I10-Beamline for Advanced Dichroism
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Abstract: Cobalt complexes with 2-(diisopropylphosphinomethyl)pyridine (PN) ligands have been synthesized with the aim of demonstrating electrocatalytic proton reduction to dihydrogen with a well-defined hydride complex of an Earth-abundant metal. Reactions of simple cobalt precursors with 2-(diisopropylphosphino-methyl)pyridine (PN) yield [CoII(PN)2(MeCN)][BF4]21, [CoIII(PN)2(H)(MeCN)][PF6]22, and [CoIII(PN)2(H)(Cl)][PF6] 3. Complexes 1 and 3 have been characterized crystallographically. Unusually for a bidentate PN ligand, all three exhibit geometries with mutually trans phosphorus and nitrogen ligands. Complex 1 exhibits a distorted square-pyramidal geometry with an axial MeCN ligand in a low-spin electronic state. In complexes 2 and 3, the PN ligands lie in a plane leaving the hydride trans to MeCN or chloride, respectively. The redox behavior of the three complexes has been studied by cyclic voltammetry at variable scan rates and by spectroelectrochemistry. A catalytic wave is observed in the presence of trifluoroacetic acid (TFA) at an applied potential close to the Co(II/I) couple of 1. Bulk electrolysis of 1, 2, or 3 at a potential of ca. −1.4 V vs E(Fc+/Fc) in the presence of TFA yields H2 with Faradaic yields close to 100%. A catalytic mechanism is proposed in which the pyridine moiety of a PN ligand acts as a pendant proton donor following opening of the chelate ring. Additional mechanisms may also operate, especially in the presence of high acid concentration where speciation changes.
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Dec 2020
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[25186]
Abstract: We present a structural and magnetic study of two batches of polycrystalline LiNi0.8Mn0.1Co0.1O2 (commonly known as Li NMC 811), a Ni-rich Li ion battery cathode material, using elemental analysis, X-ray and neutron diffraction, magnetometry, and polarized neutron scattering measurements. We find that the samples, labeled S1 and S2, have the composition Li1–xNi0.9+x–yMnyCo0.1O2, with x = 0.025(2), y = 0.120(2) for S1 and x = 0.002(2), y = 0.094(2) for S2, corresponding to different concentrations of magnetic ions and excess Ni2+ in the Li+ layers. Both samples show a peak in the zero-field-cooled (ZFC) dc susceptibility at 8.0(2) K, but the temperature at which the ZFC and FC (field-cooled) curves deviate is substantially different: 64(2) K for S1 and 122(2) K for S2. The ac susceptibility measurements show that the transition for S1 shifts with frequency whereas no such shift is observed for S2 within the resolution of our measurements. Our results demonstrate the sample dependence of magnetic properties in Li NMC 811, consistent with previous reports on the parent material LiNiO2. We further establish that a combination of experimental techniques is necessary to accurately determine the chemical composition of next-generation battery materials with multiple cations.
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Dec 2020
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I19-Small Molecule Single Crystal Diffraction
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Jonathan
Bould
,
Kamil
Lang
,
Kaplan
Kirakci
,
Luis
Cerdán
,
Daniel
Roca-sanjuán
,
Antonio
Francés-monerris
,
William
Clegg
,
Paul G.
Waddell
,
Marcel
Fuciman
,
Tomáš
Polívka
,
Michael G. S.
