I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[13284]
Abstract: Topochemical reduction of the n = 1 Ruddlesden–Popper phases LaSrCo0.5Rh0.5O4 and LaSrNi0.5Rh0.5O4 with Zr yields LaSrCo0.5Rh0.5O3.25 and LaSrNi0.5Rh0.5O3.25, respectively. Magnetization and XPS data reveal that while the rhodium centers in LaSrCo0.5Rh0.5O3.25 and LaSrNi0.5Rh0.5O3.25 have an average oxidation state of Rh2+, these are actually mixed valence Rh(I,III) compounds, with the disproportionation of Rh2+ driven by the favorability of locating d8 Rh1+ and d6 Rh3+ cations within square-planar and square-based pyramidal coordination sites, respectively.
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Sep 2020
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B18-Core EXAFS
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Josh
Abbenseth
,
Jean-pierre H.
Oudsen
,
Bas
Venderbosch
,
Serhiy
Demeshko
,
Markus
Finger
,
Christian
Herwig
,
Christian
Würtele
,
Max C.
Holthausen
,
Christian
Limberg
,
Moniek
Tromp
,
Sven
Schneider
Diamond Proposal Number(s):
[22432]
Abstract: The splitting of dinitrogen into nitride complexes emerged as a key reaction for nitrogen fixation strategies at ambient conditions. However, the impact of auxiliary ligands or accessible spin states on the thermodynamics and kinetics of N–N cleavage is yet to be examined in detail. We recently reported N–N bond splitting of a {Mo(μ2:η1:η1-N2)Mo}-complex upon protonation of the diphosphinoamide auxiliary ligands. The reactivity was associated with a low-spin to high-spin transition that was induced by the protonation reaction in the coordination periphery, mainly based on computational results. Here, this proposal is evaluated by an XAS study of a series of linearly N2 bridged Mo pincer complexes. Structural characterization of the transient protonation product by EXAFS spectroscopy confirms the proposed spin transition prior to N–N bond cleavage.
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Sep 2020
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I11-High Resolution Powder Diffraction
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Craig I.
Hiley
,
Kenneth K.
Inglis
,
Marco
Zanella
,
Jiliang
Zhang
,
Troy D.
Manning
,
Matthew S.
Dyer
,
Tilen
Knaflič
,
Denis
Arčon
,
Frédéric
Blanc
,
Kosmas
Prassides
,
Matthew J.
Rosseinsky
Open Access
Abstract: The products of the solid-state reactions between potassium metal and tetracene (K:Tetracene, 1:1, 1.5:1, and 2:1) are fully structurally characterized. Synchrotron X-ray powder diffraction shows that only K2Tetracene forms under the reaction conditions studied, with unreacted tetracene always present for x < 2. Diffraction and 13C MAS NMR show that K2Tetracene has a crystal structure that is analogous to that of K2Pentacene, but with the cations ordered on two sites because of the influence of the length of the hydrocarbon on possible cation positions. K2Tetracene is a nonmagnetic insulator, thus further questioning the nature of reported superconductivity in this class of materials.
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Aug 2020
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I11-High Resolution Powder Diffraction
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Abstract: Metal borohydrides are a fascinating and continuously expanding class of materials, showing promising applications within many different fields of research. This study presents 17 derivatives of the hydrogen-rich ammonium borohydride, NH4BH4, which all exhibit high gravimetric hydrogen densities (>9.2 wt % of H2). A detailed insight into the crystal structures combining X-ray diffraction and density functional theory calculations exposes an intriguing structural variety ranging from three-dimensional (3D) frameworks, 2D-layered, and 1D-chainlike structures to structures built from isolated complex anions, in all cases containing NH4+ countercations. Dihydrogen interactions between complex NH4+ and BH4– ions contribute to the structural diversity and flexibility, while inducing an inherent instability facilitating hydrogen release. The thermal stability of the ammonium metal borohydrides, as a function of a range of structural properties, is analyzed in detail. The Pauling electronegativity of the metal, the structural dimensionality, the dihydrogen bond length, the relative amount of NH4+ to BH4–, and the nearest coordination sphere of NH4+ are among the most important factors. Hydrogen release usually occurs in three steps, involving new intermediate compounds, observed as crystalline, polymeric, and amorphous materials. This research provides new opportunities for the design and tailoring of novel functional materials with interesting properties.
