Site‐Preferential Substitution in All‐Inorganic Metal Halides Enabling Continuous Modulation of Self‐Trapped Exciton Emission With Near‐Unity Photoluminescence Quantum Yield

ABSTRACT Self‐trapped excitons (STEs) endow metal halides with exceptional optoelectronic potential, however, achieving seamless spectral tuning without photoluminescence (PL) quenching remains highly challenging. Herein, high‐quality Sb 3+ ‐doped Cs 4 InCl 7 is synthesized, exhibiting intense broadband STE emission centered at 584 nm with a near‐unity photoluminescence quantum yield (PLQY). Through atomic‐site‐preferential substitution of Na + for Cs + to impose directional lattice strain, long‐range continuous STE emission tuning over 42 nm is achieved without PL efficiency loss. This substitution is proposed to stiffen the framework and decrease the Stokes shift, thereby driving pronounced blue‐shifts. Static high‐pressure measurements reproduce a similar spectral trajectory, with the spectral modulation effect of Na + substitution resembles that observed at approximately 1.5 GPa. The observed invariant excitation energy, together with the strictly linear proportionalities among the emission peak, linewidth and PL lifetime, reveals a collective response of the STE’ radiative properties to same underlying lattice relaxation process. This lock‐step correlation provides profound insight into the nature of STE, demonstrating that their broad tunability is governed by a similar photophysical mechanism beyond previously established characteristics. This work presents a practical materials engineering pathway for efficiency‐retaining tunable broadband emitters. Moreover, the emerging high‐efficiency green fluorescence enables advanced photonic applications such as X‐ray imaging.