Ferroelectric, Switchable Dielectric and Nonlinear Optical Properties in Inorganic–Organic Lead-Free 1D Hybrids Based on Bi(III) and Azetidine: (C₃NH₈)₂[BiCl₅], (C₃NH₈)₂[BiB

This study investigates lead-free organic-inorganic hybrids (C₃NH₈)₂[BiCl₅] (<b>ABC</b>) and (C₃NH₈)₂[BiBr₅] (<b>ABB</b>), focusing on their structural, dielectric, ferroelectric, and optical properties. Both compounds exhibit paraelectric (<b>I</b>) to ferroelectric (<b>II</b>) phase transitions (PTs) at 230/233 K and 228/229 K, respectively, transitioning from orthorhombic (<i>Pnma</i>) to monoclinic (<i>P</i>2₁) phases, with distorted [BiX₆]^3- octahedra forming 1D chains. Quasielastic neutron scattering and solid-state ¹H NMR studies reveal the localized motion of azetidinium cations. Dielectric measurements of <b>ABC</b> and <b>ABB</b> show step-like permittivity changes by 11-12 units post-transition <b>I</b> → <b>II</b>, demonstrating reversible switching behavior. Absorption studies reveal band gaps of 3.24 eV for <b>ABC</b> and 2.76 eV for <b>ABB</b>, classifying them as insulators. Luminescence spectra at 77K display 578 nm (<b>ABC</b>) and 708 nm (<b>ABB</b>) emissions, attributed to ³P_1,0 → ¹S₀ transitions. Both compounds demonstrate stable second harmonic generation (SHG) switching of the <i>on</i>-<i>off</i> type. The switching performance is evaluated over multiple thermal cycles using the <i>t</i>_req metric, which decreases with increasing temperature change rate and indicates that <b>ABB</b>'s SHG switching is approximately 30% faster than that of <b>ABC</b>.