Heterovalent Substitution of K₂SrP₂O₇:Cr³⁺ to Achieve Anti-Thermal-Quenching Broadband Near-Infrared Luminescence

Broadband near-infrared (NIR) light sources based on phosphor-converted light-emitting diodes are highly desirable for biochemical analysis and medical diagnosis applications. However, thermal quenching remains a demanding challenge for developing efficient NIR phosphors. Herein, we report the enhancement of both quantum efficiency and thermal stability in Cr³⁺-activated K₂SrP₂O₇ phosphors through a heterovalent substitution strategy by replacing Sr²⁺ with Al³⁺ in K₂Sr_1-<i>x</i> Al _<i>x</i> P₂O₇ (0.05 ≤ <i>x</i> ≤ 0.2) to obtain optimized broadband NIR emission. Structural modulation via Al³⁺ substitution leads to the optimized composition, K₂Sr_0.88Al_0.1P₂O₇:0.02Cr³⁺, which emits across a broad NIR range of 650-1100 nm peaking at 807 nm with a full width at half-maximum of ∼130 nm under 448 nm excitation. Remarkably, its emission intensity at 150 °C remains 120% of the initial value at room temperature, demonstrating a rare antithermal-quenching behavior. Temperature-dependent XRD studies further reveal that Al³⁺ substitution effectively suppresses lattice expansion at elevated temperatures, indicating enhanced lattice stability under thermal excitation. Detailed structural and spectral analyses show that the substitution enhances local site symmetry, reduces electron-phonon coupling, increases thermally induced absorption probability, and fortifies energetic barriers against nonradiative transitions. These synergistic effects collectively endow this NIR phosphor with a superior thermal stability. Furthermore, NIR light-emitting diodes fabricated with this phosphor exhibit strong potential for applications in information identification, nondestructive detection, and night vision technologies. This study demonstrates a local structure engineering strategy for designing thermally robust Cr³⁺-activated NIR phosphors, offering valuable insights into material discovery and NIR spectroscopy device development.