Erica G. Bithell

Structure and properties of an amorphous metal-organic framework.

ZIF-4, a metal-organic framework (MOF) with a zeolitic structure, undergoes a crystal-amorphous transition on heating to 300 degrees C. The amorphous form, which we term a-ZIF, is recoverable to ambient conditions or may be converted to a dense crystalline phase of the same composition by heating to 400 degrees C. Neutron and x-ray total scattering data collected during the amorphization process are used as a basis for reverse Monte Carlo refinement of an atomistic model of the structure of a-ZIF. The structure is best understood in terms of a continuous random network analogous to that of a-SiO2. Optical microscopy, electron diffraction and nanoindentation measurements reveal a-ZIF to be an isotropic glasslike phase capable of plastic flow on its formation. Our results suggest an avenue for designing broad new families of amorphous and glasslike materials that exploit the chemical and structural diversity of MOFs.

Hybrid Nanosheets of an Inorganic–Organic Framework Material: Facile Synthesis, Structure, and Elastic Properties

We report a new 2-D inorganic-organic framework material, MnDMS [Mn 2,2-dimethylsuccinate], featuring weakly bound hybrid layers in its bulk crystals that can be readily exfoliated into nanosheets via ultrasonication. The fully exfoliated hybrid nanosheets correspond to a unilamellar thickness of about 1 nm, while the partially exfoliated nanosheets (multilayer films) exhibit a typical thickness on the order of 10 nm. We used atomic force microscopy to characterize their surface topography and to map the variation of nanomechanical properties across the surface of the delaminated nanosheets. The morphology and crystallographic orientation of the exfoliated layers were further studied by transmission electron microscopy. Additionally, we investigated the elastic anisotropy underlying the bulk host material by means of single-crystal nanoindentation, from which the critical resolved shear stress (τ(crit)) needed for the micromechanical delamination of individual layers was determined to be relatively small (≲0.4 GPa).

Mechanical Tunability via Hydrogen Bonding in Metal-Organic Frameworks with the Perovskite Architecture

Two analogous metal-organic frameworks (MOFs) with the perovskite architecture, [C(NH2)3][Mn(HCOO)3] (1) and [(CH2)3NH2][Mn(HCOO)3] (2), exhibit significantly different mechanical properties. The marked difference is attributed to their distinct modes of hydrogen bonding between the A-site amine cation and the anionic framework. The stronger cross-linking hydrogen bonding in 1 gives rise to Youngs moduli and hardnesses that are up to twice those in 2, while the thermal expansion is substantially smaller. This study presents clear evidence that the mechanical properties of MOF materials can be substantially tuned via hydrogen-bonding interactions.

Facile mechanosynthesis of amorphous zeolitic imidazolate frameworks.

A fast and efficient mechanosynthesis (ball-milling) method of preparing amorphous zeolitic imidazolate frameworks (ZIFs) from different starting materials is discussed. Using X-ray total scattering, N(2) sorption analysis, and gas pycnometry, these frameworks are indistinguishable from one another and from temperature-amorphized ZIFs. Gas sorption analysis also confirms that they are nonporous once formed, in contrast to activated ZIF-4, which displays interesting gate-opening behavior. Nanoparticles of a prototypical nanoporous substituted ZIF, ZIF-8, were also prepared and shown to undergo amorphization.

Synthesis, structure and optical properties of cerium-doped calcium barium phosphate – a novel blue-green phosphor for solid-state lighting

A new blue-green phosphor, Ca6BaP4O17:Ce3+, which can be prepared by conventional solid-state synthesis, is reported as a candidate phosphor for solid-state lighting with near-ultraviolet LEDs. Under excitation at around 400 nm, Ca6BaP4O17:Ce3+ shows strong blue-green emission with the peak position at 477 nm. Ce3+,Si4+ co-doping is found to enhance the luminous intensity, and the unique emission characteristics of this combination are studied and related to the crystal structure. Ca6BaP4O17 is an exceptional host material, which also accommodates Eu2+, emitting strong yellow light under 400 nm excitation. A fabricated white LED, combining Ca6BaP4O17:Ce3+,Si4+ with Ca6BaP4O17:Eu2+ and a red CaAlSiN3:Eu2+ phosphor, achieved a luminous efficacy of 45 lm W−1 with a color-rendering index of 93 around the correlated color temperature of 4500 K.

