Structural diversity of a series of chlorocadmate(II) and chlorocuprate(II) complexes based on benzylamine and its N-methylated derivatives

Polarity and Ferromagnetism in Two-Dimensional Hybrid Copper Perovskites with Chlorinated Aromatic Spacers

Two-dimensional (2D) organic-inorganic hybrid copper halide perovskites have drawn tremendous attention as promising multifunctional materials. Herein, by incorporating ortho-, meta-, and para-chlorine substitutions in the benzylamine structure, we first report the influence of positional isomerism on the crystal structures of chlorobenzylammonium copper(II) chloride perovskites A₂CuCl₄. 2D polar ferromagnets (3-ClbaH)₂CuCl₄ and (4-ClbaH)₂CuCl₄ (ClbaH⁺ = chlorobenzylammonium) are successfully obtained. They both adopt a polar monoclinic space group Cc at room temperature, displaying significant differences in crystal structures. In contrast, (2-ClbaH)₂CuCl₄ adopts a centrosymmetric space group P 2₁/ c at room temperature. This associated structural evolution successfully enhances the physical properties of the two polar compounds with high thermal stability, discernible second harmonic generation (SHG) signals, ferromagnetism, and narrow optical band gaps. These findings demonstrate that the introduction of chlorine atoms into the interlayer organic species is a powerful tool to tune crystal symmetries and physical properties, and this inspires further exploration of designing high-performance multifunctional copper-based materials.

Polar Ferromagnet Induced by Fluorine Positioning in Isomeric Layered Copper Halide Perovskites

We present the influence of positional isomerism on the crystal structure of fluorobenzylammonium copper(II) chloride perovskites A₂CuCl₄ by incorporating ortho-, meta-, and para-fluorine substitution in the benzylamine structure. Two-dimensional (2D) polar ferromagnet (3-FbaH)₂CuCl₄ (3-FbaH⁺ = 3-fluorobenzylammonium) is successfully obtained, which crystallizes in a polar orthorhombic space group Pca2₁ at room temperature. In contrast, both (2-FbaH)₂CuCl₄ (2-FbaH⁺ = 2-fluorobenzylammonium) and (4-FbaH)₂CuCl₄ (4-FbaH⁺ = 4-fluorobenzylammonium) crystallize in centrosymmetric space groups P2₁/c and Pnma at room temperature, respectively, displaying significant differences in crystal structures. These differences indicate that the position of the fluorine atom is a driver for the polar behavior in (3-FbaH)₂CuCl₄. Preliminary magnetic measurements confirm that these three perovskites possess dominant ferromagnetic interactions within the inorganic [CuCl₄]_∞ layers. Therefore, (3-FbaH)₂CuCl₄ is a polar ferromagnet, with potential as a type I multiferroic. This work is expected to promote further development of high-performance 2D copper(II) halide perovskite multiferroic materials.

Structural Features in Some Layered Hybrid Copper Chloride Perovskites: ACuCl4 or A2CuCl4

We present three new hybrid copper(II) chloride layered perovskites of generic composition ACuCl₄ or A₂CuCl₄, which exhibit three distinct structure types. (m-PdH₂)CuCl₄ (m-PdH₂²⁺ = protonated m-phenylenediamine) adopts a Dion-Jacobson (DJ)-like layered perovskite structure type and exhibits a very large axial thermal contraction effect upon heating, as revealed via variable-temperature synchrotron X-ray powder diffraction (SXRD). This can be attributed to the contraction of an interlayer block, via a slight repositioning of the m-PdH₂²⁺ moiety. (3-AbaH)₂CuCl₄ (3-AbaH⁺ = protonated 3-aminobenzoic acid) and (4-AbaH)₂CuCl₄ (4-AbaH⁺ = protonated 4-aminobenzoic acid) possess the same generic formula as Ruddlesden-Popper (RP) layered perovskites, A₂BX₄, but adopt different structures. (4-AbaH)₂CuCl₄ adopts a near-staggered structure type, whereas (3-AbaH)₂CuCl₄ adopts a near-eclipsed structure type, which resembles the DJ rather than the RP family. (3-AbaH)₂CuCl₄ also displays static disorder of the [CuCl₄]_∞ layers. The crystal structures of each are discussed in terms of the differing nature of the templating molecular species, and these are compared to related layered perovskites. Preliminary magnetic measurements are reported, suggesting dominant ferromagnetic interactions.

Two-Dimensional Hybrid Halide Perovskites: Principles and Promises

Hybrid halide perovskites have become the "next big thing" in emerging semiconductor materials, as the past decade witnessed their successful application in high-performance photovoltaics. This resurgence has encompassed enormous and widespread development of the three-dimensional (3D) perovskites, spearheaded by CH₃NH₃PbI₃. The next generation of halide perovskites, however, is characterized by reduced dimensionality perovskites, emphasizing the two-dimensional (2D) perovskite derivatives which expand the field into a more diverse subgroup of semiconducting hybrids that possesses even higher tunability and excellent photophysical properties. In this Perspective, we begin with a historical flashback to early reports before the "perovskite fever", and we follow this original work to its fruition in the present day, where 2D halide perovskites are in the spotlight of current research, offering characteristics desirable in high-performance optoelectronics. We approach the evolution of 2D halide perovskites from a structural perspective, providing a way to classify the diverse structure types of the materials, which largely dictate the unusual physical properties observed. We sort the 2D hybrid halide perovskites on the basis of two key components: the inorganic layers and their modification, and the organic cation diversity. As these two heterogeneous components blend, either by synthetic manipulation (shuffling the organic cations or inorganic elements) or by application of external stimuli (temperature and pressure), the modular perovskite structure evolves to construct crystallographically defined quantum wells (QWs). The complex electronic structure that arises is sensitive to the structural features that could be in turn used as a knob to control the dielectric and optical properties the QWs. We conclude this Perspective with the most notable achievements in optoelectronic devices that have been demonstrated to date, with an eye toward future material discovery and potential technological developments.