Engineering Lattice Asymmetry via a Cs + ‐Triggered Domino Effect for Ultra‐Broadband Visible Emission from a Single Sb 3 + Luminophore

ABSTRACT Achieving efficient, single‐phase ultra‐broadband visible emission under mild excitation remains a key challenge in lead‐free perovskite optoelectronics. In this work, we demonstrate that B‐site heavier alkali metals can trigger a structural “domino effect” in the layered perovskite derivative (C 4 H 14 N 2 S 2 ) 2 Cs 2 InCl 9 , leading to spontaneous symmetry breaking and ultra‐broadband photoluminescence (PL) upon Sb 3 + doping. This unique “domino effect” is initiated by the close packing of the faced‐tricapped trigonal prism CsCl 9 8− within the 2D inorganic layer and propagates via the charge‐balancing interplay of intercalated chloride anions. The bonding of Cl − with three amine groups of the biprotanated cystamine, in turn, asymmetrically influences the inorganic layer, resulting in the formation of two distinct indium chloride octahedra. Consequently, Sb 3 + doping enables ultra‐broadband visible emission through a distortion‐modulated cooperative dual‐site mechanism. Our findings establish B‐site‐driven phase transition as a powerful and heretofore overlooked strategy for developing high‐performance luminescent materials.