Synthesis, structure and optical properties of europium doped calcium barium phosphate – a novel phosphor for solid-state lighting

A new phase, Ca6BaP4O17, was found in the CaO–BaO–P2O5 phase diagram and its Eu2+-doped derivative was evaluated as a possible phosphor for solid-state lighting. Ca6BaP4O17 was prepared by conventional solid-state synthesis and its structure was solved ab initio from high-resolution, synchrotron X-ray powder diffraction data. The bandgap of Ca6BaP4O17 was estimated to be 5.79 eV. Under excitation with UV to blue light, Ca6BaP4O17:Eu2+ shows strong yellow emission with the peak position at 553 nm. A white LED, which was fabricated with an InGaN blue chip and Ca6BaP4O17:Eu2+, achieved a luminous efficacy of 31 lm W−1 with a color-rendering index of 78 around the correlated color temperature of 6500 K.

Polymorph Identification and Crystal Structure Determination by a Combined Crystal Structure Prediction and Transmission Electron Microscopy Approach

Electron diffraction offers advantages over X-ray based methods for crystal structure determination because it can be applied to sub-micron sized crystallites, and picogram quantities of material. For molecular organic species, however, crystal structure determination with electron diffraction is hindered by rapid crystal deterioration in the electron beam, limiting the amount of diffraction data that can be collected, and by the effect of dynamical scattering on reflection intensities. Automated electron diffraction tomography provides one possible solution. We demonstrate here, however, an alternative approach in which a set of putative crystal structures of the compound of interest is generated by crystal structure prediction methods and electron diffraction is used to determine which of these putative structures is experimentally observed. This approach enables the advantages of electron diffraction to be exploited, while avoiding the need to obtain large amounts of diffraction data or accurate reflection intensities. We demonstrate the application of the methodology to the pharmaceutical compounds paracetamol, scyllo-inositol and theophylline.

Determination of the Crystal Structure of a New Polymorph of Theophylline

A new approach to crystal structure determination, combining crystal structure prediction and transmission electron microscopy, was used to identify a potential new crystal phase of the pharmaceutical compound theophylline. The crystal structure was determined despite the new polymorph occurring as a minor component in a mixture with Form II of theophylline, at a concentration below the limits of detection of analytical methods routinely used for pharmaceutical characterisation. Detection and characterisation of crystallites of this new form were achieved with transmission electron microscopy, exploiting the combination of high magnification imaging and electron diffraction measurements. A plausible crystal structure was identified by indexing experimental electron-diffraction patterns from a single crystallite of the new polymorph against a reference set of putative crystal structures of theophylline generated by global lattice energy minimisation calculations.

Reconstruction strategies for structure solution using precession electron diffraction data from hybrid inorganic-organic framework materials

Precession electron diffraction has received significant recent attention, and has the potential to complement x-ray structure solution methods in situations where the latter are difficult to apply. Certain materials systems may nevertheless present challenges for solution by electron diffraction, and this is particularly true when the unit cell includes light element components. Drawing specifically on our work with hybrid inorganic-organic framework materials, we describe recent approaches to reconstructing their crystal potentials, which have led to significant improvements in the accuracy and quality of the derived potential maps for this type of structure.

The determination of the structure and composition at interfaces to atomic resolution

The prediction of the properties of modern multilayered systems provides a challenge to the accuracy to which the structure of a boundary can be characterised using the transmission electron microscope (TEM). A variety of new ways of using the TEM have recently been developed in this context, and here we describe some of the approaches which are now being applied. Particular emphasis is given to a description of the strengths and weaknesses of the Fresnel method in its potentially broad application to compositional analysis.