X-ray and neutron scattering

Abstract:

Natural optical activity (NOA), the ability of a material to rotate the plane of polarized light, has traditionally been associated with structural chirality. However, this relationship has often been oversimplified, leading to conceptual misunderstandings, particularly when attempts are made to correlate structural handedness directly with optical rotatory power. In reality, the relationship between chirality and NOA is more nuanced: optical activity can arise in both chiral and achiral crystal structures, and the sign of the rotation cannot necessarily be inferred from the handedness of the space group. In this work, we conduct a first-principles investigation of NOA in SiO2 and AlPO4 crystals, focusing on their enantiomorphic structural phase transition from high-symmetry hexagonal (P6422 or P6222) to low-symmetry trigonal (P3121 or P3221) space groups. This transition, driven by the condensation of a zone-centre Γ3 phonon mode, reverses the screw-axis type given by the space-group symbol while leaving the sign of the optical activity unchanged. By following the evolution of the structure and the optical response along the transition pathway, we clarify the microscopic origin of this behaviour. We demonstrate that the sense of optical rotation is determined not by the nominal handedness of the screw axis given in the space-group symbol but by the atomic-scale handedness of the most polarizable atoms of the structure.

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.

Abstract:

In this study we report a facile one-step, room temperature synthetic methodology for fabricating ZIF-8-coated phosphate-based glass (PBG) microspheres. ZIF-8 was grown in situ onto solid (non-porous) and porous P40 glass microspheres (composition 20Na₂O-24MgO-16CaO-40P₂O₅, average diameter 153-172 µm), confirmed by PXRD and SEM analyses, with reaction times of one hour sufficient for coating onto solid P40 microspheres. An extended reaction time of 4.5 hours resulted in more homogeneous ZIF-8 coverage on the porous P40 microspheres. The ZIF-8 layer reduced microsphere degradation in water and slowed ion release from the microspheres, demonstrating a sacrificial protective coating effect of the ZIF-8. The ZIF-8 layer also enabled Zn²⁺ release (6.9-13.6 ppm over 7 days) and demonstrated improved methylene blue loading capacity, showing promise for pH-responsive drug delivery. This adaptable synthetic method to produce P40-based MOF composites highlights the potential synergistic benefits of combining PBG microspheres (bioactivity, tuneable degradation rates) and MOFs (high surface areas and porosity), offering an underexplored strategy to new MOF@PBG composites for drug delivery, antibacterial coatings and bone repair.

Abstract:

Sr2MnO2Cu3.5S3 contains mixed-valent Mn ions Mn2+/3+ in axially elongated MnO4S2 octahedra connected via apical sulfide anions to copper-deficient antifluorite-type Cu4-δS3 layers where δ ∼ 0.5. Copper deficiency is charge-compensated by oxidation of Mn 3d states resulting in mixed-valency. The compound is tetragonal in P4/mmm at ambient temperatures (a = 4.016345(1) Å, c = 11.40708(5) Å). Below 190 K, superlattice reflections in diffraction data and an increase in resistivity, signal checkerboard charge-ordering of Mn2+ and Mn3+. The superstructure approximates to a √2a × √2a × 2c expansion of the room temperature cell in space group P42/nmc. However, satellite reflections signal a (3 + 2)D incommensurate modulation of Cu site occupancies in the Cu-deficient sulfide layers coupled with displacements of the sulfur positions; overall the superstructure below 190 K requires description in superspace group P42/nmc(a,0,0)0000(0,a,0)00s0. Analysis of total scattering measurements along with pair distribution functions supports the charge-ordered low temperature model and reveals local order of distinct Mn sites within the higher-temperature charge-disordered regime. Below TN = 27 K, long-range magnetic ordering is A-type antiferromagnetic with distinct moments for Mn2+ and Mn3+ ions directed perpendicular to the MnO2 planes and ordered ferromagnetically. Long-range antiferromagnetic order results from interlayer antiferromagnetic coupling. A metamagnetic transition at 1.1 T corresponds to a change to long-range interlayer ferromagnetic ordering via a spin-reorientation of magnetic moments and is associated with a slight decrease in the charge separation between the Mn sublattices, consistent with observations on mixed-valent perovskite and Ruddlesden-Popper-type oxide manganites.

Abstract:

Zeolitic imidazolate frameworks (ZIFs), a subclass of metal-organic frameworks (MOFs), combine high porosity and chemical tunability with a resistance to harsh conditions. Understanding their response to extreme pressure and heat is critical for application development due to the conditions under which they may be required to work or for predicting their response to any processing before use. In this study, we characterize long- and short-range order in ZIF-8 and ZIF-62 under compression using Bragg X-ray diffraction and pair distribution function (PDF) analysis for a large pressure range (up to ∼5 GPa) previously attempted in very few works. X-ray absorption fine structure analysis was carried out under high-pressure-temperature conditions to probe the medium-range order, a novelty in MOFs. ZIF-8 demonstrated a crystalline-crystalline phase transition above 0.36 GPa but no full amorphization. In ZIF-62, pore intrusion of the silicone oil pressure-transmitting medium (PTM) was observed through negative compressibility and by retention of its open-pore configuration. Full amorphization was achieved, with heating lowering the amorphization threshold. Finally, a unique distortion in both MOFs was suggested by the spectroscopic data. These results provide insight into the thermomechanical stability of crystalline ZIFs and the mechanism underlying their amorphization.