Cr³⁺-activated Ca₃LnGa₃B₄O₁₅ (Ln = Gd, Y)-highly thermally stable NIR-emitting phosphors toward plant lighting application

Borate-based photoluminescent materials (phosphors) have attracted significant interest owing to their advantages, including low synthesis temperature, stable physicochemical properties, and simplified processing. Nevertheless, borate-based near-infrared (NIR) phosphors that exhibit both efficient excitation with blue LEDs (∼450 nm) and exceptional luminescence thermal stability remain scarce. In this study, a series of NIR phosphors, Ca₃LnGa₃B₄O₁₅:Cr³⁺ (Ln = Gd, Y; x = 0.1-4%), were synthesized via the high-temperature solid-phase method. These phosphors exhibit features such as efficient excitation with blue LEDs, a broad emission band with a full width at half maximum (FWHM) of ∼140 nm, and remarkable thermal stability. Specifically, at 473 K, Ca₃GdGa₃B₄O₁₅:0.5%Cr³⁺ retains 69% of its room temperature (RT) luminescence intensity. Substituting Gd³⁺ with Y³⁺ in this host significantly enhances luminescence thermal stability, with Ca₃YGa₃B₄O₁₅:0.5%Cr³⁺ retaining 84% of its RT intensity at 473 K while concurrently achieving a 1.8-fold increase in luminous intensity. This enhancement may be attributed to increased structural rigidity and the favorable ionic radius match between Cr³⁺ and Ga³⁺, which effectively suppresses non-radiative relaxation in loosely coordinated Cr³⁺ ions. These results demonstrate that this phosphor series holds potential for applications in fields such as plant-growth lighting.