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Abstract: Porous glasses from metal-organic frameworks (MOFs) represent a new class of functional inorganic-organic glasses, which have been proposed for applications ranging from solid electrolytes to radioactive waste storage. So far, just a few zeolitic imidazolate frameworks (ZIFs), a subset of MOFs, have been reported to melt and the structural and compositional requirements for MOF melting and glass formation are poorly understood. Here, we show how the melting point of the prototypical ZIF-4/ZIF-62(M) frameworks (composition M(im)2–x(bim)x; M2+ = Co2+, Zn2+; im– = imidazolate; bim– = benzimidazolate) can be controlled systematically by adjusting the molar ratio of the two imidazolate-type linkers im– and bim–. By covering the entire range from x = 0 to x = 0.35, we unveil a delicate transition from ZIF materials showing sequential amorphization/recrystallization to derivatives exhibiting coherent melting and a liquid phase that is stable over a large temperature window. The melting point of this ZIF system is a direct function of x and can be lowered from ca. 430 °C to only 370 °C – by far the lowest melting point reported for a three-dimensional porous MOF. On the basis of our results, we postulate compositional requirements for ZIF melting and glass formation, which may guide the search for other meltable ZIFs. Moreover, gas physisorption experiments establish that the ZIF glasses adsorb technologically relevant C3 and C4 hydrocarbons. Importantly, the adsorption kinetics are much faster for propylene compared to propane and are also dependent on the im–:bim– ratio, thus demonstrating the potential of these ZIF glasses for applications in gas separation.

Open Access

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Abstract: The majority of research into metal-organic frameworks (MOFs) focuses on their crystalline nature. Recent research has revealed solid-liquid transitions within the family, which we use here to create a class of functional, stable and porous composite materials. Described herein is the design, synthesis, and characterisation of MOF crystal-glass composites, formed by dispersing crystalline MOFs within a MOF-glass matrix. The coordinative bonding and chemical structure of a MIL-53 crystalline phase are preserved within the ZIF-62 glass matrix. Whilst separated phases, the interfacial interactions between the closely contacted microdomains improve the mechanical properties of the composite glass. More significantly, the high temperature open pore phase of MIL-53, which spontaneously transforms to a narrow pore upon cooling in the presence of water, is stabilised at room temperature in the crystal-glass composite. This leads to a significant improvement of CO2 adsorption capacity.

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Abstract: While titanium‐based metal–organic frameworks (MOFs) have been widely studied for their (photo)catalytic potential, only a few TiIV MOFs have been reported owing to the high reactivity of the employed titanium precursors. The synthesis of COK‐47 is now presented, the first Ti carboxylate MOF based on sheets of TiIVO6 octahedra, which can be synthesized with a range of different linkers. COK‐47 can be synthesized as an inherently defective nanoparticulate material, rendering it a highly efficient catalyst for the oxidation of thiophenes. Its structure was determined by continuous rotation electron diffraction and studied in depth by X‐ray total scattering, EXAFS, and solid‐state NMR. Furthermore, its photoactivity was investigated by electron paramagnetic resonance and demonstrated by catalytic photodegradation of rhodamine 6G.

Open Access

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Abstract: Sodium-ion batteries are seen as an increasingly attractive option for large-scale energy storage, the rising price, and limited availability of lithium being seen as a major challenge for the future of the dominant lithium systems. One of the major bottlenecks for the commercialisation of sodium-ion technology is to find a suitable anode material. This search is hampered owing to major gaps in our understanding of how these systems work. The major objective of this thesis is to gain a better understanding of the mechanisms by which sodium inserts into different anode materials. Two systems are studied: hard carbons are widely viewed as the most promising anode material in the near future, yet the sodium insertion mechanism is still widely debated. Secondly, tin represents one of the highest capacity anode materials, able to achieve a 847 mAhg − 1 −1 , with numerous electrochemical features suggesting an interesting (dis)charge mechanism. It has previously been shown that both of these systems proceed through disordered phases, and as such, we focus on pair distribution function analysis (PDF) and solid-state NMR as local structure probes for our structural characterisation. Furthermore, to avoid relaxation effects in metastable intermediates, much of this work uses \operando methods. We present evidence from \operando 2 3 23 Na NMR that sodium inserts into hard carbon in a two-stage mechanism: the first stage is consistent with charge localisation; after which the ions become progressively more metallic. We further investigate the structure of the pristine carbon, using PDF to demonstrate that the graphene-like fragments exhibit significant curvature. This work is then extended to a number of different carbons, both commercially made and synthesised from glucose. We present a novel method to fit PDF data for the pristine materials: using curved aperiodic, stacked graphene layers to generate the simplest model (fewest number of atoms) that explains all features of the data. \Operando measurements are presented that demonstrate expansion of the shortest C--C bonds during the first electrochemical process, along with the formation of sodium nano-clusters during the second. We then apply this novel multi-modal approach to tin anodes. We find that sodium insertion begins by conversion into NaSn 2 2 , a phase consisting of stanene layers separated by sodium ions. This is then broken down into an amorphous phase, where reverse Monte Carlo refinements show that the predominant tin connectivity is chains. Further reaction with sodium results in structures containing Sn−-Sn dumbbells, which interconvert through a solid-solution mechanism. Finally, we show that Na 15 15 Sn 4 4 , can store additional sodium atoms as an off-stoichiometry compound (Na 15+x 15+x Sn 4 4 ). The sodium removal (charge) process is shown to begin by forming Sn--Sn dumbbells in a structure similar to the discharge phase, but with a lower sodium content (Na 2 2 Sn). Following this, the structure loses all long-range order. We show, through refinements of suitable models against \operando PDF data, that the dumbbells are retained, but later transform into a second amorphous phase consisting of tin tetrahedra. Finally, we show that β β -tin reforms, with no intermediate analogous to NaSn 2 2 . We additionally demonstrate a substantial degree of kinetic control in these (de)alloying.

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Abstract: K0.5Bi0.5TiO3 (KBT) – one of the few perovskite-like ferroelectric compounds with room-temperature tetragonal symmetry – differs from other members of this family (BaTiO3 and PbTiO3) by the presence of a disordered mixture of K and Bi on cuboctahedral sites. This disorder is expected to affect local atomic displacements and their response to an applied electric field. We have derived nanoscale atomistic models of KBT by refining atomic coordinates to simultaneously fit neutron/X-ray total-scattering and extended X-ray absorption fine-structure data. Both Bi and Ti ions were found to be offset relative to their respective oxygen cages in the high-temperature cubic phase; in contrast, the coordination environment of K remained relatively undistorted. In the cubic structure, Bi displacements prefer the <100> directions and the probability-density distribution of Bi features six well-separated sites; a similar preference exists for the much smaller Ti displacements although the split sites for Ti could not be resolved. The cation displacements are correlated, yielding polar nanoregions, while on average the structure appears as cubic. The cubic<->tetragonal phase transition involves both order-disorder and displacive mechanisms. A qualitative change in the form of the Bi probability-density distribution occurs in the tetragonal phase on cooling to room temperature because Bi displacements “branch off” to <111> directions. This change, which preserves the average symmetry, is accompanied by the development of nanoscale polar heterogeneities that exhibit significant deviations of their polarization vectors from the average polar axis.