimer in the corundum crystal structure is electronically present and forms an a1ga1g molecular singlet in the low-temperature insulating phase. Upon heating, we observe a considerable spectral weight transfer to lower energies with orbital reconstruction. The insulator-metal transition may be viewed as a transition from a solid of isolated Ti-Ti molecules into a solid of electronically partially broken dimers, where the Ti ions acquire additional hopping in the a-b plane via the egπ channel, the opening of which requires consideration of the multiplet structure of the on-site Coulomb interaction.