Processes of the excitation energy migration and transfer in Ce 3+-doped alkali gadolinium phosphates studied with time-resolved photoluminescence spectroscopy technique

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Spectral-kinetic characteristics of Gd 3+ and Ce 3+ luminescence from a series of Ce 3+-doped alkali gadolinium phosphates of MGdP 4O 12 type (M=Li, Na, Cs) have been studied within 4.2–300 K temperature range using time-resolved luminescence spectroscopy techniques. The processes of energy migration along the Gd 3+ sub-lattice and energy transfer between the Gd 3+ and Ce 3+ ions have been investigated. Peculiarities of these processes have been compared for MGdP 4O 12 phosphate hosts with different alkali metal ions. A contribution of different levels from the 6P j multiplet of the lowest Gd 3+ excited state into the energy migration and transfer processes has been clarified. The phonon-assisted occupation of high-energy 6P 5/2, 3/2 levels by Gd 3+ in the excited 6P j state has been revealed as a shift of Gd 3+ 6P j→ 8S 7/2 emission into the short-wavelength spectral range upon the temperature increase. The relaxation of excited Gd 3+ via phonon-assisted population of Gd 3+ 6P 5/2 level (next higher one to the lowest excited 6P 7/2) is supposed to be responsible for the rise in probability of energy migration within the Gd 3+ sub-lattice initiating the Gd 3+→Ce 3+ energy transfer at T<150 K, whereas further intensification of Gd 3+→Ce 3+ energy transfer at T>150 K is explained by the increase in probability of Gd 3+ relaxation into the highest 6P 3/2 level of the 6P j multiplet. An efficient reversed Ce 3+→Gd 3+ energy transfer has been revealed for the studied phosphates at 4.2 K.