Abstract
The luminescence spectra of Pr3+ in Na5La(MO4)4 (M=Mo, W) at 4.2 K have been investigated. One- and two-phonon vibronic replicas are observed in the excitation spectra, suggesting that the contribution of the Delta process (Franck-Condon replicas) to the vibronic coupling strength of Pr3+ is significant. The transition probability Avib for the vibronic lines in the 3H4 to 3P0 excitation spectra of Pr3+ in several isostructural (scheelite-like) host lattices scales with the transition probability for the zero-phonon line. This is consistent with a vibronic coupling in which the Delta process dominates. The transition probabilities Avib for the 3H4 to 3P2 excitation and the 3P0 to 3F2 emission transitions, and the vibronic spectra of Pr3+ in Na5La(MO4)4 give additional support for this conclusion.