Enlarging Environmental Asymmetry by the Site Tailoring to Enhance Luminescence Properties of Sm³⁺ in the (Gd,Na)₃(Ga,In)₂(Ga,Ge)₃O₁₂ Garnet

A structural design strategy was employed to develop NaGd₂Ga₂InGe₂O₁₂:Sm³⁺ (NGGIG:Sm³⁺) garnet phosphors, optimizing the local coordination environment to enhance luminescence performance. By tailoring site occupation, Sm³⁺ ions were incorporated into an asymmetric crystal field, where a statistical probability model confirmed that ∼97% of Sm³⁺ ions reside in distorted coordination environments, promoting electric dipole(ED) transitions. The phosphors exhibit a high red-to-orange (<i>R</i>/<i>O</i>) intensity ratio of 2.97, contributing to enhanced red emission. CIE chromaticity coordinates of (0.61, 0.39) indicate a significant shift toward the red region, and the phosphors retain over 70% of their luminescence intensity at 400 K, demonstrating good thermal stability. These characteristics make NGGIG:Sm³⁺ a promising candidate for high-performance red phosphors in solid-state lighting applications, highlighting the effectiveness of site engineering in garnet structures for optimizing luminescence efficiency and stability.