NONRADIATIVE RELAXATION PROCESSES OF THE PR3+ ION IN SOLIDS

The P-3(0) --> D-1(2) non-radiative decay rates A(NR) of Pr3+ and the transition probabilities A(VIB) for the vibronic H-3(4) --> P-3(0) exc itation transitions of Pr3+ were measured at 4.2 K for Pr3+ in several host lattices, ranging from hosts where the non-radiative decay is ne gligible (e.g. LiYF4:Pr, A(NR) = 50 s(-1)) to hosts where it dominates (e.g. Lu2O3:Pr, A(NR) greater than or equal to 10(7) s(-1)). The comp ounds investigated can be divided into two groups concerning the order of the multiphonon relaxation: 6-7 and 4-5 phonons. Besides the well- known dependence on the number of phonons involved in the radiationles s transition, the multiphonon relaxation rates are observed to be grea tly enhanced by increasing covalency and/or decreasing Pr-ligand dista nces. The enhanced multiphonon relaxation is ascribed to a stronger el ectron-phonon coupling. The increase in the electron-phonon coupling s trength is confirmed by the increase of the vibronic transition probab ilities A(VIB) in the same sequence as the A(NR) rates. The observed h ost lattice dependence of A(NR) can be qualitatively accounted for by a nonlinear multiphonon relaxation mechanism. Under certain conditions (viz. low-lying 4f5d states, and a force constant stronger in the 4f5 d state than in the 4f(2) states) fast P-3(0) --> D-1(2) non-radiative relaxation via the 4f5d state becomes the dominant relaxation mechani sm, even at 4.2 K.