Neighboring Cation Co‐Substitution Engineering Strategy for Synergistic Energy Transfer for Efficient Broadband Near‐Infrared Luminescence in Lu _3‐x Ca _x Al _5‐x Si _x O ₁₂ :Ce ³⁺ , Cr <

Abstract Despite significant progress in the development of Cr 3+ ‐activated near‐infrared (NIR) phosphors, achieving highly efficient, thermally stable, and cost‐effective NIR phosphors remains a challenge. In this work, the Lu 3‐x Ca x Al 5‐x Si x O 12 :0.05Cr 3+ (CaxLAS:0.05Cr 3+ , 0 ≤ x ≤ 1.5) solid‐solution phosphors are synthesized through neighboring cation co‐substitution (Lu 3+ + Al 3+ → Ca 2+ + Si 4+ ). Structural Rietveld refinement, electron paramagnetic resonance, and fluorescence decay analyses confirm that the co‐substitution indirectly relaxes the [(Al/Cr)O 6 ] octahedral framework, enabling a dramatic transition in Cr 3+ emission from deep‐red sharp‐line spectra (687 nm) to broadband NIR emission (766 nm), while the full width at half maximum broadens from 25 to 172 nm. Notably, the external quantum efficiency (EQE) increases from 4.52% to 23.5%. These are attributed to the change in the mode of transition from the spin‐forbidden 2 E g → 4 A 2g to the spin‐allowed 4 T 2g → 4 A 2g and the formation of Cr 3+ pairs. Furthermore, by introducing Ce 3+ as a sensitizer, an EQE of 45.6% is achieved in the optimized Ca1.5LAS:0.08Ce 3+ ,0.05Cr 3+ composition. The phosphor retains 69.5% of its initial photoluminescence intensity at 427 K, demonstrating good thermal stability. This work not only develops a high‐performance solid‐solution phosphor for applications in non‐destructive testing and silicon solar cell efficiency enhancement but also highlights strategic approaches for advancing NIR phosphor design.