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

Phase Transition, Dielectric Properties, and Ionic Transport in the [(CH₃)₂NH₂]PbI₃ Organic–Inorganic Hybrid with 2H-Hexagonal Perovskite Structure

In this work, we focus on [(CH₃)₂NH₂]­PbI₃, a member of the [AmineH]­PbI₃ series of hybrid organic–inorganic compounds, reporting a very easy mechanosynthesis route for its preparation at room temperature. We report that this [(CH₃)₂NH₂]­PbI₃ compound with 2H-perovskite structure experiences a first-order transition at ≈250 K from hexagonal symmetry <i>P</i>6₃/<i>mmc</i> (HT phase) to monoclinic symmetry <i>P</i>2₁/<i>c</i> (LT phase), which involves two cooperative processes: an off-center shift of the Pb²⁺ cations and an order–disorder process of the N atoms of the DMA cations. Very interestingly, this compound shows a dielectric anomaly associated with the structural phase transition. Additionally, this compound displays very large values of the dielectric constant at room temperature because of the appearance of a certain conductivity and the activation of extrinsic contributions, as demonstrated by impedance spectroscopy. The large optical band gap displayed by this material (<i>E</i>_g = 2.59 eV) rules out the possibility that the observed conductivity can be electronic and points to ionic conductivity, as confirmed by density functional theory calculations that indicate that the lowest activation energy of 0.68 eV corresponds to the iodine anions, and suggests the most favorable diffusion paths for these anions. The obtained results thus indicate that [(CH₃)₂NH₂]­PbI₃ is an electronic insulator and an ionic conductor, where the electronic conductivity is disfavored because of the low dimensionality of the [(CH₃)₂NH₂]­PbI₃ structure