Emma R. Barney

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.

Thermal Amorphization of Zeolitic Imidazolate Frameworks

Zeolitic imidazolate frameworks (ZIFs) are a family of metal–organic frameworks (MOFs) that display network topologies analogous to those seen in zeolites whereby the zeolitic building blocks of corner-sharing SiO4 tetrahedra are replicated by MN4 tetrahedra (M=metal) linked by imidazolate anions. Over 100 distinct ZIF phases adopting 40 network types currently exist. Interest has focused mainly on the tuneable gas sorption and separation properties of these porous materials, though their potential for catalytic activity is starting to be explored. The retention of thermal stability derived from their zeolitic structures makes them particularly attractive candidates for practical applications. Inorganic zeolites are known to undergo pressureor temperature-induced amorphization. Depending on the heating/pressurization rate, the amorphous materials thus formed can retain some aspects of crystalline topology, and consequently possess a lower configurational entropy than true glasses. Polyamorphism (the presence of structurally isomeric amorphous phases differing in density and entropy) has been identified both experimentally and theoretically in these materials. Given the comparison often drawn between between ZIFs and zeolites (ascribed to the common subtended angles of ca. 1458 at the metal-bridging species, see Figure 1a), it is not surprising that reports of pressure-induced ZIF phase transitions and amorphization exist, albeit at pressures far lower than those of their zeolitic counterparts. Recently, we reported an amorphous ZIF (a-ZIF) with a network topology comparable to that of silica glass, formed by thermal amorphization of the crystalline Zn-based ZIF-4 framework. Further heating of the a-ZIF yielded the dense ZIF-zni. The mechanical properties of the a-ZIF, studied using nanoindentation, were found to be isotropic and intermediate between ZIF-4 and ZIF-zni. Remarkably, the amorphization temperature is comparable to that of purely inorganic zeolites. Here, we show that a-ZIF and crystalline ZIF-zni can also be prepared by heating two structural isomers of ZIF-4 (see Figure 1c). Zn-based ZIF-1 and ZIF-3, possessing the BCT and DFT zeolitic topologies, undergo amorphization and recrystallization to ZIF-zni at similar temperatures to that of ZIF-4. The same process occurs in a cobalt analogue of ZIF-4 (Co-ZIF-4), yielding an amorphous MOF containing a spinactive transition-metal ion. We also show that five ZIFs incorporating substituted imidazolate bridging ions do not undergo thermal amorphization. These frameworks—ZIF-8, -9, -11, -14, and ZIF-bqtz—adopt four different network topologies and possess three different substituted imidazolate species. Solvothermal reaction of Zn(NO3)2 and imidazole (Im) under varying conditions yielded single-crystal samples of ZIF-1, ZIF-4, Co-ZIF-4, ZIF-8, ZIF-9, and ZIF-11 of typical size 0.2! 0.2! 0.1 mm [3,19] whilst a polycrystalline powder sample of ZIF-3 was prepared by a liquid-mixing method. Liquid-assisted grinding was used to synthesize polycrystalline samples of ZIF-14 and ZIF-bqtz. For ZIF-1, -3, and -4 (Co, Zn) thermogravimetric analysis shows that the structure-directing agent and solvent molecules trapped within the porous cavities of the frameworks are Figure 1. a) The similar Si-O-Si and Zn-Im-Zn linkages in zeolites and ZIFs, respectively. b) A snapshot of the continuous random network (CRN) topology of the a-ZIF gained from reverse Monte Carlo (RMC) modeling. c) Representative views of the expanded unit cells of ZIF1 (left), ZIF-3 (center), and ZIF-4 (right).

Extracting the pair distribution function from white-beam X-ray total scattering data

A general method is described for reducing white-beam X-ray total scattering raw data to the differential scattering cross section and pair distribution function. The method incorporates corrections for X-ray fluorescence, Bremsstrahlung radiation, polarization, attenuation, multiple scattering and sample container scattering, and invokes the Krogh-Moe and Norman method to put the data on an absolute scale. An accurate method to convert the differential scattering cross section to the pair distribution function is also described, and a rigorous and revised Lorch function is proposed for removing the effects of Fourier truncation oscillations. The method can be equally applied to synchrotron X-ray data, where the data analysis can be simpler than at a laboratory source.

