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

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 (C₄H₁₄N₂S₂)₂Cs₂InCl₉, leading to spontaneous symmetry breaking and ultra-broadband photoluminescence (PL) upon Sb³ ⁺ doping. This unique "domino effect" is initiated by the close packing of the faced-tricapped trigonal prism CsCl₉ ^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, Sb³ ⁺ 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.