Processes of the excitation energy migration and transfer in Ce-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 ⁶P_j multiplet of the lowest Gd³⁺ 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 Gd³⁺ in the excited ⁶P_j state has been revealed as a shift of Gd³⁺⁶P_j→⁸S_7/2 emission into the short-wavelength spectral range upon the temperature increase. The relaxation of excited Gd³⁺ via phonon-assisted population of Gd³⁺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 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 ⁶P_3/2 level of the ⁶P_j multiplet. An efficient reversed Ce³⁺→Gd³⁺ energy transfer has been revealed for the studied phosphates at 4.2 K.