Structures of Uncharacterised Polymorphs of Gallium Oxide from Total Neutron Diffraction

A structural investigation is reported of polymorphs of Ga(2)O(3) that, despite much interest in their properties, have hitherto remained uncharacterised due to structural disorder. The most crystalline sample yet reported of γ-Ga(2)O(3) was prepared by solvothermal oxidation of gallium metal in ethanolamine. Structure refinement using the Rietveld method reveals γ-Ga(2)O(3) has a defect Fd3m spinel structure, while pair distribution function analysis shows that the short-range structure is better modelled with local F43m symmetry. In further solvothermal oxidation reactions a novel gallium oxyhydroxide, Ga(5)O(7)(OH), is formed, the thermal decomposition of which reveals a new, transient gallium oxide polymorph, κ-Ga(2)O(3), before transformation into β-Ga(2)O(3). In contrast, the thermal decomposition of Ga(NO(3))(3)·9H(2)O first forms ε-Ga(2)O(3) and then β-Ga(2)O(3). Examination of in situ thermodiffraction data shows that ε-Ga(2)O(3) is always contaminated with β-Ga(2)O(3) and with this knowledge a model for its structure was deduced and refined--space group P6(3)mc with a ratio of tetrahedral/octahedral gallium of 2.2:1 in close-packed oxide layers. Importantly, thermodiffraction provides no evidence for the existence of the speculated bixbyite structured δ-Ga(2)O(3); at the early stages of thermal decomposition of Ga(NO(3))(3)·9H(2)O the first distinct phase formed is merely small particles of ε-Ga(2)O(3).

A structural investigation of the alkali metal site distribution within bioactive glass using neutron diffraction and multinuclear solid state NMR

The atomic-scale structure of Bioglass and the effect of substituting lithium for sodium within these glasses have been investigated using neutron diffraction and solid state magic angle spinning (MAS) NMR. Applying an effective isomorphic substitution difference function to the neutron diffraction data has enabled the Na-O and Li-O nearest-neighbour correlations to be isolated from the overlapping Ca-O, O-(P)-O and O-(Si)-O correlations. These results reveal that Na and Li behave in a similar manner within the glassy matrix and do not disrupt the short range order of the network former. Residual differences are attributed solely to the variation in ionic radius between the two species. Successful simplification of the 2 < r (Å) < 3 region via the difference method has enabled all the nearest neighbour correlations to be deconvolved. The diffraction data provides the first direct experimental evidence of split Na-O nearest-neighbour correlations in these melt quench bioactive glasses, and an analogous splitting of the Li-O correlations. The observed correlations are attributed to the metal ions bonded either to bridging or to non-bridging oxygen atoms. (23)Na triple quantum MAS (3QMAS) NMR data corroborates the split Na-O correlations. The structural sites present will be intimately related to the release properties of the glass system in physiological fluids such as plasma and saliva, and hence to the bioactivity of the material. Detailed structural knowledge is therefore a prerequisite for optimizing material design.

Superior photoluminescence (PL) of Pr 3+ -In, compared to Pr 3+ -Ga, selenide-chalcogenide bulk glasses and PL of optically-clad fiber

The photoluminescent-(PL)-properties of Pr³⁺-ions in indium-containing selenide-chalcogenide bulk-glasses are found to be superior when compared with gallium-containing analogues. We observe circa doubling of mid-infrared (MIR) PL intensity from 3.5 to 6 μm for bulk glasses, pumped at 1.55 μm wavelength, and an increased excited state lifetime at 4.7 μm. PL is reported in optically-clad fiber. Ga addition is well known to enhance RE³⁺ solubility and PL behavior, and is believed to form ([RE³⁺]-Se-[Ga(III)]) in the glasses. Indium has the same outer electronic-structure as gallium for solvating the RE-ions. Moreover, indium is heavier and promotes lower phonon energy locally around the RE-ion, thereby enhancing the RE-ion PL behavior, as observed here.

