Ferroelectricity, Piezoelectricity, and Unprecedented Starry Ferroelastic Patterns in Organic-Inorganic (CH3C(NH2)2)3[Sb2X9] (X = Cl/Br/I) Hybrids.

In this study, we present a novel class of lead-free hybrid antimony halides incorporating the acetamidinium cation, with the chemical compositions: (CH3C(NH2)2)3[Sb2Cl9] (ACA), (CH3C(NH2)2)3[Sb2Br9] (ABA), and (CH3C(NH2)2)3[Sb2I9] (AIA) . Despite their seemingly analogous chemical formulations, these compounds exhibit diverse physical characteristics, predominantly dictated by the differences in their metal-halide architectures. Indeed, the anionic frameworks of ACA and AIA are reminiscent of well-known ferroelectric materials, with ACA distinguished by its piezoelectric and ferroelastic characteristics, underpinned by a buckled honeycomb two-dimensional (2D) layers of antimony chloride. Conversely, AIA is characterized by its ferroelectric attribute, with discrete bioctahedral units forming a zero-dimensional (0D) structure. A surprising structural deviation constitutes the anionic sublattice of ABA, which amalgamates features from both ACA and AIA, yielding an unprecedented hybrid two-component (0D + 2D) anionic architecture. The ferroelectric properties of AIA have been demonstrated through pyroelectric current measurements and hysteresis loop analyses. Additionally, the noncentrosymmetric nature of ACA and AIA has been corroborated by second harmonic generation experiments. The piezoelectricity of ACA was confirmed using piezoresponse force microscopy (PFM). Furthermore, observations under a polarizing microscope revealed distinct ferroelastic properties in both ACA and ABA, characterized by well-defined and abundant star patterns previously observed only in simple oxides and alloys.