Ferroelectric, Switchable Dielectric and Nonlinear Optical Properties in Inorganic-Organic Lead-Free 1D Hybrids Based on Bi(III) and Azetidine: (CNH)[BiCl], (CNH)[BiBr].

This study investigates lead-free organic-inorganic hybrids (CNH)[BiCl] () and (CNH)[BiBr] (), focusing on their structural, dielectric, ferroelectric, and optical properties. Both compounds exhibit paraelectric () to ferroelectric () phase transitions (PTs) at 230/233 K and 228/229 K, respectively, transitioning from orthorhombic () to monoclinic (2) phases, with distorted [BiX] octahedra forming 1D chains. Quasielastic neutron scattering and solid-state H NMR studies reveal the localized motion of azetidinium cations. Dielectric measurements of and show step-like permittivity changes by 11-12 units post-transition → , demonstrating reversible switching behavior. Absorption studies reveal band gaps of 3.24 eV for and 2.76 eV for , classifying them as insulators. Luminescence spectra at 77K display 578 nm () and 708 nm () emissions, attributed to P → S transitions. Both compounds demonstrate stable second harmonic generation (SHG) switching of the - type. The switching performance is evaluated over multiple thermal cycles using the metric, which decreases with increasing temperature change rate and indicates that 's SHG switching is approximately 30% faster than that of .