Processes of the excitation energy migration and transfer in Ce3+-doped alkali gadolinium phosphates studied with time-resolved photoluminescence spectroscopy technique

Spectral-kinetic characteristics of Gd3+ and Ce3+ luminescence from a series of Ce3+-doped alkali gadolinium phosphates of MGdP4O12 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 Gd3+ sub-lattice and energy transfer between the Gd3+ and Ce3+ ions have been investigated. Peculiarities of these processes have been compared for MGdP4O12 phosphate hosts with different alkali metal ions. A contribution of different levels from the 6Pj multiplet of the lowest Gd3+ excited state into the energy migration and transfer processes has been clarified. The phonon-assisted occupation of high-energy 6P5/2, 3/2 levels by Gd3+ in the excited 6Pj state has been revealed as a shift of Gd3+ 6Pj→8S7/2 emission into the short-wavelength spectral range upon the temperature increase. The relaxation of excited Gd3+ via phonon-assisted population of Gd3+ 6P5/2 level (next higher one to the lowest excited 6P7/2) is supposed to be responsible for the rise in probability of energy migration within the Gd3+ sub-lattice initiating the Gd3+→Ce3+ energy transfer at T<150 K, whereas further intensification of Gd3+→Ce3+ energy transfer at T>150 K is explained by the increase in probability of Gd3+ relaxation into the highest 6P3/2 level of the 6Pj multiplet. An efficient reversed Ce3+→Gd3+ energy transfer has been revealed for the studied phosphates at 4.2 K.