I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[21804]
Abstract: Complex metal hydrides are a fascinating and continuously expanding class of materials with many properties relevant for solid-state hydrogen and ammonia storage and solid-state electrolytes. The crystal structures are often investigated using powder X-ray diffraction (PXD), which can be ambiguous. Here, we revisit the crystal structure of Y(11BD4)3·3ND3 with the use of neutron diffraction, which, in comparison to previous PXD studies, provides accurate information about the D positions in the compound. Upon cooling to 10 K, the compound underwent a polymorphic transition, and a new monoclinic low-temperature polymorph denoted as α-Y(11BD4)3·3ND3 was discovered. Furthermore, the series of Y(11BH4)3·xNH3 (x = 0, 3, and 7) were also investigated with inelastic neutron scattering and infrared spectroscopy techniques, which provided information of the local coordination environment of the 11BH4– and NH3 groups and unique insights into the hydrogen dynamics. Partial deuteration using ND3 in Y(11BH4)3·xND3 (x = 3 and 7) allowed for an unambiguous assignment of the vibrational bands corresponding to the NH3 and 11BH4– in Y(11BH4)3·xNH3, due to the much larger neutron scattering cross section of H compared to D. The vibrational spectra of Y(11BH4)3·xNH3 could roughly be divided into three regions: (i) below 55 meV, containing mainly 11BH4– librational motions, (ii) 55–130 meV, containing mainly NH3 librational motions, and (iii) above 130 meV, containing 11B–H and N–H bending and stretching motions.
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Jul 2021
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Open Access
Abstract: Understanding the adsorption and photoactivity of acetic acid and trimethyl acetic acid on TiO2 surfaces is important for improving the performance of photocatalysts and dye-sensitized solar cells. Here we present a structural study of adsorption on rutile TiO2(100)-1 × 1 and -1 × 3 using Scanning Tunnelling Microscopy and Density Functional Theory calculations. Exposure of both terminations to acetic acid gives rise to a ×2 periodicity in the [001] direction (i.e., along Ti rows), with a majority ordered c(2 × 2) phase in the case of the 1 × 1 termination. The DFT calculations suggest that the preference of c(2 × 2) over the 2 × 1 periodicity found for TiO2(110)-1 × 1 can be attributed to an increase in interadsorbate Coulomb repulsion. Exposure of TiO2(100)-1 × 1 and -1 × 3 to trimethyl acetic acid gives rise to largely disordered structures due to steric effects, with quasi-order occurring in small areas and near step edges where these effects are reduced.
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Jun 2021
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B18-Core EXAFS
I15-1-X-ray Pair Distribution Function (XPDF)
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Diamond Proposal Number(s):
[18595, 18923, 21525]
Abstract: Phosphate-based glasses (PBGs) are traditionally prepared using the high-temperature melt-quenching (MQ) route or via the more recent sol–gel (SG) method that requires the use of organic solvents. The coacervation method represents an excellent inexpensive and green alternative to MQ and SG, being performed in aqueous solution and at room temperature. Coacervation is particularly applicable for the production of PBGs designed for biomedical applications because it allows for the inclusion of temperature-sensitive molecules and does not require the use of toxic solvents. Whereas the atomic structure of the MQ and SGPBGs is known, the atomic structure of those prepared via coacervation has yet to be investigated. In this study, a comprehensive advanced structural characterization has been performed on phosphate-based glasses in the system P2O5–CaO–Na2O–Ag2O (Ag2O mol % = 0, 1, 3, 5, 9, and 14) prepared via the coacervation method. Glasses within this system should find application as bioresorbable biomaterials thanks to their ability to release bioactive ions in a controlled manner. In particular, they possess antibacterial properties, inferred by the release of Ag+ over time. High-energy X-ray diffraction (HEXRD), 31P and 23Na solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR), and X-ray absorption Spectroscopy (XAS) at the Ag K-edge were used to probe the atomic structure of the glasses after drying in vacuum and after calcination at 300 °C. The length of the polyphosphate chains in the solid state appears to be independent of silver concentration; however, significant degradation of these chains is seen after calcination at 300 °C. Atomic-scale characterisation results indicate that the structure of these glasses is akin to that of other silver-doped phosphate glasses prepared using the MQ and SG methods. This suggests that phosphate-based glasses prepared using milder and greener conditions may have similar chemical and physical properties such as solubility, biocompatibility, and antibacterial properties.
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May 2021
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[21899]
Abstract: Organic charge transfer complexes (CTCs) with near-infrared absorption received growing interest in the past years, but the details of their photophysics, especially in thin films, remain largely unknown. We combined experimental and computational methods to thoroughly investigate and compare CTCs formed by tetracene with 2,2′-(perfluoronaphthalene-2,6-diylidene)dimalononitrile and 2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane, respectively. Using ultrafast transient absorption spectroscopy, the photophysics of these small bandgap CTCs was revealed, which is dominated by a sub-picosecond relaxation of the excitons back to the ground state. In equimolar blends, tetracene singlet fission is suppressed while in blends with excess of tetracene reduced lifetimes of tetracene, singlet and triplet excitons were found.
