Small electron polarons bound to interstitial tantalum defects in lithium tantalate

The absorption features of optically generated, short-lived small bound electron polarons are inspected in congruent lithium tantalate, ${\rm LiTaO}_3$ (LT), in order to address the question whether it is possible to localize electrons at interstitial ${\rm Ta_V}$:${\rm V_{Li}}$ defect pairs by strong, short-range electron-phonon coupling. Solid-state photoabsorption spectroscopy under light exposure and density functional theory are used for an experimental and theoretical access to the spectral features of small bound polaron states and to calculate the binding energies of the small bound ${\rm Ta}_{\rm Li}^{4+}$ (antisite) and ${\rm Ta}_{\rm V}^{4+}$:${\rm V_{Li}}$ (interstitial site) electron polarons. As a result, two energetically well separated ($\Delta E \approx 0.5\,{\rm eV}$) absorption features with a distinct dependence on the probe light polarization and peaking at $1.6\,{\rm eV}$ and $2.1\,{\rm eV}$ are discovered. We contrast our results to the interpretation of a single small bound ${\rm Ta}_{\rm Li}^{4+}$ electron state with strong anisotropy of the lattice distortion and discuss the optical generation of interstitial ${\rm Ta}_{\rm V}^{4+}$:${\rm V_{Li}}$ small polarons in the framework of optical gating of ${\rm Ta}_{\rm V}^{4+}$:${\rm Ta}_{\rm Ta}^{4+}$ bipolarons. We can conclude that the appearance of carrier localization at $\mathrm{Ta_V}$:${\rm V_{Li}}$ must be considered as additional intermediate state for the 3D hopping transport mechanisms at room temperature in addition to ${\rm Ta_{Li}}$, as well, and, thus, impacts a variety of optical, photoelectrical and electrical applications of LT in nonlinear photonics. Furthermore, it is envisaged that LT represents a promising model system for the further examination of the small-polaron based photogalvanic effect in polar oxides with the unique feature of two, energetically well separated small polaron states.