Are Mixed-Halide Ruddlesden-Popper Perovskites Really Mixed?

Mixing bromine and iodine within lead halide perovskites is a common strategy to tune their optical properties. This comes at the cost of instability, as illumination induces halide segregation and degrades device performances. Hence, understanding the behavior of mixed-halide perovskites is crucial for applications. In 3D perovskites such as MAPb(Br _x I_1-x )₃ (MA = methylammonium), all of the halide crystallographic sites are similar, and the consensus is that bromine and iodine are homogeneously distributed prior to illumination. By analogy, it is often assumed that Ruddlesden-Popper layered perovskites such as (BA)₂MAPb₂(Br _x I_1-x )₇ (BA = butylammonium) behave alike. However, these materials possess a much wider variety of halide sites featuring diverse coordination environments, which might be preferentially occupied by either bromine or iodine. This leaves an open question: are mixed-halide Ruddlesden-Popper perovskites really mixed? By combining powder and single-crystal diffraction experiments, we demonstrate that this is not the case: bromine and iodine in RP perovskites preferentially occupy different sites, regardless of the crystallization speed.

Syntheses, structure and properties of a new series of organic-inorganic Hg-based halides: adjusting halogens resulted in huge performance mutations

Three new organic-inorganic hybrid perovskite (OIHP) halides, [N(CH3)4]HgCl0.63Br2.37 (I), [N(CH3)4]HgBrI2 (II) and [N(CH3)4]HgCl0.45I2.55 (III), were synthesized by a hydrothermal reaction. They feature different crystal structures, in which both II and III are isomorphic and contain a one-dimensional chain with organic cation [N(CH3)4]+ interspersed in the space, whereas II has a similar one-dimensional chain but significantly different spatial arrangement due to the enhanced hydrogen bond interaction. The experimental results show that the divergent second-order nonlinear optical (NLO) effect from Br(Cl) to I and the arrangement of anion groups change dramatically from the presence of hydrogen bonds to the absence of hydrogen bonds, leading to a sharply increased NLO response of II and III (18 and 25 times that of I) compared with that of I. Moreover, the phase matching ability disappeared and the band gap decreased significantly. Meanwhile, a high temperature phase transition was observed in II and III, which is rare in common OIHPs. All these results indicate that the regulation of halogen bonds plays a crucial role in the structural and property mutations of OIHP halides.

Bright and Near-Unity Polarized Light Emission Enabled by Highly Luminescent Cu2I2-Dimer Cluster-Based Hybrid Materials

As one fundamental property of light, polarization has a huge impact in quantum optics and optoelectronics through light-matter interactions. However, the bright and near-unity polarized light emissions in the visible range by solid crystalline materials are scantly realized. Here, we report well-defined quasi two-dimensional (2D) hybrid crystals based on the linear alignment of Cu₂I₂-dimer/bidentate ligand hybrid clusters for achieving bright and near-unity linearly polarized light emissions. Using first-principle calculations, we demonstrate that the superaligned transition dipole moments are the key for the observed excellent polarized light emissions. To further enhance the photoluminescence (PL) polarization degree, we fabricate Cu₂I₂-dimer-based hybrid nanobelts, which display high PL quantum yield (up to 64%) and ultrahigh PL polarization degree (∼0.99). Our reported copper iodine cluster-based luminescent hybrid materials for bright and highly polarized light emissions will have great potential for future quantum optics applications.

Acentric Organic-Inorganic Hybrid Halide [N(CH3)4]2HgBr2I2 Featuring an Isolated [HgBr2I2]2- Tetrahedron and Second-Order Nonlinearity

A new hybrid compound [N(CH₃)₄]₂HgBr₂I₂, obtained by a hydrothermal reaction, crystallized in the noncentrosymmetric space group P2₁2₁2₁. Its structure contains an isolated asymmetric [HgBr₂I₂]^2- tetrahedron with net polarization, connected by hydrogen bonds to form pseudo-one-dimensional chain structures. Moreover, its optical band gap, nonlinear optical (NLO) property, fluorescence property, and thermal property were characterized in detail. A rare high-temperature phase transition was observed in the compound. In addition, theoretical calculations were performed to elaborate the relation between electronic state, band structure, and their nonlinear optical response. These results indicate that [N(CH₃)₄]₂HgBr₂I₂ is a new potential candidate for future photoelectronic applications in fluorescence and nonlinear optics.

