Crystal Structure and Properties of Acetamidinium Lead Bromide: A New Member of the A3PbBr5 Halide Family

Abstract

A new hybrid lead bromide, acetamidinium lead bromide (Aca₃PbBr₅), representing a previously unreported member of the A₃PbBr₅ halide family, has been synthesized and characterized. Single-crystal X-ray diffraction reveals a monoclinic structure (space group P21/c) composed of one-dimensional chains of corner-sharing PbBr₆ octahedra. Aca₃PbBr₅ exhibits broad room-temperature photoluminescence and is thermally stable up to 250 °C. To place this phase in the context of the A₃PbBr₅ family, we performed a comparative crystal-chemical analysis of Aca₃PbBr₅, FA₃PbBr₅, and Im₃PbBr₅ and introduced a set of intrinsic cation descriptors (steric footprint, hydrogen-bond donor density, and hydrogen-bond donor-field asymmetry) that rationalize how the A-site cation directs the structural response of the inorganic framework. Density functional theory calculations show that Aca₃PbBr₅ and FA₃PbBr₅ remain largely inorganic-dominated near the band edges, while Im₃PbBr₅ displays substantially stronger cation participation. These results demonstrate how cation geometry governs the structural and optical behavior of A₃PbBr₅ bromides and provide a useful basis for the targeted design of broadband-emissive low-dimensional haloplumbates.