I15-1-X-ray Pair Distribution Function (XPDF)
|
Diamond Proposal Number(s):
[22774]
Abstract: To understand the importance of the crystallite size on the stabilization of metastable tetragonal ZrO2, ultra-fine ZrO2 nanocrystallites were synthesized via: (i) the precipitation method in supercritical water using nitrate precursors, (ii) the sol-gel method in a supercritical ethanol-water mixture and (iii) the borderline non-hydrolytic sol-gel route in supercritical ethanol using propoxide precursors. The obtained nanocrystals displayed a variation of the monoclinic versus tetragonal molar frac-tions from 100 wt. % down to ≈ 10 wt. % of monoclinic. This variation was concomitant with an overall size decrease of the nanocrystals, ranging from 7 to 2 nm depending on the synthesis procedures. Phase contents were quantified by refinement analysis of X-ray scattering datasets, and crosschecked with Raman spectroscopy. Our results suggest that an upper limit of ≈ 90 wt. %, of tetragonal ZrO2 phase is possible, even for ultra-fine nanoparticles (2 nm). These findings thus question the exist-ence of any critical size limit below which stabilization of pure t-ZrO2 is attainable at low temperatures.
|
Sep 2020
|
|
I09-Surface and Interface Structural Analysis
|
Jack E. N.
Swallow
,
Christian
Vorwerk
,
Piero
Mazzolini
,
Patrick
Vogt
,
Oliver
Bierwagen
,
Alexander
Karg
,
Martin
Eickhoff
,
Jörg
Schörmann
,
Markus R.
Wagner
,
Joseph William
Roberts
,
Paul R.
Chalker
,
Matthew J.
Smiles
,
Philip
Murgatroyd
,
Sara
Mohamed
,
Zachary W.
Lebens-higgins
,
Louis F. J.
Piper
,
Leanne A. H.
Jones
,
Pardeep K.
Thakur
,
Tien-lin
Lee
,
Joel B.
Varley
,
Juergen
Furthmüller
,
Claudia
Draxl
,
Tim D.
Veal
,
Anna
Regoutz
Diamond Proposal Number(s):
[21430, 24670]
Abstract: The search for new wide band gap materials is intensifying to satisfy the need for more advanced and energy effcient power electronic devices. Ga2O3 has emerged as an alternative to SiC and GaN, sparking a renewed interest in its fundamental properties beyond the main β-phase. Here, three polymorphs of Ga2O3, α, β, and ε, are investigated using X-ray diffraction, X-ray photoelectron and absorption spectroscopy, and ab initio theoretical approaches to gain insights into their structure - electronic structure relationships. Valence and conduction electronic structure as well as semi-core and core states are probed, providing a complete picture of the influence of local coordination environments on the electronic structure. State-of-the-art electronic structure theory, including all-electron density functional theory and many-body perturbation theory, provide detailed understanding of the spectroscopic results. The calculated spectra provide very accurate descriptions of all experimental spectra and additionally illuminate the origin of observed spectral features. This work provides a strong basis for the exploration of the Ga2O3 polymorphs as materials at the heart of future electronic device generations.
|
Sep 2020
|
|
I11-High Resolution Powder Diffraction
|
Diamond Proposal Number(s):
[14809]
Abstract: Potassium-ion batteries (KIB) are a promising complementary technology to lithium-ion batteries because of the comparative abundance and affordability of potassium. Currently, the most promising KIB chemistry consists of a potassium manganese hexacyanoferrate (KMF) cathode, a Prussian blue analog, and a graphite anode (723Whl−1 and 359Whkg−1 at 3.6V). No electrolyte has yet been formulated that is concurrently stable at the high operating potential of KMF (4.02V vs K+/K) and compatible with K+ intercalation into graphite, currently the most critical hurdle to adoption. Here we combine a KMF cathode and a graphite anode with a KFSI in Pyr1,3FSI ionic liquid electrolyte for the first time and show unprecedented performance. We use high-throughput techniques to optimize the KMF morphology for operation in this electrolyte system, achieving 119 mA h g−1 at 4 V vs K+/K and a coulombic efficiency >99.3%. In the same ionic liquid electrolyte graphite shows excellent electrochemical performance and we demonstrate reversible cycling by operando XRD. These results are a significant and essential step forward towards viable potassium-ion batteries.
|
Aug 2020
|
|
I11-High Resolution Powder Diffraction
|
Diamond Proposal Number(s):
[13284]
Abstract: Na-for-Rb cation exchange followed by K-for-Na cation exchange of RbNdM2O7 (M = Nb, Ta) yields the corresponding, metasta-ble KNdM2O7 phases. Synchrotron X-ray and neutron powder diffraction data, combined with powder SHG data, reveal that the KNdM2O7 phases adopt a polar structure (space group Im2m) consisting of NdM2O7 double perovskite sheets stacked in a (0, ½, z) manner with K+ cations ordered within the 6-coordinate prismatic inter-layer sites. The perovskite double sheets adopt an (a0b+c0/a0-b+c0) tilting distortion, however unlike other A’AB2O7 phases this distortion is not the origin of the non-centrosymmetric structure, which is attributed to a second-order Jahn-Teller distortion of the MO6 units. First-principles DFT calculations confirm the polar Im2m phase is more stable than the corresponding centrosymmetric alternative. The role of the A’- and A- cations in di-recting the stacking patterns and tilting distortions of A’AB2O7 phases is discussed with reference to hybrid improper ferroelectric behavior.
