X-ray and neutron scattering

Abstract:

Crystalline metal-organic frameworks (MOFs) exhibit enormous potential application in gas separation, thanks to their highly porous structures and precise pore size distributions. Nevertheless, the inherent limitations in mechanical stability of crystalline MOFs cause challenges in processing MOF powders into bulky structures, particularly for membrane filtrations. Melt-quenched MOF glasses boast excellent processability due to liquid-like properties. However, the melting process diminishes the inherent porosity, leading to reduced gas adsorption capacities and lower gas diffusion coefficients. In this work, we demonstrated that enhancing the porosity of MOF glasses is achievable through topological engineering on the crystalline precursors. Crystalline zeolitic imidazolate frameworks (ZIFs) with large 12-membered rings pores, including AFI and CAN topology, were synthesized by using both structure-directing agents and mixed organic ligands. The large pores are partially preserved in the melt-quenched glass as evidenced by high-pressure CO₂ absorption at 3000 kPa. The a_gAFI-[Zn(Im)_1.68(bIm)_0.32] glass was then fabricated into self-supported membranes, which shows high gas separation performance, for example, CO₂ permeance of 3.7 × 10⁴ GPU with a CO₂/N₂ selectivity of 14.8.

Abstract:

ABX₃-type hybrid organic-inorganic structures have recently emerged as a new class of meltable materials. Here, by the use of phenylphosphonium derivatives as A cation, we study liquid- and glass-forming behavior of a new family of hybrid structures, (RPh₃P)[Mn(dca)₃] (R = Me, Et, Ph; dca = dicyanamide). These new compounds melt at 196-237 °C (T_m) and then vitrify upon cooling to room temperature, forming glasses. In situ glass formation of this new family of materials was probed on a large scale using a variable-temperature PXRD experiment. Structure analyses of the crystalline and the glasses were carried out by solid-state nuclear magnetic resonance spectroscopy and synchrotron X-ray total scattering techniques for using the pair distribution function. The mechanical properties of the glasses produced were evaluated showing promising durability. Thermal and electrical conductivities showed low thermal conductivities (κ ∼ 0.07-0.09 W m^-1 K^-1) and moderate electrical conductivities (σ ∼ 10^-4-10^-6 S m^-1) at room temperature, suggesting that by the precise control of the A cation, we can tune meltable hybrid structures from moderate conductors to efficient thermal insulators. Our results raise attention on the practical use of this new hybrid material in applications including, e.g., photovoltaic devices to prevent light-deposited heat (owing to low κ_RT), energy harvesting thermoelectric, etc., and advance the structure-property understanding.

Abstract:

A special issue of Acta Crystallographica Section B reports the great progress made recently in the determination, reporting, and archiving of magnetic structures, of which there are now more than 2000. The infrastructure needed to support the field is now in place. The special issue also highlights new science made possible by these developments.