I11-High Resolution Powder Diffraction
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Mathias
Jorgensen
,
Patrick T.
Shea
,
Anton W.
Tomich
,
Joel B.
Varley
,
Marnik
Bercx
,
Sergio
Lovera
,
Radovan
Cerny
,
Wei
Zhou
,
Terrence J.
Udovic
,
Vincent
Lavallo
,
Torben
Jensen
,
Brandon C.
Wood
,
Vitalie
Stavila
Abstract: Solid-state ion conductors based on closo-polyborate anions combine high ionic conductivity with a rich array of tunable properties. Cation mobility in these systems is intimately related to the strength of the interaction with the neighboring anionic network and the energy for reorganizing the coordination polyhedra. Here, we explore such factors in solid electrolytes with two anions of the weakest coordinating ability, [HCB11H5Cl6]– and [HCB11H5Br6]–, and a total of eleven polymorphs are identified for their lithium and sodium salts. Our approach combines ab initio molecular dynamics, synchrotron X-ray powder diffraction, differential scanning calorimetry, and AC impedance measurements to investigate their structures, phase-transition behavior, anion orientational mobilities, and ionic conductivities. We find that M(HCB11H5X6) (M = Li, Na, X = Cl, Br) compounds exhibit order-disorder polymorphic transitions between 203 and 305 °C, and display Li and Na superionic conductivity in the disordered state. Through detailed analysis, we illustrate how cation disordering in these compounds originates from a competitive interplay among the lattice symmetry, the anion reorientational mobility, the geometric and electronic asymmetry of the anion, and the polarizability of the halogen atoms. These factors are compared to other closo-polyborate-based ion conductors to suggest guidelines for optimizing the cation-anion interaction for fast ion mobility. This study expands the known solid-state, poly(carba)borate-based materials capable of liquid-like ionic conductivities, unravels the mechanisms responsible for fast ion transport, and provides insights into the development of practical superionic solid electrolytes.
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Jan 2020
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I15-1-X-ray Pair Distribution Function (XPDF)
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Timothee
Stassin
,
Ivo
Stassen
,
Joao
Marreiros
,
Alexander John
Cruz
,
Rhea
Verbeke
,
Min
Tu
,
Helge
Reinsch
,
Marcel
Dickmann
,
Werner
Egger
,
Ivo F. J.
Vankelecom
,
Dirk E.
De Vos
,
Rob
Ameloot
Diamond Proposal Number(s):
[17279]
Abstract: A simple solvent- and catalyst-free method is presented for the synthesis of the large-pore metal-organic framework (MOF) MAF-6 (RHO-Zn(eIm)2) based on the reaction of ZnO with 2-ethylimidazole vapor at temperatures ≤ 100 °C. By translating this method to a chemical vapor deposition (CVD) protocol, crystalline films of a large-pore material could be deposited for the first time entirely from the vapor phase. A combination of PALS and Kr physisorption measurements confirmed the porosity of these MOF-CVD films and the size of the MAF-6 supercages (diam. ~2 nm), in close agreement with powder data and calculations. MAF-6 powders and films were further characterized by XRD, TGA, SEM, FTIR, PDF and EXAFS. The exceptional uptake capacity of MAF-6 in comparison to ZIF-8 is demonstrated by vapor-phase loading of a molecule larger than the ZIF-8 windows.
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Jan 2020
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Abstract: In recent years a considerable amount of effort has been put into a better understanding of the correlation of structure and electrochemical properties in Li- and Mn-rich NCM layered oxide Li ion battery cathode materials, such as the intensely investigated Li1.2Ni0.15Co0.1Mn0.55O2 composition. A gradual transformation from a trigonal R-3m layered structure towards a cubic Fd-3m spinel structure during electrochemical cycling results in an unwanted decay of the mean charge and discharge voltages, called ‘voltage fade’. This transformation proceeds via an interim phase, which is characterized by the local de-ordering of cations and the introduction of various lattice defects after the formation of the initially well-ordered material. In this study, these structural changes are studied in detail on a long-range atomic scale by synchrotron radiation powder diffraction as well as on a very-local atomic scale by 7Li nuclear magnetic resonance and X-ray absorption spectroscopy. A structural reordering was induced by a mild thermal treatment (150 °C – 300 °C) in lithiated (discharged to 2.0 V) as well as in delithiated (charged to 4.7 V) electrodes, which results either in a partial recovery of the initial well-ordered state or in an intensification of the structural degradation towards a spinel-type cation ordering, respectively. The structural reordering thus obtained was again studied on a long-range and local atomic scale and correlated with the electrochemical properties. In order to complement the experiment an electrochemically highly fatigued electrode (300 cycles, discharged to 2.0 V) showing a pronounced voltage fade was thermally treated, which resulted again in a partial recovery of the initial well-ordered structure and its accompanying electrochemical properties. Finally, the results are summarized in a model explaining the influence of the local cation ordering, lattice defects and the oxygen sublattice on the electrochemical properties, such as the oxygen redox activity and the voltage fade.
