  Rutile TiO2 is a paradigmatic transition metal oxide with applications in
optics, electronics, photocatalysis, etc., that are subject to pervasive
electron-phonon interaction. To understand how energies of its electronic
bands, and in general semiconductors or metals where the frontier orbitals have
a strong d-band character, depend on temperature, we perform a comprehensive
theoretical and experimental study of the effects of electron-phonon (e-p)
interactions. In a two-photon photoemission (2PP) spectroscopy study we observe
an unusual temperature dependence of electronic band energies within the
conduction band of reduced rutile TiO2, which is contrary to the well
understood sp-band semiconductors and points to a so far unexplained dichotomy
in how the e-p interactions affect differently the materials where the frontier
orbitals are derived from the sp- and d-orbitals. To develop a broadly
applicable model, we employ state-of-the-art first-principles calculations that
explain how phonons promote interactions between the Ti-3d orbitals of the
conduction band within the octahedral crystal field. The characteristics
differences in e-p interactions experienced by the Ti 3d-orbitals of rutile
TiO2 crystal lattice are contrasted with the more familiar behavior of the Si
2s-orbitals of stishovite SiO2 polymorph, in which the frontier 2s-orbital
experience a similar crystal field with the opposite effect...
