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Saint Andrews, United Kingdom

Playford H.Y.,University of Warwick | Playford H.Y.,Rutherford Appleton Laboratory | Hannon A.C.,Rutherford Appleton Laboratory | Tucker M.G.,Rutherford Appleton Laboratory | And 5 more authors.
Journal of Physical Chemistry C | Year: 2014

Solvothermal oxidation of metallic gallium in monoethanolamine for 72 h at 240 °C yields a crystalline sample of γ-Ga2O3 (∼30 nm crystallites). While Rietveld refinement (cubic spinel structure, Fd3̄m; a = 8.23760(9) Å) reveals that Ga occupies two pairs of octahedral and tetrahedral sites (ideal spinel and nonspinel), it provides no information about their local distribution, which cannot be statistical owing to the short Ga-Ga contacts produced if neighboring ideal spinel and nonspinel sites are simultaneously occupied. To create an atomistic model to reconcile this situation, a 6 × 6 × 6 supercell of the crystal structure is constructed and refined against neutron total scattering data using a reverse Monte Carlo (RMC) approach. This accounts well for the local as well as long-range structure and reveals significant local distortion in the octahedral sites that resembles the structure of thermodynamically stable β-Ga 2O3. 71Ga solid-state NMR results reveal a octahedral:tetrahedral Ga ratio that is consistent with the model obtained from RMC. Nanocrystalline samples of γ-Ga2O3 are produced by either a short solvothermal reaction (240 °C for 11 h in diethanolamine; ∼15 nm crystallites) or by precipitation from an ethanolic solution of gallium nitrate (∼5 nm crystallites). For these samples, the Bragg scattering profile is broadened by their smaller crystallite size, consistent with transmission electron microscopy results, and analysis of the relative Bragg peak intensities provides evidence that a greater proportion of tetrahedral versus octahedral sites are filled. In contrast, neutron total scattering shows the same average Ga-O distance with decreasing particle size, consistent with 71Ga solid-state NMR results that indicate that all samples contain the same overall proportion of octahedral:tetrahedral Ga. It is postulated that increased occupation of tetrahedral sites within the smaller crystallites is balanced by an increased proportion of octahedral surface Ga sites, owing to termination by bound solvent or hydroxide. © 2014 American Chemical Society.

Ashbrook S.E.,St Andrews Center for Magnetic Resonance | Dawson D.M.,St Andrews Center for Magnetic Resonance | Seymour V.R.,St Andrews Center for Magnetic Resonance
Physical Chemistry Chemical Physics | Year: 2014

Microporous materials, having pores and channels on the same size scale as small to medium molecules, have found many important applications in current technologies, including catalysis, gas separation and drug storage and delivery. Many of their properties and functions are related to their detailed local structure, such as the type and distribution of active sites within the pores, and the specific structures of these active sites. Solid-state NMR spectroscopy has a strong track record of providing the requisite detailed atomic-level insight into the structures of microporous materials, in addition to being able to probe dynamic processes occurring on timescales spanning many orders of magnitude (i.e., from s to ps). In this Perspective, we provide a brief review of some of the basic experimental approaches used in solid-state NMR spectroscopy of microporous materials, and then discuss some more recent advances in this field, particularly those applied to the study of crystalline materials such as zeolites and metal-organic frameworks. These advances include improved software for aiding spectral interpretation, the development of the NMR-crystallography approach to structure determination, new routes for the synthesis of isotopically-labelled materials, methods for the characterisation of host-guest interactions, and methodologies suitable for observing NMR spectra of paramagnetic microporous materials. Finally, we discuss possible future directions, which we believe will have the greatest impact on the field over the coming years. © 2014 the Owner Societies.

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