Rotational Flexibility in Dication Drives Ambient Temperature Ferroelectricity in an Organic-Inorganic Hybrid Halide

Organic-inorganic hybrid halides (OIHHs) have gained attention as potential ferroelectric materials due to structure-property synergy of the organic and inorganic constituents. This study introduces an unusual Ag(I)-based ternary OIHH, (4,4'-bpy)Ag₂Br₄, featuring rotational flexibility in the organic dication to induce asymmetry into the structure. The compound crystallizes in a monoclinic crystal system with a non-centrosymmetric polar P2₁ space group at room-temperature and undergoes a structural phase transition to a centrosymmetric phase (P2₁/c) at Curie temperature (T_c) of 330 K which was further supported by differential scanning calorimetry (DSC), second harmonic generation (SHG) signals, dielectric anomaly, current-voltage (I-V) profiles, and X-ray photoelectron spectroscopy (XPS) data. Ferroelectricity is confirmed through polarization-electric field (P-E) hysteresis loops and piezoresponse force microscopy (PFM), exhibiting switchable polar domains. Density functional theory (DFT) calculations revealed electronic structures of the ferroelectric and paraelectric phases, identified the (β-AgBr₂)_n ^n- inorganic anionic chain contributing to the net polarization, and in general, complemented the experimental results. Comparative studies with structurally analogous Ag(I)-based OIHHs lacking dication rotational freedom endorse the critical role of organic flexibility in driving ferroelectricity. This study provides insights into the role of organic dications in controlling ferroelectric behavior and offers a promising pathway for developing coinage metal-based OIHH ferroelectric materials.