Londesborough
Diamond Proposal Number(s):
[22240]
Abstract: We present the first examples of alkylated derivatives of the macropolyhedral boron hydride, anti-B18H22, which is the gain medium in the first borane laser. This new series of ten highly stable and colorless organic–inorganic hybrid clusters are capable of the conversion of UVA irradiation to blue light with fluorescence quantum yields of unity. This study gives a comprehensive description of their synthesis, isolation, and structural characterization together with a delineation of their photophysical properties using a combined theoretical and experimental approach. Treatment of anti-B18H221 with RI (where R = Me or Et) in the presence of AlCl3 gives a series of alkylated derivatives, Rx-anti-B18H22–x (where x = 2 to 6), compounds 2–6, in which the 18-vertex octadecaborane cluster architectures are preserved and yet undergo a linear “polyhedral swelling”, depending on the number of cluster alkyl substituents. The use of dichloromethane solvent in the synthetic procedure leads to dichlorination of the borane cluster and increased alkylation to give Me11-anti-B18H9Cl211, Me12-anti-B18H8Cl212, and Me13-anti-B18H7Cl213. All new alkyl derivatives are highly stable, extremely efficient (ΦF = 0.76–1.0) blue fluorophores (λems = 423–427 nm) and are soluble in a wide range of organic solvents and also a polystyrene matrix. The Et4-anti-B18H18 derivative 4b crystallizes from pentane solution in two phases with consequent multiabsorption and multiemission photophysical properties. An ultrafast transient UV–vis absorption spectroscopic study of compounds 4a and 4b reveals that an efficient excited-state absorption at the emission wavelength inhibits the laser performance of these otherwise remarkable luminescent molecules. All these new compounds add to the growing portfolio of octadecaborane-based luminescent species, and in an effort to broaden the perspective on their highly emissive photophysical properties, we highlight emerging patterns that successive substitutions have on the molecular size of the 18-vertex borane cluster structure and the distribution of the electron density within.
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Nov 2020
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Abstract: The layered perovskite (MA)2PbI2(SCN)2 (MA = CH3NH3+) is a member of an emerging series of compounds derived from hybrid organic–inorganic perovskites. Here, we successfully synthesized (MA)2PbI2–xBrx(SCN)2 (0 ≤ x < 1.6) by using a solid-state reaction. Despite smaller bromide substitution for iodine, 1% linear expansion along the a axis was observed at x ∼ 0.4 due to a change of the orientation of the SCN– anions. Diffuse reflectance spectra reveal that the optical band gap increases by the bromide substitution, which is supported by the DFT calculations. Curiously, bromine-rich compounds where x ≥ 0.8 are light sensitive, leading to partial decomposition after ∼24 h. This study demonstrates that the layered perovskite (MA)2PbI2(SCN)2 tolerates a wide range of bromide substitution toward tuning the band gap energy.
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Nov 2020
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[13284, 18786]
Abstract: Sr2CrO2Cr2As2 and Ba2CrO2Cr2As2 with Cr2+ ions in CrO2 sheets and in CrAs layers crystallize with the Sr2Mn3Sb2O2 structure (space group I4/mmm, Z = 2) and lattice parameters a = 4.00800(2) Å, c = 18.8214(1) Å (Sr2CrO2Cr2As2) and a = 4.05506(2) Å, c = 20.5637(1) Å (Ba2CrO2Cr2As2) at room temperature. Powder neutron diffraction reveals checkerboard-type antiferromagnetic ordering of the Cr2+ ions in the arsenide layers below TN1_Sr, of 600(10) K (Sr2CrO2Cr2As2) and TN1_Ba 465(5) K (Ba2CrO2Cr2As2) with the moments initially directed perpendicular to the layers in both compounds. Checkerboard-type antiferromagnetic ordering of the Cr2+ ions in the oxide layer below 230(5) K for Ba2CrO2Cr2As2 occurs with these moments also perpendicular to the layers, consistent with the orientation preferences of d4 moments in the two layers. In contrast, below 330(5) K in Sr2CrO2Cr2As2, the oxide layer Cr2+ moments are initially oriented in the CrO2 plane; but on further cooling, these moments rotate to become perpendicular to the CrO2 planes, while the moments in the arsenide layers rotate by 90° with the moments on the two sublattices remaining orthogonal throughout [behavior recently reported independently by Liu et al. [Liu et al. Phys. Rev. B 2018, 98, 134416]]. In Sr2CrO2Cr2As2, electron diffraction and high resolution powder X-ray diffraction data show no evidence for a structural distortion that would allow the two Cr2+ sublattices to couple, but high resolution neutron powder diffraction data suggest a small incommensurability between the magnetic structure and the crystal structure, which may account for the coupling of the two sublattices and the observed spin reorientation. The saturation values of the Cr2+ moments in the CrO2 layers (3.34(1) μB (for Sr2CrO2Cr2As2) and 3.30(1) μB (for Ba2CrO2Cr2As2)) are larger than those in the CrAs layers (2.68(1) μB for Sr2CrO2Cr2As2 and 2.298(8) μB for Ba2CrO2Cr2As2) reflecting greater covalency in the arsenide layers.