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Jan 2020
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[18786]
Abstract: By tuning the tolerance factor, $t$, of the Ruddlesden--Popper oxide Ca$_2$MnO$_4$ through isovalent substitutions we show that the uniaxial coefficient of linear thermal expansion (CLTE) of these systems can be systematically changed through large negative to positive values. High-resolution X-ray diffraction measurements show that the magnitude of uniaxial negative thermal expansion (NTE) increases as $t$ decreases across the stability window of the NTE phase. Transitions to phases with positive thermal expansion (PTE) are found to occur at both the high-$t$ and low-$t$ limits of stability. First-principles calculations demonstrate that reducing $t$ enhances the contribution to thermal expansion from the lowest frequency phonons, which have the character of octahedral tilts and have negative mode Gr\"uneisen parameter components along the NTE axis. By tuning $t$ to the lower edge of the NTE phase stability window, we are hence able to maximise the amplitudes of these vibrations and thereby maximise NTE with a CLTE of -8.1~ppm/K at 125~K. We also illustrate, at the other end of the phase diagram, that an enhancement in compliance of these materials associated with the rotational instability provides another mechanism by which NTE could be yet further enhanced in this and related systems.
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Dec 2019
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Abstract: Mixed anion compounds in the Fm-3m vacancy ordered perovskite structure were synthesised and characterised experimentally and computationally with a focus on compounds where A = Cs+. Pure anion Cs2SnX6 compounds were formed with X = Cl, Br and I using a room temperature solution phase method. Mixed anion compounds were formed as solid solutions of Cs2SnCl6 and Cs2SnBr6 and a second series from Cs2SnBr6 and Cs2SnI6. Single phase structures formed across the entirety of both composition series, with no evidence of long range anion ordering observed by diffraction. A distortion of the cubic A2BX6 structure was identified in which the spacing of the BX6 octahedra changes to accommodate the A site cation without reduction of overall symmetry. Optical band gap values varied with anion composition between 4.89 eV in Cs2SnCl6 to 1.35 eV in Cs2SnI6, but proved highly non-linear with changes in composition. In mixed halide compounds it was found that lower energy optical transitions appeared that were not present in the pure halide compounds, and this could be attributed to lowering of the local symmetry within the tin halide octahedra. The electronic structure was characterised by photoemission spectroscopy, and Raman spectroscopy revealed vibrational modes in the mixed halide compounds that could be assigned to particular mixed halide octahedra. This analysis was used to determine the distribution of octahedra types in mixed anion compounds, which was found to be consistent with a near-random distribution of halide anions throughout the structure, although some deviations from random halide distribution were noted in mixed iodide-bromide compounds, where the larger iodide anions preferentially adopted trans configurations.
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Nov 2019
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I11-High Resolution Powder Diffraction
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Jacinthe
Gamon
,
Benjamin B.
Duff
,
Matthew S.
Dyer
,
Christopher
Collins
,
Luke M.
Daniels
,
T. Wesley
Surta
,
Paul M.
Sharp
,
Michael W.
Gaultois
,
Frédéric
Blanc
,
John B.
Claridge
,
Matthew
Rosseinsky
Abstract: With the goal of finding new lithium solid electrolytes by a combined computational-experimental method, the exploration of the Li-Al-O-S phase field resulted in the discovery of a new sulphide Li3AlS3. The structure of the new phase was determined through an approach combining synchrotron X-ray and neutron diffraction with 6Li and 27Al magic angle spinning nuclear magnetic resonance spectroscopy, and revealed a highly ordered cationic polyhedral network within a sulphide anion hcp-type sublattice. The originality of the structure relies on the presence of Al2S6 repeating dimer units consisting of two edge-shared Al tetrahedra. We find that, in this structure type consisting of alternating tetrahedral layers with Li-only polyhedra layers, the formation of these dimers is constrained by the Al/S ratio of 1/3. Moreover, by comparing this structure to similar phases such as Li5AlS4 and Li4.4Al0.2Ge0.3S4 ((Al+Ge)/S = 1/4), we discovered that the Al2S6 dimers not only influence atomic displacements and Li polyhedral distortions, but also determine the overall Li polyhedral arrangement within the hcp lattice, leading to the presence of highly ordered vacancies in both the tetrahedral and Li-only layer. AC-impedance measurements revealed a low lithium mobility, which is strongly impacted by the presence of ordered vacancies. Finally, a composition-structure-property relationship understanding was developed to explain the extent of lithium mobility in this structure type.
