Non-resonant energy transfer between Tb3+and Eu3+in the cubic hexachloroelpasolite crystals Cs2NaTb1-xEuxCl6(x=0.01-0.15)

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Abstract
Luminescence decay curves have been measured for the Eu3+ emission and Tb3+ emission in the cubic crystals Cs2NaTb1-xEuxCl6 at 80 and 293 K using pulsed dye laser excitation into the 5D4 state of Tb3+ and the 5D1 state of Eu3+. For excitation of Tb3+, efficient energy transfer occurs to the 5D0 state (but not the 5D1 state) of Eu3+ and there is fast energy migration within the 5D4 state. When excitation is into the 5D1 state of Eu3+, phonon-assisted coupling to the Tb3+ ions results in enhanced relaxation to the 5D0 state but the lifetime of 5D0 is not affected by the presence of Tb3+. In all cases the non-resonant energy transfer involves creation or annihilation of phonons which have odd parity with respect to the metal centers and cannot be explained within the usual Born-Oppenheimer treatment of phonon-assisted energy transfer. The mechanism of the energy transfer processes is considered in detail.