Applicability of the shell model for energy transfer

In the cubic hexachloroelpasolite crystals Cs2NaSmxEuyGd1-x-yCl6 the emission from the 4G5/2 state of Sm3+ is strongly quenched by cross-relaxation and energy-transfer processes. At temperatures above 100 K the energy-transfer rate consists of non-resonant contributions due to electric dipole vibronic-electric dipole vibronic interaction and a near-resonant contribution where both the donor and acceptor transitions are magnetic-dipole allowed. The near-resonant energy-transfer rate is calculated and the small discrepancy between the theoretical and experimental values shows the applicability of the shell model for highly symmetric crystals. (orig.)