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Aug 2020
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I15-1-X-ray Pair Distribution Function (XPDF)
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Diamond Proposal Number(s):
[7288, 17685]
Abstract: Mixed-metal cyanides (Cu1/2Au1/2)CN, (Ag1/2Au1/2)CN, and (Cu1/3Ag1/3Au1/3)CN adopt an AuCN-type structure in which metal-cyanide chains pack on a hexagonal lattice with metal atoms arranged in sheets. The interactions between and within the metal-cyanide chains are investigated using density functional theory (DFT) calculations, 13C solid-state NMR (SSNMR), and X-ray pair distribution function (PDF) measurements. Long-range metal and cyanide order is found within the chains: (−Cu–NC–Au–CN−)∞, (−Ag–NC–Au–CN−)∞, and (−Cu–NC–Ag–NC–Au–CN−)∞. Although Bragg diffraction studies establish that there is no long-range order between chains, X-ray PDF results show that there is local order between chains. In (Cu1/2Au1/2)CN and (Ag1/2Au1/2)CN, there is a preference for unlike metal atoms occurring as nearest neighbors within the metal sheets. A general mathematical proof shows that the maximum average number of heterometallic nearest-neighbor interactions on a hexagonal lattice with two types of metal atoms is four. Calculated energies of periodic structural models show that those with four unlike nearest neighbors are most favorable. Of these, models in space group Immm give the best fits to the X-ray PDF data out to 8 Å, providing good descriptions of the short- and medium-range structures. This result shows that interactions beyond those of nearest neighbors must be considered when determining the structures of these materials. Such interactions are also important in (Cu1/3Ag1/3Au1/3)CN, leading to the adoption of a structure in Pmm2 containing mixed Cu–Au and Ag-only sheets arranged to maximize the numbers of Cu···Au nearest- and next-nearest-neighbor interactions.
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Jul 2020
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I15-Extreme Conditions
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Juan A.
Sans
,
Rosario
Vilaplana
,
E. Lora
Da Silva
,
Catalin
Popescu
,
Vanesa P.
Cuenca-gotor
,
Adrián
Andrada-chacón
,
Javier
Sánchez-benitez
,
Oscar
Gomis
,
André L. J.
Pereira
,
Plácida
Rodríguez-hernández
,
Alfonso
Muñoz
,
Dominik
Daisenberger
,
Braulio
Garcia-domene
,
Alfredo
Segura
,
Daniel
Errandonea
,
Ravhi S.
Kumar
,
Oliver
Oeckler
,
Philipp
Urban
,
Julia
Contreras-garcia
,
Francisco J.
Manjón
Abstract: High pressure X-ray diffraction, Raman scattering, and electrical measurements, together with theoretical calculations, which include the analysis of the topological electron density and electronic localization function, evidence the presence of an isostructural phase transition around 2 GPa, a Fermi resonance around 3.5 GPa, and a pressure-induced decomposition of SnSb2Te4 into the high-pressure phases of its parent binary compounds (α-Sb2Te3 and SnTe) above 7 GPa. The internal polyhedral compressibility, the behavior of the Raman-active modes, the electrical behavior, and the nature of its different bonds under compression have been discussed and compared with their parent binary compounds and with related ternary materials. In this context, the Raman spectrum of SnSb2Te4 exhibits vibrational modes that are associated but forbidden in rocksalt-type SnTe; thus showing a novel way to experimentally observe the forbidden vibrational modes of some compounds. Here, some of the bonds are identified with metavalent bonding, which were already observed in their parent binary compounds. The behavior of SnSb2Te4 is framed within the extended orbital radii map of BA2Te4 compounds, so our results pave the way to understand the pressure behavior and stability ranges of other “natural van der Waals” compounds with similar stoichiometry.
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Jul 2020
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[15781, 17433]
Abstract: Metal closo-borates have recently received significant attention due to their potential applications as solid-state ionic conductors. Here, the synthesis, crystal structures, and properties of (NH4)2B10H10·xNH3 (x = 1/2, 1 (α and β)) and (NH4)2B12H12·xNH3 (x = 1 and 2) are reported. In situ synchrotron radiation powder X-ray diffraction allows for the investigation of structural changes as a function of temperature. The structures contain the complex cation N2H7+, which is rarely observed in solid materials, but can be important for proton conductivity. The structures are optimized by density functional theory (DFT) calculations to validate the structural models and provide detailed information about the hydrogen positions. Furthermore, the hydrogen dynamics of the complex cation N2H7+ are studied by molecular dynamics simulations, which reveals several events of a proton transfer within the N2H7+ units. The thermal properties are investigated by thermogravimetry and differential scanning calorimetry coupled with mass spectrometry, revealing that NH3 is released stepwise, which results in the formation of (NH4)2BnHn (n = 10 and 12) during heating. The proton conductivity of (NH4)2B12H12·xNH3 (x = 1 and 2) determined by electrochemical impedance spectroscopy is low but orders of magnitude higher than that of pristine (NH4)2B12H12. The thermal stability of the complex cation N2H7+ is high, up to 170 °C, which may provide new possible applications of these proton-rich materials.
