David Keen

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.

Abstract:

We present a model of x-ray thermal diffuse scattering (TDS) from a cubic polycrystal with an arbitrary crystallographic texture, based on the classic approach of Warren [B. E. Warren, Acta Crystallogr. 6, 803 (1953)]. We compare the predictions of our model with femtosecond x-ray diffraction patterns gathered from ambient and dynamically compressed rolled copper foils obtained at the High Energy Density instrument of the European X-Ray Free-Electron Laser facility and find that the texture-aware TDS model yields more accurate results than does the conventional powder model owed to Warren. Nevertheless, we further show: with sufficient angular detector coverage, the TDS signal is largely unchanged by sample orientation and in all cases strongly resembles the signal from a perfectly random powder; shot-to-shot fluctuations in the TDS signal resulting from grain-sampling statistics are at the percent level, in stark contrast to the fluctuations in the Bragg-peak intensities (which are over an order of magnitude greater); and TDS is largely unchanged even following texture evolution caused by compression-induced plastic deformation. We conclude that TDS is robust against texture variation, making it a flexible temperature diagnostic applicable just as well to off-the-shelf commercial foils as to ideal powders.

Abstract:

We present a direct route to prepare a family of MOF glasses without a meltable crystalline precursor, in contrast to the conventional melt-quenching approach. This one-step synthesis uses the linker itself as the reaction medium under an inert atmosphere, enabling the incorporation of highly hydrolytically unstable M(II) centers. This route produces high-purity iron (II) MOF glasses avoiding the oxidation and partial degradation commonly associated with the conventional melt-quenching process. The transparent glassy monoliths of formula Fe(im)_2-x(bim)_x, denoted as dg-MUV-29 (dg = direct-glass), can be prepared with different amounts of imidazole and benzimidazole as well as with linkers with diverse functionalities (NH₂, CH₃, Br, and Cl). The absence of magnetic impurities allows us to study the magnetic properties of the MOF glass itself and show that MOF glasses are good model systems for topologically-disordered amorphous antiferromagnets. We also present the functional advantages of direct-glass synthesis by creating free-standing films of glassy MOFs and integrating them in optoelectronic devices. Direct-glass synthesis is thus a powerful route to exploit the true functional potential of glassy MOFs, not only realizing further classes of MOF glasses but also unveiling properties that can be accessed with these materials.

Abstract:

Understanding the relationship between crystal structure, bonding and thermal transport is critical for the discovery of materials with ultralow thermal conductivities. Materials in the bismuthinite-aikinite series, Cu_1-x□_xPb_1-xBi_1+xS₃ (0 ≤ x ≤ 1), in which a Bi³⁺ cation and a vacancy (□) are progressively substituted by a Pb²⁺ and a Cu⁺ cation, exhibit ultralow thermal conductivities (∼0.5 W m^-1K^-1 for x < 1). Here, we investigate the effect of decreasing the Pb²⁺ and Cu⁺ content on the crystal structure and properties of Cu_1-x□_xPb_1-xBi_1+xS₃ (x = 0, 0.33, 0.6 and 0.83). These materials exhibit two-channel thermal transport, with non-propagating phonons being the dominant contribution. Neutron diffraction data reveal that intermediate compositions crystallize in the krupkaite structure (x = 0.5, P2₁ma), instead of the end-member aikinite structure (x = 0, Pnma). Pair distribution function (PDF) analysis reveals that the disordering of vacancies and cations deviates significantly from that expected for a statistical distribution and that, at a local level, copper-rich and copper-poor regions occur. Reducing the Pb²⁺ and Cu⁺ content results in lattice softening, which may be attributed to the increased concentration of vacancies in copper-poor regions. Moreover, the persistence of short Pb²⁺-Cu⁺ distances in the copper-rich regions is likely to facilitate the cooperative interaction between lone pairs and rattling Cu⁺ cations that leads to phonon scattering. These findings provide crucial insights into the effect of the local structure on the phonon transport and highlight the potential of local-structure design to achieve high thermoelectric performance in crystalline solids.

Abstract:

High-quality total scattering data, a key tool for understanding atomic-scale structure in disordered materials, require stable instrumentation and access to high momentum transfers. This is now routine at dedicated synchrotron instrumentation using high-energy X-ray beams, but it is very challenging to measure a total scattering dataset in less than a few microseconds. This limits their effectiveness for capturing structural changes that occur at the much faster timescales of atomic motion. Current X-ray free-electron lasers (XFELs) provide femtosecond-pulsed X-ray beams with maximum energies of ∼24 keV, giving the potential to measure total scattering and the attendant pair distribution functions (PDFs) on femtosecond timescales. We demonstrate that this potential has been realized using the HED scientific instrument at the European XFEL and present normalized total scattering data for 0.35 Å^-1 < Q < 16.6 Å^-1 and their PDFs from a broad spectrum of materials, including crystalline, nanocrystalline and amorphous solids, liquids and clusters in solution. We analyzed the data using a variety of methods, including Rietveld refinement, small-box PDF refinement, joint reciprocal-real-space refinement, cluster refinement and Debye scattering analysis. The resolution function of the setup is also characterized. We conclusively show that high-quality data can be obtained from a single ∼30 fs XFEL pulse for multiple different sample types. Our efforts not only significantly increase the existing maximum reported Q range for an S(Q) measured at an XFEL but also mean that XFELs are now a viable X-ray source for the broad community of people using reciprocal-space total scattering and PDF methods in their research.