  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.