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Jul 2020
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B18-Core EXAFS
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Ian D.
Johnson
,
Gene
Nolis
,
Kit
Mccoll
,
Yimin A.
Wu
,
Daisy
Thornton
,
Linhua
Hu
,
Hyun Deog
Yoo
,
John W.
Freeland
,
Furio
Corà
,
Jeremy K.
Cockcroft
,
Ivan P.
Parkin
,
Robert F.
Klie
,
Jordi
Cabana
,
Jawwad A.
Darr
Diamond Proposal Number(s):
[14239]
Abstract: While commercial Li-ion batteries offer the highest energy densities of current rechargeable battery technologies, their energy storage limit has almost been achieved. Therefore, there is considerable interest in Mg batteries, which could offer increased energy densities in comparison to Li-ion batteries if a high-voltage electrode material, such as a transition-metal oxide, can be developed. However, there are currently very few oxide materials which have demonstrated reversible and efficient Mg2+ insertion and extraction at high voltages; this is thought to be due to poor Mg2+ diffusion kinetics within the oxide structural framework. Herein, the authors provide conclusive evidence of electrochemical insertion of Mg2+ into the tetragonal tungsten bronze V4Nb18O55, with a maximum reversible electrochemical capacity of 75 mA h g–1, which corresponds to a magnesiated composition of Mg4V4Nb18O55. Experimental electrochemical magnesiation/demagnesiation revealed a large voltage hysteresis with charge/discharge (1.12 V vs Mg/Mg2+); when magnesiation is limited to a composition of Mg2V4Nb18O55, this hysteresis can be reduced to only 0.5 V. Hybrid-exchange density functional theory (DFT) calculations suggest that a limited number of Mg sites are accessible via low-energy diffusion pathways, but that larger kinetic barriers need to be overcome to access the entire structure. The reversible Mg2+ intercalation involved concurrent V and Nb redox activity and changes in crystal structure, as confirmed by an array of complementary methods, including powder X-ray diffraction, X-ray absorption spectroscopy, and energy-dispersive X-ray spectroscopy. Consequently, it can be concluded that the tetragonal tungsten bronzes show promise as intercalation electrode materials for Mg batteries.
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Jul 2020
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I15-1-X-ray Pair Distribution Function (XPDF)
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Abstract: Catalyst promoters are often used to tune or improve performance with little understanding as to how they work. Here we present a fundamental evaluation of cobalt‐manganese interactions in Fischer‐Tropsch (FT) catalysis and evaluate a new bifunctional catalyst for CO dissociation and CO insertion mechanisms. FT has gained considerable attention recently due to the potential to convert bio or municipal waste feeds into commercial fuels and chemicals. Products are commonly highly linear paraffins, but here we show how the role of manganese can tune selectivity for linear olefins and alcohols in a copper, iron and alkali metal free cobalt bifunctional catalyst. These products also have significant commercial value for lubricants, plasticizers, detergents and base chemicals. Advanced catalyst characterization is shown with in situ techniques (XRD, EXAFS, PDF & TEM), while the FT products are fully analysed by NMR, GC and GCxGC. The role of manganese during catalyst synthesis is reviewed while fundamental understanding of the formation of a mixed metal oxide spinel is presented. The Co–Mn interactions change during catalyst reduction, with EXAFS showing cobalt metal and MnO as the main species present.
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Jun 2020
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I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[14825]
Abstract: Single crystal X-ray diffraction has been used to study the CO2 absorption sites in a microporous Cu-MOF, [CuI2(py-pzpypz)2(μ-CN)2]n (1) (where py-pzpypz = 4-(4-pyridyl)-2,5-dipyrazyl-pyridine), which features zigzag-shaped channels, at a range of CO2 pressures (1, 5, and 10 bar) and at two temperatures (240 and 298 K). Unlike the acetonitrile molecules in the as-synthesized MOF, 1·MeCN, the CO2 molecules in 1·nCO2 (n = 0.8, 0.7, 0.45) are preferentially centered on the vertices of each zig and zag, which allows for weak (azine) C–H···OCO interactions with the H atoms on the electron-deficient pyrazine and pyridine rings of the MOF.
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Apr 2020
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