Glass Formation from Two-Dimensional Hybrid Organic-Inorganic Perovskites

The crystal-liquid-glass phase transition has been achieved in hybrid framework materials such as coordination polymers (CPs) and metal-organic frameworks (MOFs) in the past decade¹. Despite the absence of long-range order, these hybrid glasses preserve the integrity of the organic and inorganic basic building blocks, showing diverse attractive advantages that are complementary to their crystalline counterparts². To expand the scope of the relatively new hybrid glass family, here we turn to hybrid organic-inorganic perovskites (HOIPs).<br/>HOIPs occupy a prominent position in the field of materials chemistry due to their fascinating optoelectronic properties³. The prototypical HOIPs are three-dimensional (3D) frameworks with a general chemical formula of ABX₃, in which A is a monovalent organic cation, B is a metal ion, and X is an anion. The B-site ions are six-coordinated with X-site anions to form a corner-sharing network of BX₆ octahedra, and the larger A-site cations are caged in the 3D framework. Additionally, two-dimensional (2D) HOIPs are also known, where larger organic A-site cations can separate the BX₆ octahedral layers as spacers, and these materials typically exhibit higher chemical stability. Several examples of glasses formed from HOIPs have emerged since 2021, yet they have been limited to a few HOIP structures to date^4,5. Substantial challenges remain, including the discovery, characterization, and exploitation of glasses formed from HOIPs³.<br/>Firstly, further investigation into characterizing the liquid state and demonstrating the melting process is clearly warranted, as the glass-formation mechanism of HOIPs remains unclear. Focusing on a two-dimensional HOIP material (<i>S</i>-NEA)₂PbBr₄ (<i>S</i>-NEA = <i>S</i>-(-)-1-(1-naphthyl)ethylammonium) we investigated its thermal properties and glass-formation behaviors. Detailed structural studies were performed to gain insights into its melting mechanism and vitrification process using various techniques including electron microscope, terahertz (THz)/far-IR, total scattering and high-temperature NMR. Mechanical and optical properties of crystalline (<i>S</i>-NEA)₂PbBr₄ and the corresponding <i>a</i>_g(<i>S</i>-NEA)₂PbBr₄ (<i>a</i>_g = melt-quenched glass) materials were also explored. This study provides insights into the nature of glass formation and build structure-relationships, promoting the development of HOIP glasses for a broad range of prospective applications.<br/>Secondly, conventional melt-quenching approach for glass formation requires a stable liquid state at high temperature, which is absent in most HOIPs. Here we report the first mechanochemically-induced crystal-glass transformation of HOIPs, which provides a rapid, green, and efficient route to their glassy phase. Both a melting and a non-melting crystalline HOIP were successfully converted to amorphous solids which exhibit glass transitions. Time-resolved in-situ ball-milling synchrotron powder diffraction was employed to study the microstructural evolution of amorphisation, which showed that the crystallite size reaches a comminution limit before the amorphisation process is complete, indicating that energy is further accumulated as crystal defects. Total scattering experiments revealed the limited short-range order of HOIP glasses, and their optical properties were studied by photoluminescence spectroscopy. This study provides another pathway to prepare HOIP glasses and demonstrates how mechanochemistry offers further opportunities to explore the glassy state of HOIP materials.<br/><br/><br/>[1] T. D. Bennett and S. Horike, <i>Nat. Rev. Mat.</i>, 3, 431-440, 2018<br/>[2] N. Ma and S. Horike, <i>Chem. Rev</i>., 122, 4163−4203, 2022<br/>[3] C. Ye, L. N. McHugh, C. Chen, <i>et al</i>. <i>Angew. </i><i>Chem. Int. Ed.</i>, 62, e2023024, 2023<br/>[4] B. K. Shaw, A. R. Hughes, M. Ducamp, <i>et al</i>. <i>Nat Chem</i>., 13, 778, 2021<br/>[5] A. Singh, M. K. Jana and D. B. Mitzi, <i>Adv. Mater</i>., 33, 2005868, 2021