Stable Polyhedron-Cluster-Based Rigid Fluoride Framework toward Zero-Thermal-Quenching Near-Infrared Emission for Prolonged LED Applications

Anextremely thermally stable Cr3+-doped fluorideCs(2)NaAl(3)F(12):Cr3+ NIR phosphorwith a [Al6Na4F45] cluster-connected3D rigid framework has been first demonstrated. Development of highly thermally stable broadband near-infrared(NIR) luminescence materials is crucial for advancing the prolongedstable application of smart NIR light sources. In this study, a zero-thermal-quenchingand reversible temperature-dependent broadband NIR-emitting Cs2NaAl3F12:Cr3+ phosphor isdemonstrated, benefiting from its stable polyhedron-cluster-buildingrigid structure. The excellent thermal stability of Cs2NaAl3F12:Cr3+ is rooted in its stable[Al6Na4F45] cluster building unit,which provides a rigid structure with a weak electron-phononcoupling effect and a wide band gap with a huge thermal activatedbarrier. Such characteristics are well revealed by multiple studieson crystal structure, electronic structure, Huang-Rhys factor S, configuration coordinate model, and Debye temperature.The incorporation of Li or K instead of Na weakens the luminescencethermal stability, directly proving the importance of the stable [Al6Na4F45] cluster for stable Cr3+ substitution and rigid structure construction. Furthermore, Cs2NaAl3F12:Cr3+ presents muchsuperior thermal stability compared to traditional rigid garnet-typefluorides Na3X2Li3F12:Cr3+ (X = Al, Ga, In). A high-power NIR LED is presented, utilizingthe high quantum efficiency (& SIM;71%) and extremely thermallystable broadband NIR emission around 750 nm of Cs2NaAl3F12:Cr3+. It realizes clear vein andcartilage imaging in the human hand, demonstrating its potential inmedical diagnosis applications. This result provides important insightsfor designing new-type rigid crystal structures using stable polyhedronclusters as basic units, advancing the development of highly thermallystable NIR-emitting phosphors.