Sodium-Based Germanate Garnet Phosphors: Fundamentals, Luminescence Regulation and Applications

Garnet-structured compounds have long been recognized as versatile host platforms for luminescent phosphors, owing to their robust chemical stability and highly tunable cationic sublattices. Conventional aluminate and gallate garnets, however, generally face intrinsic limitations, including severe concentration quenching at high dopant levels, relatively high phonon energy, and unsatisfactory efficiency for long-wavelength near-infrared (NIR) emission. In recent years, sodium-based germanate garnets, constructed by introducing aliovalent Na+ into dodecahedral sites combined with Ge4+ substitution in tetrahedral frameworks, have emerged as a promising branch of garnet phosphors. The aliovalent Na+ incorporation brings unique structural effects, including energy migration blocking, coordination environment distortion, and defect level modulation, which endow the materials with advantages in mitigating concentration quenching, boosting energy transfer efficiency, and enhancing thermal stability. This review systematically summarizes the crystal structure classification and luminescence fundamentals of sodium-based germanate garnet systems, and introduces mainstream synthesis techniques represented by the high-temperature solid-state method. Focusing on representative host systems, the luminescence characteristics and energy transfer mechanisms of both rare earth- and transition metal-doped systems are elaborated in detail, followed by a summary of four representative performance regulation strategies: cationic disorder engineering, crystal field engineering, defect engineering and dual-site cooperative regulation. The multifunctional applications of these materials in white light-emitting diodes, plant growth lighting, fluorescence temperature sensing, NIR imaging, and information encryption are also presented. Finally, existing challenges and future research perspectives are proposed to provide guidance for the development of high-performance garnet phosphors.