Self-Trapping in Mixed Crystal –Clustering, Dimensionality, Percolation–

Self-trapping of a Frenkel exciton in binary mixed crystal A c B 1- c , with excitation energy difference Δ = ε B - ε A and exciton-phonon coupling constant g , is studied. Impurity-assisted self-trapping is possible through cooperation of Δ and g even when neither of them alone is enough for localization. In this situation, one expects amalgamated absorption spectrum representing unrelaxed extended exciton and Stokes-shifted broad emission band representing relaxed localized exciton. We study whether relaxed exciton is localized (S-state) at a particular site of, or extended (B-state) throughout, the A -cltuster of various sizes, and how such cluster state changes, as c increases, into free (F) state extended throughout the crystal. The results are compared with recent experiments on TlBr c Cl 1- c . Sudden appearance of edge emission (due to F) at c ∼0.3, at the cost of broad band (due to S and B), is ascribed to the combined effect of size-dependent discontinuity of S-B transition in Br-clusters and their percolation.