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.