Achieving Ultrahigh Thermal Stability in Cr3+-Activated Garnet Phosphors through Electron Migration between Thermally Coupled Levels

Recently, Cr³⁺-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 Lu₂CaGa₄SnO₁₂:Cr³⁺ 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+xCa_1-xGa_4+xSn_1-xO₁₂:Cr³⁺ (x = 0-1.0) solid solutions with tunable thermal quenching performance have been designed. It is found that the fluorescence intensity ratio (FIR) of ⁴T₂ → ⁴A₂ to ²E → ⁴A₂ [I(⁴T₂)/I(²E)] transitions (i.e. electron occupation) decreases monotonously with increasing [Lu³⁺-Ga³⁺] co-substitution, resulting from a strengthened crystal field strength and increased energy difference between ⁴T₂ and ²E energy levels. Benefiting from the various thermal population and energy difference Δ', the PL thermal quenching behavior of Lu_2+xCa_1-xGa_4+xSn_1-xO₁₂:Cr³⁺ can be adjusted easily, and the corresponding mechanism is explored in detail. Most notably, the emission intensity of Lu_2+xCa_1-xGa_4+xSn_1-xO₁₂:Cr³⁺ at 425 K can reach up to 142% compared with that at 300 K, which may be the best for Cr³⁺-activated NIR phosphors. This work may provide an alternative path for the development of thermally stable broadband NIR phosphors.