Atomic structure of Ca40+XMg25Cu35−X metallic glasses

The atomic structures of four Ca40+XMg25Cu35−X (X = 0, 5, 10, and 20 at. %) ternary metallic glasses have been determined using a synergistic combination of neutron diffraction, ab initio molecular dynamics (MD) simulation, and constrained reverse Monte Carlo modeling. It is described as close-packing of efficiently packed Cu-centered clusters that have Ca, Mg, and Cu atoms in the first coordination shell. The close-packed arrangement of the clusters provides a characteristic medium range order in these alloys. An average coordination number (CN) of 10 (with about 5–7 Ca, 2–3 Mg, and 1–2 Cu atoms) is most common for the Cu-centered clusters. The average coordination numbers around Mg and Ca are 12–13 (∼6–8 Ca, 3 Mg, and 1–4 Cu) and 13–15 (7–9 Ca, 3–4 Mg, and 2–5 Cu), respectively, and they are composition dependent. Strong interaction of Cu with Mg and Ca results in pair bond shortening. Icosahedral short range order does not dominate in these amorphous alloys, although polytetrahedral packing and five-fo...

Activation and local structural stability during the thermal decomposition of Mg/Al-hydrotalcite by total neutron scattering

The activation of synthetic hydrotalcite, the carbonated layered double hydroxide (LDH) with ratio Mg2+/Al3+ = 3 and structural formula [Mg6Al2(OH)16]2+·CO32−n(H2O), has been investigated using neutron and X-ray diffraction. In situneutron diffraction was used to follow the structural phase transformations during the thermal decomposition (calcination) of hydrotalcite under vacuum in the temperature range 298–723 K, and during which the residual gas evolved by the sample was analysed by mass spectrometry. Detailed structural information of the LDH and mixed metal oxides (MMOs) was extracted from both the Bragg peaks and the total scattering. These two analysis techniques provide complementary insight into the relevant transition mechanisms, since Bragg diffraction originates from long-range periodicities within the samples while total scattering reveals the subtleties of the local atomic environment. This latter information is particularly important for our understanding of catalytic activity since it elucidates the local metal coordination. We find that, during the calcination process, the local environment around the metal centres is robust, as the various stages during the phase transition have identical local structures. The implications of these new results for the nature of the MMOs is discussed in relation to the well-studied, reverse, rehydration reaction, and the high propensity of trivalent Al ions to migrate to tetrahedrally-coordinated lattice sites.

Characterisation of phosphate coacervates for potential biomedical applications

In this study, amorphous (Na2O) x (CaO)0.50− x (P2O5)0.50·yH2O (where x = ∼0.15 and y = ∼3) samples were prepared by a coacervate method. Thermal analysis showed that two types of water molecules were present in the coacervate structures: one type loosely bound and the other part of the phosphate structure. Structural studies using Fourier transform infrared spectroscopy (FTIR) and X-ray total diffraction revealed the samples to have very similar structures to melt-quenched glasses of comparable composition. Furthermore, no significant structural differences were observed between samples prepared using calcium nitrate as the calcium source or those prepared from calcium chloride. A sample containing ∼1 mol% Ag2O was prepared to test the hypothesis that calcium phosphate coacervate materials could be used as delivery agents for antibacterial ions. This sample exhibited significant antibacterial activity against the bacterium Psuedomonas aeruginosa. FTIR data revealed the silver-doped sample to be structurally akin to the analogous silver-free sample.

The local environment of Dy3+in selenium-rich chalcogenide glasses

The environment of Dy3+ is investigated when it is added as DyCl3 or Dy foil into two base glasses, Ge16.5As19−xGaxSe64.5, where x = 3 or 10 at%, at doping levels between 0 and 3000 parts per million by weight (ppmw) Dy3+. Extended X-ray Absorption Fine Structure demonstrates the glasses doped with Dy foil, or less than 1000 ppmw Dy3+ as DyCl3, contain Dy ions that are fully incorporated into the glass network and are coordinated by 7–8 Se atoms. However, when the DyCl3 dopant is present in concentrations ≥1000 ppmw Dy3+ the environment is dominated by Dy–Cl bonds. Furthermore, these Dy–Cl environments are nanocrystalline, retaining chemical order beyond the first coordination shell. By comparison with XRD and FTIR results, we report that the presence of α-Ga2Se3 crystallites in the glass, and the increased optical scattering in the fibres, are both related to the presence of DyCl3 crystallites.