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) excitation transitions of Pr3+ were measured at 4.2 K for Pr3+ in several host lattices, ranging from hosts where the non-radiative decay is negligible (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 compounds 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 radiationless transition, the multiphonon relaxation rates are observed to be greatly enhanced by increasing covalency and/or decreasing Pr-ligand distances. The enhanced multiphonon relaxation is ascribed to a stronger electron-phonon coupling. The increase in the electron-phonon coupling strength is confirmed by the increase of the vibronic transition probabilities A(VIB) in the same sequence as the A(NR) rates. The observed host 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 4f5d 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 mechanism, even at 4.2 K.