Achieving Ultrahigh Thermal Stability in Cr 3+ -Activated Garnet Phosphors through Electron Migration between Thermally Coupled Levels.

Recently, Cr 3+ -activated near-infrared (NIR) phosphors have received much more attention due to their excellent photoluminescence (PL) properties. However, most of them suffer from poor thermal stability which limits further application. Herein, a novel Lu 2 CaGa 4 SnO 12 :Cr 3+ phosphor with broadband NIR emission (λ em = 750 nm) is synthesized successfully. Despite the good luminescence property, its PL intensity decreases obviously with temperature ( I 425 K = 79%). To improve the thermal stability, a series of Lu 2+ x Ca 1- x Ga 4+ x Sn 1- x O 12 :Cr 3+ ( x = 0-1.0) solid solutions with tunable thermal quenching performance have been designed. It is found that the fluorescence intensity ratio (FIR) of 4 T 2 → 4 A 2 to 2 E → 4 A 2 [ I ( 4 T 2 )/ I ( 2 E)] transitions (i.e. electron occupation) decreases monotonously with increasing [Lu 3+ -Ga 3+ ] co-substitution, resulting from a strengthened crystal field strength and increased energy difference between 4 T 2 and 2 E energy levels. Benefiting from the various thermal population and energy difference Δ', the PL thermal quenching behavior of Lu 2+ x Ca 1- x Ga 4+ x Sn 1- x O 12 :Cr 3+ can be adjusted easily, and the corresponding mechanism is explored in detail. Most notably, the emission intensity of Lu 2+ x Ca 1- x Ga 4+ x Sn 1- x O 12 :Cr 3+ at 425 K can reach up to 142% compared with that at 300 K, which may be the best for Cr 3+ -activated NIR phosphors. This work may provide an alternative path for the development of thermally stable broadband NIR phosphors.