Oxygen-vacancy driven electron localization and itinerancy in rutile-based TiO${}_2$

Abstract

Oxygen-deficient TiO${}_2$ in the rutile structure as well as the Ti${}_3$O${}_5$ Magn{é}li phase is investigated within the charge self-consistent combination of density functional theory (DFT) with dynamical mean-field theory (DMFT). It is shown that an isolated oxygen vacancy (V${}_{\mathrm{O}}$) in titanium dioxide is not sufficient to metallize the system at low temperatures. In a semiconducting phase, an in-gap state is identified at $\varepsilon_{\rm IG}^{\hfill}\sim -0.75\,$eV\, in excellent agreement with experimental data. Band-like impurity levels, resulting from a threefold V${}_{\mathrm{O}}$-Ti coordination as well as entangled $(t_{2g},e_g)$ states, become localized due to site-dependent electronic correlations. Charge localization and strong orbital polarization occur in the V${}_{\mathrm{O}}$-near Ti ions, which details can be modified by a variation of the correlated subspace. At higher oxygen vacancy concentration, a correlated metal is stabilized in the Magn{é}li phase. A V${}_{\mathrm{O}}$-defect rutile structure of identical stoichiometry shows key differences in the orbital-resolved character and the spectral properties. Charge disproportionation is vital in the oxygen-deficient compounds, but obvious metal-insulator transitions driven or sustained by charge order are not identified.

11 pages, 11 figures