Unexpected Quenching in Luminescence With Smaller‐Ion Substitution: Structural and Phononic Insights Into a Trigonal Phosphor

ABSTRACT The substitution of host cations with smaller isovalent ions is an established route to improve luminescence through lattice contraction. Yet, its efficacy is not universal, and the underlying mechanisms for its failure remain poorly understood. This study reveals an anomaly in trigonal Ba 3 SbAl 3 Si 2 O 14 :Bi 3+ , where substituting Ba 2+ with smaller Sr 2+ severely degrades both quantum efficiency and thermal stability. Counterintuitively, this performance decline occurs alongside global lattice contraction and a reduced Huang‐Rhys factor, which signals weakened electron‐phonon coupling. We demonstrate that this paradox arises from a transition in the dominant quenching pathway. In the partially substituted regime, performance is governed by a potent, athermal static quenching mechanism induced by cation disorder, as confirmed by the formation energy of isolated Sr Ba defects and non‐exponential decay dynamics. At full substitution, while static quenching is suppressed, the rigid Sr‐host generates high‐energy phonon modes that, despite a high activation energy, accelerate the attempt frequency for non‐radiative decay. Our findings delineate a crucial design rule that a rigid average structure can be undermined by deleterious local strain, and the benefits of reduced electron‐phonon coupling can be overwhelmed by a high‐energy phonon environment. This insight provides a critical structural perspective for advancing high‐performance phosphors through rational compositional design.