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] (<b>ACA</b>), (CH₃C(NH₂)₂)₃[Sb₂Br₉] (<b>ABA</b>), and (CH₃C(NH₂)₂)₃[Sb₂I₉] (<b>AIA</b>) . 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 <b>ACA</b> and <b>AIA</b> are reminiscent of well-known ferroelectric materials, with <b>ACA</b> distinguished by its piezoelectric and ferroelastic characteristics, underpinned by a buckled honeycomb two-dimensional (2D) layers of antimony chloride. Conversely, <b>AIA</b> 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 <b>ABA</b>, which amalgamates features from both <b>ACA</b> and <b>AIA</b>, yielding an unprecedented hybrid two-component (0D + 2D) anionic architecture. The ferroelectric properties of <b>AIA</b> have been demonstrated through pyroelectric current measurements and hysteresis loop analyses. Additionally, the noncentrosymmetric nature of <b>ACA</b> and <b>AIA</b> has been corroborated by second harmonic generation experiments. The piezoelectricity of <b>ACA</b> was confirmed using piezoresponse force microscopy (PFM). Furthermore, observations under a polarizing microscope revealed distinct ferroelastic properties in both <b>ACA</b> and <b>ABA</b>, characterized by well-defined and abundant star patterns previously observed only in simple oxides and alloys.