  Femto-/nanosecond pulse-induced, red and near-infrared absorption is studied
in $\mathrm{LiNb}_{1-x}\mathrm{Ta}_{x}\mathrm{O}_3$ (LNT) solid solutions with
the goal to probe the intrinsic defect structure via the formation, transport
and recombination of optically generated small bound electron polarons with
strong coupling to the lattice. As a result, long-lived transients are
uncovered for LNT which exceed lifetimes of LN and LT by a factor of up to 100
over the entire range of investigated compositions. At the same time, the
starting amplitude varies in the range of
$\alpha_\mathrm{li}^0\approx10-100\,\mathrm{m}^{-1}$ as a function of $x$ and
exceed the ones of LN and LT by a factor of up to ten. The results are
interpreted in the model of three-dimensional small polaron hopping transport
considering the simultaneous presence of three different types of small bound
polarons, in particular of small electron $\mathrm{Nb}_\mathrm{Li}^{4+}$ and
$\mathrm{Ta}_\mathrm{Li}^{4+}$ antisite polarons, and of small electron
$\mathrm{Ta}_\mathrm{V}^{4+}$ interstitial polarons. We conclude that the
differences between LNT, LN, and LT may point to model systems that consist of
one (LN), two (LT) and three (LNT) intrinsic defect centers for electron
localization.
