Kinetic Study of the Low Temperature Transformation of Co(HCOO)₃[(CH₃)₂NH₂]

The conventional model-fitting approach assumes a fixed mechanism throughout the reaction and extracts a single value of the apparent activation energy and pre-exponential factor. This approach was found to be too simplistic because the values of Arrhenius parameters obtained in such a way are an average that does not reflect changes in the reaction mechanism and kinetics with the extent of conversion. In this work, kinetic analysis of a low temperature solid-state phase transformation observed in a metal organic framework is performed on differential scanning calorimetry (DSC) data obtained from Co(HCOO)₃[(CH₃)₂NH₂]. The approach used here consists of fitting a mathematical expression without caring of its physical meaning. An important feature of this model is that allows for heat capacity baseline subtraction. Once the proposed model resulted to mathematically explain the curves obtained at three cooling rates for the extent of conversion, a physical meaning was investigated by testing how some physical assumptions match the mathematical model. In the end, non-Arrhenius kinetics was found, which is consistent with the experiments performed at three cooling rates and which makes use of a limited number of physical parameters: the ideal thermodynamic equilibrium transformation temperature, an energy barrier parameter, the peak temperature, and an asymmetry factor, which in this case is dependent on the cooling rate

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

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

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