Based on a model of coupled processes with differently time-dependent decay kinetics we present a critical review on photoluminescence (PL) and transient absorption (TA) experiments in undoped and Mg or Fe-doped LiNbO ₃ , together with a comprehensive interpretation of visible radiative and parallel non-radiative decay processes on timescales ranging from 50 ns up to minutes. Analogies and peculiarities of the kinetics of mobile self-trapped and pinned excitons are investigated and compared with those of hopping polarons in the same system. Exciton hopping with an activation energy of ≈0.18 eV is shown to govern the lifetime and quenching of the short PL component above 100 K. Strong interaction between excitons and dipolar pinning defects explains the exorbitant lifetimes and large depinning energies characterizing delayed TA components in doped LiNbO ₃ , while restricted hopping of the pinned excitons is proposed to play a role in strongly delayed PL in LiNbO ₃ :Mg exhibiting a narrowed emission band due to locally reduced electron–phonon coupling. Atomistic models of pinned excitons are proposed corresponding to charge-compensated dipolar defects predicted by theories of dopant incorporation in LiNbO ₃ and are systematically assigned to absorption bands observed near the UV edge. Excitation in these bands is shown to lead directly to pinned exciton states confirming also the previously proposed two-step exciton-decay scenario in LiNbO ₃ :Fe. Weak intrinsic sub-80 ns luminescence in congruent LiNbO ₃ is explained as an opposite effect of enhanced electron–phonon coupling for excitons pinned on Nb _Li antisite defects. The comparison of the different observed stretching behaviors in the paradigmatic system LiNbO ₃ provides an intuitive picture of the underlying physical processes. The findings are relevant not only for holographic and non-linear optical applications of LiNbO ₃ but are of general interest also for the treatment of stretched exponential or other time-dependent kinetics in complex condensed systems ranging from nanocrystals and polymers to liquids and biophysical systems.

The recent Special Issue on lithium niobate (LiNbO₃) is dedicated to Prof. Schirmer and his topics and contains nineteen papers, out of which seven review various aspects of intrinsic and extrinsic defects in single crystals, thin films, and powdered phases; six present brand-new results of basic research, including two papers on Li(Nb,Ta)O₃ mixed crystals; and the remaining six are related to various optical and/or thin film applications.

The present review is intended for a broader audience interested in the resolution of the several decades-long controversy on the possible role of oxygen-vacancy defects in LiNbO₃. Confronting ideas of a selected series of papers from classical experiments to brand new large-scale calculations, a unified interpretation of the defect generation and annealing mechanisms governing processes during thermo- and mechanochemical treatments and irradiations of various types is presented. The dominant role of as-grown and freshly generated Nb antisite defects as traps for small polarons and bipolarons is demonstrated, while mobile lithium vacancies, also acting as hole traps, are shown to provide flexible charge compensation needed for stability. The close relationship between LiNbO₃ and the Li battery materials LiNb₃O₈ and Li₃NbO₄ is pointed out. The oxygen sublattice of the bulk plays a much more passive role, whereas oxygen loss and Li₂O segregation take place in external or internal surface layers of a few nanometers.

For a better understanding of the outstanding real-time photorefractive medium ${\mathrm{LiNbO}}_3$:(Bi,Mg) and excitonic processes in similar systems in general, stoichiometric ${\mathrm{LiNbO}}_3$ (SLN) crystals grown by two different methods and doped by Bi, Fe, or (Bi,Mg) have been investigated using low-intensity near-UV irradiation and absorption spectroscopy. Tracking down the incorporation of dopants and impurities including Fe and H by chemical and IR-UV spectroscopic analyses we assign the various spectroscopic features to specific defect complex types. Absorption bands near 4 eV overlapping the UV band edge and polarized along Nb–O bonds are ascribed to the generation of self-trapped excitons (STEs) pinned to dipolar defects consisting of $\mathrm{B}{\mathrm{i}}^{3+}$ or $\mathrm{F}{\mathrm{e}}^{3+}$ ions and their charge compensators, becoming exceptionally long-lived in the presence of lone pairs on ${\mathrm{Bi}}^{3+}$ ions. Near-UV illumination induces persistent additional absorption bands at ≈2.7 and ≈3.4 eV in Bi-single-doped crystals which can be eliminated by a short annealing above $\approx 150{}^{\circ }\mathrm{C}$ or fade within a few months in the dark. These and similar photochromic bands in SLN:(Bi,Mg) are attributed to further excitations of pinned STEs, resulting in charge release. Shortening of the buildup time of the photochromic bands in (Bi,Mg)-codoped crystals is ascribed to the absence of ${\mathrm{Nb}}_{\mathrm{Li}}$ antisite complexes serving as exciton sinks in under-threshold LN. Due to its markedly different properties an additional stable narrow band at 3.24 eV always present in SLN:(Bi,Mg) is attributed to fast reversible charge transfer processes from ${\mathrm{O}}^{2–}$ to $\mathrm{B}{\mathrm{i}}^{5+}$ ions on the Nb site. Processes on widely differing timescales are predicted for one- and two-photon excitations involving different excited states, relevant for further optimization of the system.

The energy levels of Dy³⁺ ions have been determined in lithium yttrium borate (Li₆Y(BO₃)₃) single crystals in a wide spectral range between 3000 and 40,000 cm⁻¹ using optical absorption and luminescence spectroscopy, which also allow for an analysis of the ground state. The crystal field splittings of the ⁶H_15/2 ground state and all excited states up to the ⁴F_7/2 manifold were obtained at a low temperature, based on luminescence (T = 4.2–78 K) and absorption (T = 8–100 K) measurements, respectively. The numbers of experimentally observed Stark sublevels are in agreement with those expected theoretically for Dy³⁺ ions occupying a single low symmetry (C₁) site.