This paper reports on a study of the luminescence emitted by Li 6 Gd(BO 3 ) 3 : Ce 3+ crystals under selective photoexcitation to lower excited states of the host ion Gd 3+ and impurity ion Ce 3+ within the 100-500 K temperature interval, where the mechanisms of migration and relaxation of electronic excitation energy have been shown to undergo noticeable changes. The monotonic 10-15 fold increase in intensity of the luminescence band at 3.97 eV has been explained within a model describing two competing processes, namely, migration of electronic excitation energy over chains of Gd 3+ ions and vibrational energy relaxation between the 6 I j and 6 P j levels. It has been shown that radiative transitions in Ce 3+ ions from the lower excited state 5d 1 to 2 F 5/2 and 2 F 7/2 levels of the ground state produce two photoluminescence bands, at 2.08 and 2.38 eV (Ce1 center) and 2.88 and 3.13 eV (Ce2 center). Possible models of the Ce1 and Ce2 luminescence centers have been discussed.

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