Apparent Colossal Dielectric Constants in Nanoporous Metal Organic Frameworks

In this work, we show that the hybrid material Co₂(1,4-bdc)₂(dabco)·[4DMF·1H₂O], shows an apparent colossal dielectric constant at room temperature (ε′_r ≈ 5000 at 300 K for ν = 100 Hz). Nevertheless, such response does not imply colossal polarizability processes, as its dielectric constant is not purely intrinsic, but is greatly enhanced by the activation of extrinsic dielectric effects close to room temperature associated to the diffusion of numerous guest molecules through the channels. If such extrinsic contributions are eliminated or reduced, the values of the dielectric constant turn to be much smaller, as observed in the closely related Co₂(1,4-bdc-NH₂)₂(dabco)·[7/2DMF·1H₂O], Co₂(1,4-ndc)₂(dabco) ·[3DMF·2H₂O] and Ni₂(1,4-bdc)₂(dabco)·[3DMF·1/2H₂O] compounds. Therefore, we warn about the imperious necessity of distinguishing between intrinsic and extrinsic effects in electrically inhomogenous MOF materials that display a certain conductivity in order to adequately interpret their dielectric behavior

Room-Temperature Polar Order in [NH₄][Cd(HCOO)₃] - A Hybrid Inorganic–Organic Compound with a Unique Perovskite Architecture

We report on the hybrid inorganic–organic ammonium compound [NH₄]­[Cd­(HCOO)₃], which displays a most unusual framework structure: instead of the expected 4⁹·6⁶ topology, it shows an ABX₃ perovskite architecture with the peculiarity and uniqueness (among all the up-to-date reported hybrid metal formates) that the Cd ions are connected only by <i>syn</i>–<i>anti</i> formate bridges, instead of <i>anti</i>–<i>anti</i> ones. This change of the coordination mode of the formate ligand is thus another variable that can provide new possibilities for tuning the properties of these versatile functional metal–organic framework materials. The room-temperature crystal structure of [NH₄]­[Cd­(HCOO)₃] is noncentrosymmetric (S.G.: <i>Pna</i>2₁) and displays a polar axis. DFT calculations and symmetry mode analysis show that the rather large polarization arising from the off-center shift of the ammonium cations in the cavities (4.33 μC/cm²) is partially canceled by the antiparallel polarization coming from the [Cd­(HCOO)₃]⁻ framework, thus resulting in a net polarization of 1.35 μC/cm². As shown by second harmonic generation studies, this net polarization can be greatly increased by applying pressure (<i>P</i>_max = 14 GPa), an external stimulus that, in turn, induces the appearance of new structural phases, as confirmed by Raman spectroscopy