|
Aug 2020
|
|
|
|
Abstract: While the transport of ions and electrons in conventional Li-ion battery cathode materials is well understood, our knowledge of the phonon (heat) transport is still in its infancy. We present a first-principles theoretical investigation of the chemical trends in the phonon frequency dispersion, mode lifetimes, and thermal conductivity in the series of layered lithium transition-metal oxides Li(NixMnyCoz)O2 (x + y + z = 1). The oxidation and spin states of the transition metal cations are found to strongly influence the structural dynamics. Calculations of the thermal conductivity show that LiCoO2 has highest average conductivity of 45.9 W·m–1·K–1 at T = 300 K and the largest anisotropy, followed by LiMnO2 with 8.9 W·m–1·K–1 and LiNiO2 with 6.0 W·m–1·K–1. The much lower thermal conductivity of LiMnO2 and LiNiO2 is found to be due to 1–2 orders of magnitude shorter phonon lifetimes. We further model the properties of binary and ternary transition metal combinations to examine the possible effects of mixing on the thermal transport. These results serve as a guide to ongoing work on the design of multicomponent battery electrodes with more effective thermal management.
|
Aug 2020
|
|
B18-Core EXAFS
|
Diamond Proposal Number(s):
[14239]
Abstract: The rutiles (M,Ru)O2 (M = Mg, Zn, Co, Ni, Cu) are formed directly under hydrothermal conditions at 240 °C from potassium perruthenate and either peroxides of zinc or magnesium, or poorly crystalline oxides of cobalt, nickel or copper. The polycrystalline powders consist of lath-shaped crystallites, tens of nanometres in maximum dimension. Powder neutron diffraction shows that the materials have expanded a axis and contracted c axis compared to the parent RuO2, but there is no evidence of lowering of symmetry to other AO2-type structures, supported by Raman spectroscopy. Rietveld refinement shows no evidence for oxide non-stoichiometry and provides a formula (MxRu1-x)O2 with 0.14 < x < 0.2, depending on the substituent metal. This is supported by energy-dispersive X-ray analysis on the transmission electron microscope, while Ru K-edge XANES spectroscopy shows that upon inclusion of the substituent the average Ru oxidation state is increased to balance charge. Variable temperature magnetic measurements provide evidence for atomic homogeneity of the mixed metal materials, with suppression of the high temperature antiferromagnetism of RuO2 and increased magnetic moment. The new rutiles all show enhanced electrocatalysis compared to reference RuO2 materials for oxygen evolution in 1 M H2SO4 electrolyte at 60 °C, with higher specific and mass activity (per Ru) than a low surface area crystalline RuO2, and with less Ru dissolution over 1000 cycles compared to an RuO2 with a similar surface area. Magnesium substitution provides the optimum balance between stability and activity, despite leaching of the Mg2+ into solution, and this was proved in membrane electrode assemblies.
|
Jun 2020
|
|
E01-JEM ARM 200CF
|
Diamond Proposal Number(s):
[22572, 21641]
Abstract: Porous polymer catalysts possess the potential to combine the advantages of heterogeneous and homogeneous cataly-sis, namely easy post-reaction recycling and high dispersion of active sites. Here we designed a -SO3H functionalized polyphenylene (PPhen) framework with purely sp2-hybridized carbons, which exhibited high activity in the hydration of alkynes including challenging aliphatic substrates such as 1-octyne. The superiority of the structure lies in its cova-lent crosslink in the xy-plane with π-π stacking interaction between the planes, enabling simultaneously high swella-bility and porosity (653 m2·g-1). High acidic sites density (2.12 mmol/g) was achieved at mild sulfonation condition. Similar turnover frequencies (0.015 ± 0.001 min-1) were obtained regardless of acidic density and crosslink content, suggesting high accessibility for all active sites over PPhen. In addition, the substituted benzene groups can activate alkynes through a T-shape CH/π interaction, as indicated by the 8 cm-1 and 16 cm-1 red shift of the alkyne C-H stretch-ing peak for phenylacetylene and 1-octyne repectively in the IR spectra. These advantages render PPhen-SO3H a prom-ising candidate as a solid catalyst replacing the highly toxic liquid phase acids such as the mercury salt.
