Structural Phase Transition in 0D (TMBA)₃Bi₂–₂xSb₂xCl₉. Metal Halides: Expression of the Lone Pair Effect and Polyhedral Distortion

Abstract

Low-dimensional bismuth/antimony (Bi/Sb) organic–inorganic metal halides (OIMHs) are receiving considerable attention for their rich structural variety and strong luminescence. Gaining deeper insight into how crystal structure influences light-emission behaviour is essential for tailoring these materials for practical applications. In this work we synthesized two new lead-free OIMHs, (TMBA)₃Bi₂Cl₉ and (TMBA)₃Sb₂Cl₉ [TMBA = benzyltrimethylammonium], which adopt zero-dimensional (0D) structures. The bismuth compound crystallizes in the orthorhombic space group P2₁2₁2₁, while the antimony analogue forms in the monoclinic P2₁/c space group. Both materials act as typical semiconductors, exhibiting indirect optical band gaps of about 3.2eV and 2.6 eV, respectively. Progressive substitution of Sb for Bi introduces lattice strain arising from the stereochemically active lone pair on Sb³⁺, driving a structural transformation from a orthorhombic to a monoclinic phase. The impact of this phase change on the optical response was further explored in the mixed system (TMBA)₃Bi₂–₂xSb₂xCl₉. These findings provide useful guidelines for designing and tuning zero-dimensional hybrid metal halides to enhance their luminescent properties.