Long-lived, pulse-induced absorption in $\mathrm{LiNb}_{1-x}\mathrm{Ta}_x\mathrm{O}_3$ solid solutions: the case of three intrinsic defect sites for electron localization with strong coupling

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.