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Oct 2020
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I11-High Resolution Powder Diffraction
I19-Small Molecule Single Crystal Diffraction
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Stephen P.
Argent
,
Ivan
Da Silva
,
Alex
Greenaway
,
Mathew
Savage
,
Jack
Humby
,
Andrew J.
Davies
,
Harriott
Nowell
,
William
Lewis
,
Pascal
Manuel
,
Chiu C.
Tang
,
Alexander J.
Blake
,
Michael W.
George
,
Alexander V.
Markevich
,
Elena
Besley
,
Sihai
Yang
,
Neil R.
Champness
,
Martin
Schroeder
Diamond Proposal Number(s):
[861, 11622, 15833, 9443]
Open Access
Abstract: Designing porous materials which can selectively adsorb CO2 or CH4 is an important environmental and industrial goal which requires an understanding of the host–guest interactions involved at the atomic scale. Metal–organic polyhedra (MOPs) showing permanent porosity upon desolvation are rarely observed. We report a family of MOPs (Cu-1a, Cu-1b, Cu-2), which derive their permanent porosity from cavities between packed cages rather than from within the polyhedra. Thus, for Cu-1a, the void fraction outside the cages totals 56% with only 2% within. The relative stabilities of these MOP structures are rationalized by considering their weak nondirectional packing interactions using Hirshfeld surface analyses. The exceptional stability of Cu-1a enables a detailed structural investigation into the adsorption of CO2 and CH4 using in situ X-ray and neutron diffraction, coupled with DFT calculations. The primary binding sites for adsorbed CO2 and CH4 in Cu-1a are found to be the open metal sites and pockets defined by the faces of phenyl rings. More importantly, the structural analysis of a hydrated sample of Cu-1a reveals a strong hydrogen bond between the adsorbed CO2 molecule and the Cu(II)-bound water molecule, shedding light on previous empirical and theoretical observations that partial hydration of metal−organic framework (MOF) materials containing open metal sites increases their uptake of CO2. The results of the crystallographic study on MOP–gas binding have been rationalized using DFT calculations, yielding individual binding energies for the various pore environments of Cu-1a.
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Oct 2020
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I20-Scanning-X-ray spectroscopy (XAS/XES)
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Diamond Proposal Number(s):
[11356]
Abstract: We have investigated the role of oxygen stoichiometry and structural properties in the modulation of Co valence and spin state in single-layer La2–xAxCoO4±δ (A = Sr, Ca; 0 ≤ x ≤ 1) perovskites as well as the interplay between their local structural properties and the magnetic and charge-ordering phenomena. We show the results of high angular resolution powder X-ray diffraction and Co K-edge X-ray absorption and emission spectroscopy experiments on polycrystalline and single-crystal samples. The different doping-induced changes in the Co valence and spin state by Ca (or Sr) substitution can be understood in terms of the evolving oxygen stoichiometry. For Ca doping, the interstitial oxygen excess around the La/Ca atoms in underdoped samples is rapidly lost upon increasing the Ca content. The creation of oxygen vacancies leads to the stabilization of a mixed-valence Co2.5+ independently of the Ca content. In contrast, Sr substitution leads to almost stoichiometric samples and a lower oxygen vacancy concentration, which allows higher mixed-valence states for Co up to Co2.9+. The Co mixed-valence state along the two series is fluctuating between two valence states, Co2.4+ as in La2CoO4.2 and Co2.9+ as in LaSrCoO3.91, that become periodically ordered for the charge-ordered phases around the half-doping. The X-ray emission derived spin states agree well with the Co fluctuating mixed-valence state derived from X-ray absorption spectroscopy on consideration of a distribution of high-spin Co2+ and low-spin Co3+. Furthermore, there is no quenching of the orbital contribution for the high-spin Co2+, as concluded from a comparison with macroscopic magnetization measurements. Doping holes are mainly located in the ab plane and have a strong oxygen 2p character. The major lattice distortions, which are different for Sr and Ca doping, occur along the c axis, where changes in the oxygen stoichiometry take place. Moreover, charge-order transitions are clearly shown from the anomalous increase of the c lattice parameter with an increase in the temperature above 500 K but there is no signature for a temperature-dependent spin-state transition.
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Oct 2020
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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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Diamond Proposal Number(s):
[12714, 14015]
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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