Luminescence emission of Tm3+, in YAG codoped with Cr3+, under selecti
ve (into 2E Cr3+, levels or F-3(3) Tm3, levels) and nonselective (into
4T2 Cr3+ level) excitation is dominated by three new centers. Based o
n structural and spectral characteristics, these centers are associate
d with the three possible perturbations produced by Cr3+ ions at the n
earest neighbour TM3+ ions. A selective energy transfer within these C
r3+-TM3+ pairs is observed.