Temperature dependence of small polaron population decays in iron-doped lithium niobate by Monte Carlo simulations

•The lifetime of bound polarons exhibits 3 regimes of different activation energies.•The radii of polaron traps shrink at rising temperature (trap-size effect, TSE).•The TSE explains why the β-exponent of Kohlrausch-like decays increases at rising T.•It may explain other misunderstood features of light-induced effects.

The population decay of light-induced small polarons in iron-doped lithium niobate is simulated by a Monte-Carlo method on the basis of Holstein's theory. The model considers random walks of both bound polarons (NbLi4+) and free polarons (NbNb4+) ending to deep traps (FeLi3+). The thermokinetic interplay between polaron species is introduced by trapping and de-trapping rates at niobium antisites (NbLi). The decay of the NbLi4+ population proceeds by three possible channels: direct trapping at FeLi3+ sites, hopping on niobium antisites and hopping on Nb regular sites after conversion to the free state. Up to three regimes, each one reflecting the predominance of one of these processes, appear with different activation energies in the Arrhenius plots of the decay time. The influence of FeLi and NbLi concentrations on the transition temperatures is evidenced. For both polaron species, the length of the final hop (trapping length) is found much larger than the usual hopping length and decreases at rising temperature. This trap size effect is a natural consequence of Holstein's theory and may explain some unclear features of polaron-related light-induced phenomena, such as the temperature-dependent stretching exponent of light-induced absorption decays and the anomalous increase of the photoconductivity at high doping levels.

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