Solvent-assisted mechanochemical crystallization of the metal-free perovskite solid solution (H2dabco, H2hmta)NH4(BF4)3

All-proportional solid solutions of the metal-free perovskite (H2dabco1-y, H2hmtay)(NH4)(BF4)3 ((d,h)-BF4) were crystallized via a mechanochemical method. Their molecular dynamics depend on the ratio y with a compositional boundary at y = 0.43, where H2dabco2+ was deduced to be at a dynamic disorder state, even below phase transition temperature to a plastic crystalline phase seen at y = 0.

Molecularly thin, two-dimensional all-organic perovskites

Recently, the emergence of all-organic perovskites with three-dimensional (3D) structures has expanded the potential applications of perovskite materials. However, the synthesis and utilization of all-organic perovskites in 2D form remain largely unexplored because the design principle has not been developed. We present the successful synthesis of a metal-free 2D layered perovskite, denoted as the Choi-Loh van der Waals phase (CL-v phase), with the chemical formula A2B2X4, where A represents a larger-sized cation compared to B and X denotes an anion. The CL-v phase exhibits a van der Waals gap enabled by interlayer hydrogen bonding and can be exfoliated or grown as molecularly thin 2D organic crystals. The dielectric constants of the CL-v phase range from 4.8 to 5.5 and we demonstrate their potential as gate dielectrics for thin-film transistors.

Metal-free enantiomorphic perovskite [dabcoH2]2+[H3O]+Br- 3 and its one-dimensional polar polymorph

The structure and stoichiometry of a new metal-free and ammonium-free compound [dabcoH2]2+H3O+Br- 3 (where [dabcoH2]2+ = 1,4-di-aza-bicyclo-[2.2.2]octane dication) correspond to the general formula ABX 3 characteristic of perovskites. In enantiomorphic trigonal polymorph α of [dabcoH2]2+H3O+Br- 3, the corner-sharing [H3O]Br6 octahedra combine into a 3D framework embedding [dabcoH2]2+ dications in pseudo-cubic cages. In the more dense polymorph β, the face-sharing [H3O]Br6 octahedra form 1D polyanionic columns separated by [dabcoH2]2+ dications. These different topologies correlate with different crystal fields around the cations and their different disorder types: orientational disorders of [dabcoH2]2+ dications and H3O+ cations in polymorph α and positional disorder of [H3O]+ cations in polymorph β. The orientational disorder increases the lengths of OH⋯Br hydrogen bonds in polymorph α, but NH⋯Br distances of ordered dabcoH2 dications are longer in polymorph β. The presence of polar [H3O]+ cations in [dabcoH2]2+H3O+Br- 3 polymorphs offers additional polarizability of the centres compared with analogous metal-free [dabcoH2]2+[NH4]+Br- 3 perovskite.

Structural and Functional Insights into Metal-Free Perovskites

In the past three years, metal-free perovskites have garnered significant interest as promising candidates for utilization in next-generation wearable electronics. A variety of different molecular structures for these perovskites have been designed for different applications. However, there is still no systematic understanding that can elucidate the relationship between the structural details and properties of perovskites. This would provide a helpful guide for designing a metal-free perovskite with the desired packing structure and properties. Herein, we summarize recently reported structural and functional insights into metal-free perovskites. The underlying design of the molecular structure and its role in the packing structure and resulting properties are explained. In addition, important factors and challenges in the design of a molecular structure that will be useful for future applications are discussed. This information will help enrich the library of potential structures and future applications of metal-free perovskites, which is believed to be much larger than is currently known.

Molecular solid solutions for advanced materials – homeomorphic or heteromorphic

Crystalline molecular solid solutions are discussed on the basis of homeomorphism and heteromorphism of blended molecules.

Slider-crank mechanism in a molecular crystal: conversion of linear thermal expansion of a lattice to circular rotation of a coordination chain

Slider-crank like force conversion of linear thermal expansion to rotational motion of the coordination chains were observed in the molecular crystal.

Supramolecular [Na(15-crown-5)]+ cations anchored to face-sharing octahedral lead bromide chains featuring a rotor-like one-dimensional perovskite with a reversible isostructural phase transition near room temperature

A 1D rotor-like organic perovskite, {[Na(15-crown-5)]PbBr3}n, features a high-κ nature and experiences a reversible isostructural phase transition near room temperature.