Changes in the electronic structure of Ti4O7 across the semiconductor-semiconductor-metal transitions.

Abstract

We present and discuss photoemission (PES) and O 1s x-ray-absorption spectra (XAS) of ${\mathrm{Ti}}_{4}$${\mathrm{O}}_{7}$ taken at different temperatures in the range 50--300 K. The PES taken at 300 K show Ti 3d bands at the Fermi level and O 2p bands at higher binding energies. The Ti 3d bands shift approximately 0.25 eV towards higher binding energies in the low-temperature semiconducting phase (50 K). The O 1s XAS are related, via the corresponding metal-oxygen hybridization, to the unoccupied electronic states in the conduction band. The XAS taken at 300 K reflect Ti 3d bands at threshold and Ti 4sp bands at higher photon energies. The Ti 3d bands are split by crystal-field effects into ${\mathit{t}}_{2\mathit{g}}$ and ${\mathit{e}}_{\mathit{g}}$ subbands and shift approximately 0.45 eV towards higher photon energies in the low-temperature semiconducting phase (80 K). The XAS are basically temperature independent in both the low-temperature semiconducting phase and in the high-temperature metallic phase. The main changes in the O 1s XAS appear rather suddenly at the semiconductor-metal transition around 150 K.