ABX3 (A = organic cation; B = Sn, Pb; and X = halogen) organohalide perovskites have recently attracted much attention for their photovoltaic applications. Such hybrid compounds are derived from the replacement of the inorganic monovalent metal element by an organic cation, for example, methylammonium ion (MA = CH3NH3 +) and formamidinium ion (FA= +HC­(NH2)2). In particular, since the organic cations are polar, it is interesting to investigate their possible long-range ordering and the corresponding Rashba spin-split bands. In this work, by using density functional theory calculations, we estimate the ferroelectric polarization corresponding to a complete ordering of dipole moments for the optimized structures of 12 perovskite halides, with A = MA, FA; B = Pb, Sn; X = Cl, Br, I. The adiabatic path and functional mode analysis have been discussed for all cases. The calculated values of the polarization may be as high as a conventional inorganic ferroelectric compound, such as BaTiO3. The concomitant inversion symmetry breaking, coupled to the sizable spin–orbit coupling of Pb and Sn, results in a fairly large Rashba spin-splitting effect for both valence and conduction bands. We highlight a rather anisotropic dispersion of spin–orbit split bands which gives rise to different Rashba parameters in different directions perpendicular to the polar axis in k-space. Furthermore, we found a weak and positive correlation between the magnitude of polarization and relevant spin-split band parameters. Since the mechanism for enhanced carrier lifetime in 3D Rashba materials is connected to the reduced recombination rate due to the spin-forbidden transition, our study could aid in the understanding of the fundamental physics of organometal halide perovskites and the optimization and design of materials for better performance.

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.

Density functional theory calculations were performed for the electronic and the ferroelectric properties of the bulk and the monolayer benzylammonium lead-halide (BA2PbCl4). Our calculations indicate that both the bulk and monolayer systems display a band gap of ∼3.3 eV (HSE06+SOC) and a spontaneous polarization of ∼5.4 μC/cm2. The similar physical properties of bulk and monolayer systems suggest a strong decoupling among the layers in this hybrid organic–inorganic perovskite. Both the ferroelectricity, through associated structure distortion, and the spin–orbit coupling, through splitting induced in the electronic bands, significantly influence the band gaps. Most importantly, we found for the first time in a two-dimensional hybrid organic–inorganic class of material, a peculiar spin texture topology such as a unidirectional spin–orbit field, which may lead to a protection against spin decoherence.

Hybrid perovskite crystals with organic and inorganic structural components are able to combine desirable properties from both classes of materials. Electronic interactions between the anionic inorganic framework and functional organic cations (such as chromophores or semiconductors) can give rise to unusual photophysical properties. Cyanine dyes are a well known class of cationic organic dyes with high extinction coefficients and tunable absorption maxima all over the visible and near-infrared spectrum. Here we present the synthesis and characterization of an original 1D hybrid perovskite composed of NIR-absorbing cyanine cations and polyanionic lead halide chains. This first demonstration of a cyanine-perovskite hybrid material is paving the way to a new class of compounds with great potential for applications in photonic devices.

The theoretical ferroelectric polarization of the low-temperature (monoclinic, P2 1 ) phase and the hightemperature (hexagonal, P6 3 ) phase of hydroxyapatite Ca 10 (PO 4 ) 6 (OH) 2 is calculated based on the density functional theory (DFT). In the monoclinic structure, the value of ferroelectric polarization is found to be 9.87 mC cm À2 along the [001] direction. In the hexagonal structure, the ferroelectric polarization is 7.05 mC cm À2 along the [001] direction. The main contribution to the electric polarization comes from ordered hydroxyl OH À anions for both phases, although the inorganic Ca 5 (PO 4 ) 3 apatite framework also gives a non-negligible contribution. A detailed analysis of ferroelectric polarization and structural change of the hydroxyapatite is presented for a better understanding of this important biomaterial.