|
May 2020
|
|
B18-Core EXAFS
|
Diamond Proposal Number(s):
[16478]
Abstract: The methodology for experimentally verifying the point-defect chemistry (site of substitution, valence, and charge compensation mechanisms) in manganese-doped SrTiO3 ceramics is presented for dilute and non-dilute dopant concentrations. Experimental and theoretical techniques have strengths and weaknesses depending upon defect types and concentrations, so a combinatorial characterization approach is required. Using electron-paramagnetic-resonance and X-ray-absorption-fine-structure measurements combined with density functional theory calculations, the charge compensation mechanisms and local structural relaxations for five unique manganese defect centers are identified. Mn4+, as an isovalent dopant, occupies the octahedrally coordinated Ti sites without the need for charge compensation; its smaller ionic radius relative to Ti is accommodated by isotropic contraction of the [MnO6] octahedra. Mn3+ is an aliovalent dopant that also favors the Ti sites with the [MnO6] octahedra exhibiting Jahn-Teller distortions. The charge difference associated with the Mn3+Ti4+ substitution is compensated by formation of oxygen vacancies. A more complex behavior is observed for the Mn2+ species which can occupy either the Sr or Ti sites depending on the Sr/Ti ratio. The effects of Mn concentration or high-temperature annealing on the site preference (i.e., Sr vs Ti) are negligible. The Mn2+ species on the Sr sites are strongly off-centered in the relatively large cuboctahedral cages within the oxygen framework. The dynamic nature of the Mn displacements in these configurations is confirmed using ab initio molecular dynamics simulations.
|
May 2020
|
|
I11-High Resolution Powder Diffraction
|
Diamond Proposal Number(s):
[13284]
Abstract: There has been much interest recently in hybrid improper ferroelectrics – materials which adopt polar, ferroelectric structures due to a complex tilting and twisting of the MO6 octahedra which constitute perovskite and related struc-tures. Using a combination of synchrotron X-ray powder diffraction and high-resolution neutron powder diffraction data the temperature-dependent phase transitions of a series of n = 2 Dion-Jacobson oxides have been investigated. RbNdM2O7 undergoes a transition from a polar, aˉaˉc+/-(aˉaˉc+) distorted I2cm phase to an antipolar, aˉb0cˉ/-(aˉb0)cˉ distorted Cmca phase at T = 790 K and 500 K for M = Nb and Ta respectively. There is a subsequent transition to an a0a0cˉ/a0a0-cˉ distorted I4/mcm structure at 865 K and 950 K for M = Nb and Ta respectively, before a transition to the undistorted P4/mmm aristotype structure. In contrast CsNdM2O7 undergoes a transition from a polar, aˉaˉc+ distorted P21am structure to an antipolar, aˉb0cˉ distorted C2/m phase at T = 625 K and 330 K for M = Nb and Ta respectively, with a subsequent phase transition to the undistorted P4/mmm aristotype structure at 800 K and 820 K for M = Nb and Ta respectively. A plot of Tc against the relative stability of the 4 polar Dion-Jacobson phases compared to the corre-sponding aristotype P4/mmm structures (calculated from first-principles DFT), yields a strong linear relation suggest-ing Tc is not proportional to the enthalpy change at the ferroelectric phase transition.
|
Apr 2020
|
|
B18-Core EXAFS
|
Ronghuan
Zhang
,
Paul E.
Pearce
,
Vanessa
Pimenta
,
Jordi
Cabana
,
Haifeng
Li
,
Daniel
Alves Dalla Corte
,
Artem M.
Abakumov
,
Gwenaëlle
Rousse
,
Domitille
Giaume
,
Michael
Deschamps
,
Alexis
Grimaud
Diamond Proposal Number(s):
[12559]
Abstract: Water electrolysis is considered as a promising way to store and convert excess renewable energies into hydrogen, which is of high value for many chemical transformation processes such as the Haber-Bosch process. However, to allow for the wide-spread of the polymer electrolyte membrane water electrolysis (PEMWE) technology, the main challenge lies in the design of robust catalysts for oxygen evolution reaction (OER) under acidic conditions since most of transition metal-based oxides un-dergo structural degradation under these harsh acidic conditions. To broaden the variety of candidate materials as OER cata-lysts, a cation-exchange synthetic route was recently explored to reach crystalline pronated iridates with unique structural properties and stability. In this work, a new protonated phase H3.6IrO4∙3.7H2O, prepared via Sr2+/H+ cation exchange at room temperature starting from the parent Ruddlesden-Popper Sr2IrO4 phase, is described. This is the first discovery of crystalline protonated iridate forming from a perovskite-like phase, adopting a layered structure with apex-linked IrO6 octahedra. Fur-thermore, H3.6IrO4∙3.7H2O is found to possess not only an enhanced specific catalytic activity, superior to that of other perov-skite-based iridates, but also a mass activity comparable to that of nanosized IrOx particles, while showing an improved cata-lytic stability owing to its ability to reversibly exchange protons in acid.
|
Mar 2020
|
|