Advances and Property Investigations of an Organic-Inorganic Ferroelectric: (diisopropylammonium)2[CdBr4]

The preparation of materials featuring more than one ferroelectric phase represents a promising strategy for controlling electrical properties arising from spontaneous polarization, since it offers an added advantage of temperature-dependent toggling between two different ferroelectric states. Here, we report on the discovery of a unique ferroelectric–ferroelectric transition in diisopropylammonium tetrabromocadmate (DPAC, (C₆H₁₆N)₂[CdBr₄]) with a T_c value of 244 K, which is continuous in nature. Both phases crystallize in the same polar orthorhombic space group, Iab2. The temperature-resolved second-harmonic-generation (SHG) measurements using 800 nm femtosecond laser pulses attest to the polar structure of DPAC on either side of the phase transition (PT). The dc conductivity parameters were estimated in both solid phases. The anionic substructure is in the form of [CdBr₄]^2– discrete complexes (0D), while in the voids of the structure, the diisopropylammonium cations are embedded. The ferroelectric properties of phases I and II have been confirmed by the reversible pyroelectric effect as well as by P–E loop investigations. On the basis of the dielectric responses, the molecular mechanism of the PT at 244 K has been postulated to be of mixed type with an indication of its displacive nature.

A novel ferroelectric crystal of (C₆H₁₆N)₂[CdBr₄] has been synthesized, and a description of its properties (thermal, structural, electric, second-harmonic generation) is presented. The ferroelectric properties in phases I and II have been successfully confirmed by the reversible pyroelectric effect as well as by P−E hysteresis loop investigations and SHG properties.

Motional freedom of dimethylammonium ions in a cyanoelpasolite, [(CH3)2NH2]2KCo(CN)6, which exhibits phase transition associated with a distinct change in dielectric property

The temperature dependence of the ¹H nuclear magnetic resonance spin-lattice relaxation time T₁ of [(CH₃)₂NH₂]₂KCo(CN)₆ and the partially deuterated analogue [(CD₃)₂NH₂]₂KCo(CN)₆ has been reported. A change in the molecular motion of organic cations through the first-order phase transition at T_c = 246 K has been discussed. Although this first-order transition has been reported as an isosymmetric transition, in which the space group of the crystal does not change, the number of the ¹⁴N nuclear quadrupole resonance line changed from three in the high-temperature phase to twelve in the low-temperature phase indicating the lowering of crystal symmetry.

Dielectric phase transition of an A2BX4-type perovskite with a pentahedral to octahedral transformation

Organic-inorganic hybrid compounds that undergo reversible dielectric phase transitions are a very attractive class of smart materials due to their wide applications in data storage, data communication and signal sensing. Here, a piperidine ring, C₅H₁₁N, was introduced into the inorganic lead halide perovskite scaffold to obtain three hybrid perovskite compounds, [C₅H₁₂N]₂PbCl₄ (1), [C₅H₁₂N]₂PbBr₄ (2), and [C₅H₁₂N]PbI₃ (3). When compound 2 and compound 3 feature static two-dimensional (2D) and one-dimensional (1D) perovskite structures, respectively, it is striking that compound 1 shows a reversible pentahedral to octahedral transformation. It undergoes an above-room-temperature dielectric phase transition at T_c≅ 352 K, wherein the high dielectric constant is more than twice the low dielectric constant. Structural analysis shows that 1 undergoes a phase transition from the space group Pnma at the low temperature phase (LTP) to C2/c at the high temperature phase (HTP). The phase transition originates from the order-disorder conversion of piperidinium cations. It is interesting to note that, the Pb²⁺ cations in the inorganic moieties change from five-coordinate at the LTP to six-coordinate at the HTP. The discovery of dielectric phase transition hybrid organic-inorganic lead halide perovskite materials further enhances the potential applications of high temperature responsive dielectric switchable materials.