Non-radiative relaxation processes of the Pr 3+ ion in solids

Abstract

The 3P 0 → 1D 2 non-radiative decay rates A NR of Pr 3+ and the transition probabilities A VIB for the vibronic 3H 4 → 3P 0 excitation transitions of Pr 3+ were measured at 4.2 K for Pr 3+ in several host lattices, ranging from hosts where the non-radiative decay is negligible (e.g. LiYF 4:Pr, A NR = 50 s −1) to hosts where it dominates (e.g. Lu 2O 3:Pr, A NR ⩾ 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 4f 5d states, and a force constant stronger in the 4f 5d state than in the 4f 2 states) fast 3P 0 → 1D 2 non-radiative relaxation via the 4f 5d state becomes the dominant relaxation mechanism, even at 4.2 K.