Crystal‐Field‐Driven Broadband‐to‐Narrowband Transition in Cr ³⁺ ‐Activated La ₃ (Sc _{1‐ x} Al _x ) ₂ Ga ₃ O ₁₂ Garnet‐Perovskite Solid Solutions

Abstract Transition‐metal activators such as Cr ³⁺ offer an exceptional platform for probing the interplay between crystal structure and electronic transitions in solid‐state luminescent materials. Here, a composition‐controlled La ₃ Sc ₂ Ga ₃ O ₁₂ : Cr system is demonstrated that undergoes a reversible transformation from a garnet ( Ia d ) to a perovskite ( Pm m ) structure through progressive Al ³⁺ substitution. The resulting lattice contraction strengthens the octahedral crystal field around Cr ³⁺ , driving a transition from broadband near‐infrared (NIR) emission (800 nm, ⁴ T ₂ → ⁴ A ₂ ) to narrowband deep‐red luminescence (731 nm, ² E→ ⁴ A ₂ ). Structural refinements and spectroscopic analyses reveal a field‐induced crossover between spin‐allowed and spin‐forbidden transitions, accompanied by suppressed electron‐phonon coupling and a lifetime extension from microseconds to milliseconds. The configurational‐coordinate model further links lattice vibrations to emission dynamics. Beyond mechanistic insight, the broadband NIR emitter enables nondestructive imaging, whereas the deep‐red perovskite matches phytochrome absorption for plant photoregulation. This study establishes a unified structure‐field‐emission relationship and presents a general strategy for tuning broadband‐to‐narrowband transitions in transition‐metal‐activated oxides via crystal‐field modulation.