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[BiBr₅]

High Resolution Image Download MS PowerPoint Slide This study investigates lead-free organic–inorganic hybrids (C 3 NH 8 ) 2 [BiCl 5 ] ( ABC ) and (C 3 NH 8 ) 2 [BiBr 5 ] ( ABB ), focusing on their structural, dielectric, ferroelectric, and optical properties. Both compounds exhibit paraelectric ( I ) to ferroelectric ( II ) phase transitions (PTs) at 230/233 K and 228/229 K, respectively, transitioning from orthorhombic ( Pnma ) to monoclinic ( P 2 1 ) phases, with distorted [BiX 6 ] 3– octahedra forming 1D chains. Quasielastic neutron scattering and solid-state 1 H NMR studies reveal the localized motion of azetidinium cations. Dielectric measurements of ABC and ABB show step-like permittivity changes by 11–12 units post-transition I → II, demonstrating reversible switching behavior. Absorption studies reveal band gaps of 3.24 eV for ABC and 2.76 eV for ABB, classifying them as insulators. Luminescence spectra at 77K display 578 nm ( ABC ) and 708 nm ( ABB ) emissions, attributed to 3 P 1,0 → 1 S 0 transitions. Both compounds demonstrate stable second harmonic generation (SHG) switching of the on – off type. The switching performance is evaluated over multiple thermal cycles using the t req metric, which decreases with increasing temperature change rate and indicates that ABB ’s SHG switching is approximately 30% faster than that of ABC .