0D Bismuth(III)-Based Hybrid Ferroelectric: Tris(acetamidinium) Hexabromobismuthate(III)

Ferroelectric properties of haloantimonates­(III) and halobismuthates­(III) have been detected for as much as 40 structures belonging to 7 different types of anionic networks, with RMX₄, R₂MX₅, R₃M₂X₉, and R₅M₂X₁₁ stoichiometries being the most frequently reported to host these properties. We report on the first ferroelectric of the halobismuthate­(III) family with a R₃MX₆ stoichiometry, that is, tris­(acetamidinium)­hexabromobismuthate­(III), (CH₃C­(NH₂)₂)₃[BiBr₆] (ABB), characterized by a one-component organic network. While the stoichiometry and crystal packing of ABB might seem uncomplicated, the temperature-resolved structural and spectroscopic studies paint a different picture in which rich polymorphism in the solid state occurs between tetragonal (paraelastic) and triclinic (ferroelastic) crystal phases: I (P4₂/n) → II (P1̅) at 272/277 K (cooling/heating), II (P1̅) → III (P1̅) at 207 K, and III (P1̅) → IV (P1) at 98/127 K. The ferroelectric properties of phase IV have been confirmed by the pyroelectric current and hysteresis loop measurements; additionally, the acentric symmetry has been further supported by second harmonic generation measurements. Crystallographic analysis of phase III reveals the antiparallel alignment of acetamidinium dipoles, pointing to the antiferrroelectric nature of this phase. In turn, the character of the ferroelectric transition (III → IV) should be considered as “displacive” for both cationic and anionic substructures.) In this report, we also explore the two-photon absorption property of ABB at 800 nm, a property that is unexplored for any halobismuthate­(III) thus far. We also present periodic ab initio calculations for ABB crystals. The Berry-phase approach at the Hartree–Fock and density functional theory (DFT-D3) method levels is employed for spontaneous polarization calculations. The origin of ferroelectric polarization is studied using DFT-D3 and RHF electronic structure calculations, emphasizing the relationship between P_s and the relative orientation of organic/inorganic components.