Potentially Ferroelectric {Cu^IIL₂}_n Based Two-Dimensional Framework Exhibiting High Polarization and Guest-Assisted Dielectric Anomaly

Ferroelectrics in metal–organic materials have attracted recent interest owing to their synthetic simplicity and tunable nature. Utilizing isomeric dipodal phosphoramide ligands, L¹ [PhPO­(NH⁴Py)₂] and L² [PhPO­(NH²Py)₂], two new Cu^IIL₂ derivatives, 1 ({[CuL¹₂(H₂O)₂]·(NO₃)₂·(H₂O)_1.5·(CH₃OH)}_∞) and 2 ([CuL²₂]·(NO₃)₂), were synthesized. Compound 1 crystallizes in a noncentrosymmetric polar space group Cc as a two-dimensional framework, and 2 is a centrosymmetric complex. Electrical hysteresis (P–E loop) measurements on 1 at room temperature gave the remnant (P_r) and saturation (P_s) polarization values of 27.96 and 21.79 μC·cm^–2, respectively, which are the highest among all of the known metal–organic ferroelectric materials. Also, the P_r value obtained for 1 is comparable to that of barium titanate and higher than most of the organic, polymeric, and inorganic ferroelectric materials. The permittivity measurements on 1 and 2 result in high dielectric constant values of 186.3 and 53.24, respectively, at 1 Hz frequency at room temperature. Temperature-dependent permittivity measurement on 1 yields a dielectric anomaly peak at 40 °C due to phase transition assisted by desolvation. The existence of phase transition is further confirmed by differential scanning calorimetry, powder X-ray diffraction, and polarized light microscopy. A comparison of the P–E loops of 1, 1_desolvated, and 1_resolvated suggest that gasification/release of the solvate molecules from the packing structure affects the polarization in 1.