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 SbIII center. Furthermore, on the basis of analysis, possible routes are suggested to enhance ferroelectric polarization in this class of compounds.