Potentially Ferroelectric {CuIIL2}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, L-1 [PhPO((NHPy)-Py-4)(2)] and L-2 [PhPO((NHPy)-Py-2)(2)], two new (CuL2)-L-II derivatives, 1 ({[CuL21(H2O)(2)]center dot(NO3)(2)center dot(H2O)(1.5)center dot(CH3OH)}(infinity)) and 2 ([CuL22]center dot(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 mu C. 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 degrees 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.