Zeolitic imidazolate frameworks (ZIFs) feature complex phase transitions, including polymorphism, melting, vitrification, and polyamorphism. Experimentally probing their structural evolution during transitions involving amorphous phases is a significant challenge, especially at the medium-range length scale. To overcome this challenge, here we first train a deep learning-based force field to identify the structural characteristics of both crystalline and non-crystalline ZIF phases. This allows us to reproduce the structural evolution trend during the melting of crystals and formation of ZIF glasses at various length scales with an accuracy comparable to that of ab initio molecular dynamics, yet at a much lower computational cost. Based on this approach, we propose a new structural descriptor, namely, the ring orientation index, to capture the propensity for crystallization of ZIF-4 (Zn(Im)2, Im = C3H3N2−) glasses, as well as for the formation of ZIF-zni (Zn(Im)2) out of the high-density amorphous phase. This crystal formation process is a result of the reorientation of imidazole rings by sacrificing the order of the structure around the zinc-centered tetrahedra. The outcomes of this work are useful for studying phase transitions in other metal-organic frameworks (MOFs) and may thus guide the development of MOF glasses.
The structure of melt-quenched zeolitic imidazole framework (ZIF) glasses can provide insights into their glass-formation mechanism. We directly detected short-range disorder in ZIF glasses using ultrahigh-field zinc-67 solid-state nuclear magnetic resonance spectroscopy. Two distinct Zn sites characteristic of the parent crystals transformed upon melting into a single tetrahedral site with a broad distribution of structural parameters. Moreover, the ligand chemistry in ZIFs appeared to have no controlling effect on the short-range disorder, although the former affected their phase-transition behavior. These findings reveal structure-property relations and could help design metal-organic framework glasses.
more » « less- Award ID(s):
- 1855176
- NSF-PAR ID:
- 10141518
- Publisher / Repository:
- American Association for the Advancement of Science (AAAS)
- Date Published:
- Journal Name:
- Science
- Volume:
- 367
- Issue:
- 6485
- ISSN:
- 0036-8075
- Page Range / eLocation ID:
- p. 1473-1476
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
ABSTRACT -
In hybrid materials, a high-symmetry lattice is decorated by low-symmetry building blocks. The result is an aperiodic solid that hosts many nearly-degenerate disordered configurations. Using the perovskite methylammonium lead iodide (MAPbI 3 ) as a prototype hybrid material, we show that the inherent disorder renders the conventional phonon picture of transport insufficient. Ab initio molecular dynamics and analysis of the spectral energy density reveal that vibrational carriers simultaneously exhibit features of both classical phonons and of carriers typically found in glasses. The low frequency modes retain elements of acoustic waves but exhibit extremely short lifetimes of only a few tens of picoseconds. For higher frequency modes, strong scattering due to rapid motion and reconfiguration of the organic cation molecules induces a loss of definition of the wave vector. Lattice dynamics shows that these carriers are more akin to diffusons – the nonwave carriers in vitreous materials – and are the dominant contributors to thermal conduction near room temperature. To unify the framework of glassy diffusons with that of phonons scattered at the ultimate limit, three-phonon interactions resolved from first-principles expose anharmonic effects two orders of magnitude higher than in silicon. The dominant anharmonic interactions occur within modes of the PbI 6 octahedral framework itself, as well as between modes of the octahedral framework and modes localized to the MA molecules. The former arises from long-range interactions due to resonant bonding, and the latter from polar rotor scattering of the MA molecules. This establishes a clear microscopic connection between symmetry-breaking, dynamical disorder, anharmonicity, and the loss of wave nature in MAPbI 3 .more » « less
-
Sodalite zeolitic-imidazole frameworks (ZIFs) show great potential due to their effective aperture sizes suitable for small gas separations. Numerous efforts have, therefore, been made in tuning their effective aperture sizes to control and enhance their molecular sieving properties. Herein, we present a new strategy to finely tune the effective aperture size of CdIF-1, a cadmium-substituted ZIF-8 analogue, based on thermal amorphization. Among several ZIF-8 analogues screened, CdIF-1 was found to be the only one that could be thermally amorphized. The controlled amorphization reduced the long-range structural order while preserving the short-range order, thereby systematically densifying the ZIF structure and consequently affecting its effective aperture. Meanwhile, it was found that amorphization enhanced the flexibility of the framework, resulting in accessible pores at temperatures above 273 K. As compared to its crystalline counterpart, partially amorphized CdIF-1 showed significantly improved diffusion and adsorption selectivities of n -C 4 H 10 /i-C 4 H 10 ( i.e. , 1.5 → 40.7 and 1.1 → 4.9, respectively), likely due to the amorphization-induced tuning of its effective aperture size.more » « less
-
Recent demonstrations of melting in the metal–organic framework (MOF) family have created interest in the interfacial domain between inorganic glasses and amorphous organic polymers. The chemical and physical behaviour of porous hybrid liquids and glasses is of particular interest, though opportunities are limited by the inaccessible melting temperatures of many MOFs. Here, we show that the processing technique of flux melting, ‘borrowed’ from the inorganic domain, may be applied in order to melt ZIF-8, a material which does not possess an accessible liquid state in the pure form. Effectively, we employ the high-temperature liquid state of one MOF as a solvent for a secondary, non-melting MOF component. Differential scanning calorimetry, small- and wide-angle X-ray scattering, electron microscopy and X-ray total scattering techniques are used to show the flux melting of the crystalline component within the liquid. Gas adsorption and positron annihilation lifetime spectroscopy measurements show that this results in enhanced, accessible porosity to a range of guest molecules in the resultant flux melted MOF glass.more » « less
-
more » « less
Abstract Features intrinsic to disorder and network aspects are ubiquitous in structural glasses. Among this important class of materials, chalcogenide glasses are special—they are built of short‐range covalent forces, making them simpler than silicate glasses that possess mixed ionic and covalent forces. Selenium‐based glasses also display complex elastic phase transitions that have been described from various models, including mean‐field approaches to molecular simulations. These point to the presence of two sharply defined elastic phase transitions, a rigidity and stress transition that are non‐mean‐field in character, and separate the three distinct topological phases of flexible, isostatically rigid, and stressed‐rigid. This article reviews the physics of these glassy networks. The elastic phases and glass transition temperature are explained on a molecular level in terms of topological constraint theory (TCT), connectivity, and the open degrees of freedom. The broader aspects of TCT in relation to phase change materials, high‐
k dielectrics, and cements are also commented upon.
