Switching Ti Valence in ${\mathrm{SrTiO}}_3$ by a dc Electric Field

A (001) ${\mathrm{SrTiO}}_3$ wafer has been investigated in situ at room temperature under application of a static electric field of varying polarity by fluorescence x-ray absorption near edge structure (XANES) analysis at the Sr-$K$ and Ti-$K$ absorption edges. The XANES spectra show a clear shift of the Ti-$K$ absorption edge energy. The shift is attributed to a change of the Ti valence state in a volume invoked by diffusion of the oxygen ions and vacancies. No shift was observed for the Sr-$K$ absorption edge energy. Theoretical calculations support these findings.

Figure 1

Schematic representation of the experimental setup with incident angle $\omega$ of the exciting x rays.

Figure 2

XANES spectra for photon energies at (a) the Sr-$K$ and (b) the Ti-$K$ (ROI: 4.3…5.0 keV) absorption edges under application of a dc electric field. The inset in (a) gives an enlarged view of the Sr-$K$ absorption edge region. An exemplary XANES spectrum for the completely integrated energy range (ROI: 3.1…8.5 keV) of the fluorescence yield showing the well-known preedge features of Ti is depicted in the inset of (b).

Figure 3

Sketch of the supercell used for the $\mathrm{LSDA}+U$ calculation. The different Ti environments and the oxygen vacancy are indicated.