Lone-Pair-Electron-Driven Ionic Displacements in a Ferroelectric Metal-Organic Hybrid

A displacive-type mechanism, which accounts for the occurrence of ferroelectricity in most inorganic ferroelectrics, is rarely found in molecule-based ferroelectrics. Its role is often covered by the predominant order-disorder one. Herein, we report a lone-pair-electron-driven displacive-type ferroelectric organic-inorganic hybrid compound, [H₂dmdap][SbCl₅] (1; dmdap = N,N-dimethyl-1,3-diaminopropane). The structure of 1 features a typical zigzag chain of [SbCl₅]_∞ containing cis-connected anionic octahedra. The compound undergoes a second-order paraelectric-ferroelectric phase transition at 143 K (P2₁/c ↔ Pc) with a saturation polarization of 1.36 μC·cm^-2 and a coercive field of 3.5 kV·cm^-1 at 119 K. Theoretical study discloses the ferroelectricity mainly originating from the relative displacements of the Sb and Cl ions in the crystal lattice, which are driven by the 5s² lone-pair electrons of the Sb^III center. Furthermore, on the basis of analysis, possible routes are suggested to enhance ferroelectric polarization in this class of compounds.