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Oct 2019
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[21432]
Abstract: This work reports a systematical study on the relationship of electronic structure to oxygen evolution reaction (OER) activity of NixCo3-xO4 (x=0-1) mixed oxides. The specific OER activity is substantially increased by 16 times from 0.02 mA cm-2BET for pure Co3O4 to 0.32 mA cm-2BET for x=1 at an overpotential of 0.4 V and exhibits a strong correlation with the amount of Ni ions in +3 oxidation state. X-ray spectroscopic study reveals that inclusion of Ni3+ ions upshifts the occupied valence band maximum (VBM) by 0.27 eV toward the Fermi level (EF), and creates a new hole (unoccupied) state located ~1 eV above the EF. Such electronic features favour the adsorption of OH surface intermediates on NixCo3-xO4, resulting in enhanced OER. Furthermore, the emerging hole state effectively reduces the energy barrier for electron transfer from 1.19 eV to 0.39 eV, and thereby improves the kinetics for OER. The electronic structure features that lead to a higher OER in NixCo3-xO4 can be extended to other transition metal oxides for rational design of highly active catalysts.
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Aug 2019
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I20-Scanning-X-ray spectroscopy (XAS/XES)
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Diamond Proposal Number(s):
[16508]
Abstract: The reaction between ceria and hydrogen has been subject to numerous theoretical and experimental studies due to its importance as a catalytic material. Here we present dynamic and reversible evolution of the cerium oxidation states observed through X-ray Absorption Spectroscopy experiments in addition to the investigation of associated lattice expansion and contraction through X-ray diffraction and PDF methods. Employing a novel calculation of the temperature dependence of the Gibbs free energy through consideration of the relationship between the instantaneous thermal lattice expansion and the rate of change of the cerium oxidation state, the unusual redox chemistry is reported here. This unusual behaviour is interpreted as due to the formation of a metastable cerium oxyhydride as suggested.
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Aug 2019
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[17901]
Abstract: Sodium ion batteries based on Prussian blue analogues (PBAs) are ideal for large scale energy storage applications due to the ability to meet the huge volumes and low costs required. For Na2-xFe[Fe(CN)6]1-y.zH2O realising its commercial potential means fine control of the concentration of sodium, Fe(CN)6 vacancies and water content. To date, there is a huge variation in the literature of composition leading to variable electrochemical performance. In this work, we break down the synthesis of Prussian blue analogous (PBAs) into three steps for controlling the sodium, vacancy and water content via an inexpen-sive, scalable synthesis method. We produce rhombohedral Prussian white Na1.88(5)Fe[Fe(CN)6].0.18(9)H2O with an initial capacity of 158 mAhg-1 retaining 90% capacity after 50 cycles. Subsequent characterisation revealed that the increased polarisation on the 3 V plateau is coincident with a phase transition and reduced utilisation of the high spin Fe(III)/Fe(II) redox couple. This reveals a clear target for subsequent improvements of the material to boost long term cycling stability. These results will be of great interest for the myriad of applications PBAs are suited towards such as catalysis, magnetism, electrochromics and gas sorption.
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Aug 2019
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I19-Small Molecule Single Crystal Diffraction
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Marta
Mon
,
Rosaria
Bruno
,
Estefanía
Tiburcio
,
Aida
Grau-atienza
,
Antonio
Sepulveda-escribano
,
Enrique V
Ramos-fernandez
,
Alessio
Fuoco
,
Elisa
Esposito
,
Marcello
Monteleone
,
Johannes Carolus
Jansen
,
Joan
Cano
,
Jesus
Ferrando-soria
,
Donatella
Armentano
,
Emilio
Pardo
Diamond Proposal Number(s):
[18768]
Abstract: Understand/visualize the established interactions between gases and adsorbents is mandatory to implement better per-formant materials in adsorption/separation processes. Here we report the unique behavior of a rare example of a hemila-bile chiral three-dimensional metal-organic framework (MOF) with an unprecedented qtz-e-type topology, with formula CuII2(S,S)-hismox . 5H2O (1) (hismox = bis[(S)-histidine]oxalyl diamide). 1 exhibits a continuous and reversible breath-ing behavior, based on the hemilability of carboxylate groups from L-histidine. In-situ powder (PXRD) and single crystal X-ray diffraction (SCXRD) using synchrotron radiation allowed to unveil the crystal structures of 4 different host-guest adsorbates (Ar, N2, CO2 and C3H6@1), the rationalization of the breathing motion and unravel the mechanisms govern-ing the adsorption of these gases. Then, this information has been transferred to implement efficient separations of mix-tures of industrial and environmental relevance –CO2/N2, CO2/CH4 and C3H8/C3H6 using 1 in packed columns as the sta-tionary phase and dispersed in a mixed matrix membrane.
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Jul 2019
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