X-ray and neutron scattering

Abstract:

We use spherical neutron polarimetry to determine the ground state magnetic structure of ${\mathrm{Mn}}_3\mathrm{Sn}$ . We find that ${\mathrm{Mn}}_3\mathrm{Sn}$ adopts an inverse triangular structure with spins parallel to $〈100〉$ (type III) rather than spins parallel to $〈110〉$ (type IV). Density functional theory calculations reveal no energy difference between these two structures, suggesting that the selection is caused by subtle effects such as sixth-order anisotropy. Partial control of the magnetic domain population through a moderate magnetic field is key to distinguishing between the two models. We find that three of the six domains are approximately equally populated, while the others have negligible population. Upon entering the low temperature incommensurate phase, the domain structure is lost. The domains in this phase are decoupled from the magnetic field and can therefore not be controlled by any known method.

Abstract:

We report a scalable, water-based methodology for the direct room-temperature synthesis of porous amorphous UiO-66-type metal–organic frameworks (aMOFs), enabling the incorporation of a range of functionalised terepthalate linkers without organic solvents during framework formation. Powder X-ray diffraction and scanning electron diffraction confirm the formation of truly topologically amorphous UiO-66 derivatives, while pair distribution function (PDF) analysis shows that the amorphous frameworks retain the local structural motifs of their crystalline analogues despite the loss of long-range order. Relative to crystalline UiO-66, the directly synthesised amorphous UiO-66 exhibits a reduced but permanent porosity (BET surface area 286 vs. 997 m2 g−1 CO2-accessible pore volume 0.196 vs. 0.519 cm3 g−1), together with a high concentration of defects, consistent with a cluster : linker ratio of 1 : 5.3 compared with 1 : 6 for the ideal crystalline framework. In the esterification of levulinic acid, amorphous UiO-66 reaches 87.7% conversion to methyl levulinate after 3 h, compared with 75.5% for crystalline UiO-66, and retains 95% of its initial activity after five catalytic cycles (vs. 86% for the crystalline analogue). These results demonstrate that direct, water-based synthesis provides access to functional, porous, and highly defective amorphous UiO-66 materials with catalytic performance comparable to or exceeding that of their crystalline counterparts under the conditions studied.

Abstract:

Tin is known for its asymmetric crystal structure and numerous solid phase transitions, with molecular dynamics studies suggesting the beta to gamma phase transition exhibits a strong orientation dependence. In this study, shock compression experiments are conducted on tin single crystals and polycrystals to probe the effects of the crystal orientation on this phase transition through Hugoniot measurements, with peak pressures between 9 and 13 GPa. A strong order-of-magnitude orientation dependence of the elastic limit is found; however, the transition and post-transition behavior show at best only qualitative differences to the velocimetry profiles, with no quantitative variation. A dependence of the transition on the peak pressure is also observed. Explanations of these results based on potential transformation pathways identified through prior static high pressure work are discussed.

Abstract:

Defect engineering of metal-organic frameworks (MOFs) has been shown to impact many properties of these porous structures, including affecting the accessible pore volume as well as introducing additional active sites to modify the catalytic activity of the frameworks. However, this defect engineering has previously primarily been carried out through synthesis-based methods. Ball-milling of the frameworks presents an alternative method for the introduction of defects, which has not been largely investigated for its effects on catalysis. The complex pressure states experienced during milling result in property changes, both enhancing and diminishing defect accessibility, necessitating a detailed investigation. This work characterizes three Zirconium-based MOFs (UiO-66, MOF-808, and NU-1000), using total scattering X-ray diffraction, infrared spectroscopy, and thermal analysis to investigate their collapse and defect introduction during all stages of ball-milling. It then assesses the utility of ball-milling UiO-66 to different extents as a method for improving catalytic abilities within two reactions, the formation of propargylamine, and the conversion of glucose to fructose. The mechanical amorphization of UiO-66 led to either an increase or a decrease in catalytic ability depending on the milling time and the reaction investigated.