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

Abstract

In this work, we focus on [(CH)NH]PbI, a member of the [AmineH]PbI series of hybrid organic-inorganic compounds, reporting a very easy mechanosynthesis route for its preparation at room temperature. We report that this [(CH)NH]PbI compound with 2H-perovskite structure experiences a first-order transition at ≈250 K from hexagonal symmetry P6/mmc (HT phase) to monoclinic symmetry P2/c (LT phase), which involves two cooperative processes: an off-center shift of the Pb 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 (E = 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 [(CH)NH]PbI is an electronic insulator and an ionic conductor, where the electronic conductivity is disfavored because of the low dimensionality of the [(CH)NH]PbI structure.

Perovskites on Ice: An Additive‐Free Approach to Increase the Shelf‐Life of Triple‐Cation Perovskite Precursor Solutions

“…In either case, any excess of large organic cations (MFA + , DMFA + , DMA + ) in the perovskite is expected to modify the Goldschmidt tolerance factor of the perovskite, forcing the material to adopt a non‐photoactive phase. For large cations, these APbI 3 phases are generally similar to the aforementioned δ‐FAPbI 3 (2H polytype) phase, comprising “1D” continuous chains of face‐sharing lead‐iodide octahedra [6,23–25] . The formation of such cationic species in degraded precursor solutions therefore represents an important process that is expected to prevent the crystallisation of the precursor into a light‐absorbing perovskite phase.…”

Efficient and Stable Inverted Perovskite Solar Cells Incorporating Secondary Amines

“…[ [20][21][22] We then characterized optical properties of the samples (Figure 2c). No obvious change in the absorption spectra for x < 0.3 ( Figure S7a, Supporting Information) indicates that the DMA has negligible impact on the electronic structure.…”

Novel photoelectric material of perovskite-like (CH3)3SPbI3 nanorod arrays with high stability

“…Previous studies have shown that the introduction of large cations could enhance the stability of perovskite films, such as azetidinium [14], hydrazinium [15], ethylammonium [16], dimethylammonium [17], and trimethylammonium cations [17,18]. Meanwhile, if the organic cations are too large, the perovskite films tend to form low-dimensional structure with high stability.…”