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Mar 2021
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Open Access
Abstract: Ternary lanthanide indium oxides LnInO3 (Ln = La, Pr, Nd, Sm) were synthesized by high-temperature solid-state reaction and characterized by X-ray powder diffraction. Rietveld refinement of the powder patterns showed the LnInO3 materials to be orthorhombic perovskites belonging to the space group Pnma, based on almost-regular InO6 octahedra and highly distorted LnO12 polyhedra. Experimental structural data were compared with results from density functional theory (DFT) calculations employing a hybrid Hamiltonian. Valence region X-ray photoelectron and K-shell X-ray emission and absorption spectra of the LnInO3 compounds were simulated with the aid of the DFT calculations. Photoionization of lanthanide 4f orbitals gives rise to a complex final-state multiplet structure in the valence region for the 4fn compounds PrInO3, NdInO3, and SmInO3, and the overall photoemission spectral profiles were shown to be a superposition of final-state 4fn–1 terms onto the cross-section weighted partial densities of states from the other orbitals. The occupied 4f states are stabilized in moving across the series Pr–Nd–Sm. Band gaps were measured using diffuse reflectance spectroscopy. These results demonstrated that the band gap of LaInO3 is 4.32 eV, in agreement with DFT calculations. This is significantly larger than a band gap of 2.2 eV first proposed in 1967 and based on the idea that In 4d states lie above the top of the O 2p valence band. However, both DFT and X-ray spectroscopy show that In 4d is a shallow core level located well below the bottom of the valence band. Band gaps greater than 4 eV were observed for NdInO3 and SmInO3, but a lower gap of 3.6 eV for PrInO3 was shown to arise from the occupied Pr 4f states lying above the main O 2p valence band.
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Mar 2021
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I11-High Resolution Powder Diffraction
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Open Access
Abstract: The structure and reorientational dynamics of KB3H8 were studied by using quasielastic and inelastic neutron scattering, Raman spectroscopy, first-principles calculations, differential scanning calorimetry, and in situ synchrotron radiation powder X-ray diffraction. The results reveal the existence of a previously unknown polymorph in between the α′- and β-polymorphs. Furthermore, it was found that the [B3H8]− anion undergoes different reorientational motions in the three polymorphs α, α′, and β. In α-KB3H8, the [B3H8]− anion performs 3-fold rotations in the plane created by the three boron atoms, which changes to a 2-fold rotation around the C2 symmetry axis of the [B3H8]− anion upon transitioning to α′-KB3H8. After transitioning to β-KB3H8, the [B3H8]− anion performs 4-fold rotations in the plane created by the three boron atoms, which indicates that the local structure of β-KB3H8 deviates from the global cubic NaCl-type structure. The results also indicate that the high reorientational mobility of the [B3H8]− anion facilitates the K+ cation conductivity, since the 2-orders-of-magnitude increase in the anion reorientational mobility observed between 297 and 311 K coincides with a large increase in K+ conductivity.
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Feb 2021
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I15-Extreme Conditions
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Diamond Proposal Number(s):
[22477]
Open Access
Abstract: Defects are emerging as a key tool for fine-tuning the stimuli-responsive behavior of coordination polymers and metal–organic frameworks. Here, we study the ramifications of defects on the mechanical properties of the molecular perovskite [C(NH2)3]MnII(HCOO)3 and its defective analogue [C(NH2)3]Fe2/3III□1/3(HCOO)3, where □ = vacancy. Defects reduce the bulk modulus by 30% and give rise to a temperature-driven phase transition not observed in the nondefective system. The results highlight the opportunities that come with defect-engineering approaches to alter the mechanical properties and underlying thermodynamics, with important implications for the research on stimuli-responsive materials.
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Jan 2021
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I09-Surface and Interface Structural Analysis
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Peter
Knecht
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Paul T. P.
Ryan
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David A.
Duncan
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Li
Jiang
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Joachim
Reichert
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Peter S.
Deimel
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Felix
Haag
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Johannes T.
Kuchle
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Francesco
Allegretti
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Tien-Lin
Lee
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Martin
Schwarz
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Manuela
Garnica
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Willi
Auwärter
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Ari Paavo
Seitsonen
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Johannes V.
Barth
,
Anthoula C.
Papageorgiou
Diamond Proposal Number(s):
[24320, 17634]
Abstract: The adsorption and monolayer self-assembly of functional metal–organic blocks on solid surfaces are critical for the physicochemical properties of these low-dimensional materials. Although modern microscopy tools are very sensitive to the lateral arrangement of such blocks, they are still unable to offer directly the complete structural analysis especially for nonplanar molecules containing different atoms. Here, we apply a combinatorial approach for the characterization of such interfaces, which enables unexpected insights. An archetypal metalloporphyrin on a catalytically active surface as a function of its molecular coverage and substituent arrangement is characterized by low-energy electron diffraction, scanning probe microscopy, X-ray photoelectron spectroscopy, normal-incidence X-ray standing waves, and density functional theory. We look into Ru tetraphenyl porphyrin (Ru-TPP) on Ag(111), which is also converted into its planarized derivates via surface-assisted cyclodehydrogenation reactions. Depending on the arrangement of the phenyl substituents, the Ru atoms have distinct electronic structures and the porphyrin macrocycles adapt differently to the surface: saddle shape (pristine Ru-TPP) or bowl shape (planarized Ru-TPP derivates). In all cases, the Ru atom resides close to the surface (2.59/2.45 Å), preferably located at hollow sites and in the interface between the plane of the porphyrin macrocycle and the Ag surface. For the more flexible pristine Ru-TPP, we identify an additional self-assembled structure, allowing the molecular density of the self-assembled monolayer to be tuned within ∼20%. This precise analysis is central to harnessing the potential of metalloporphyrin/metal interfaces in functional systems.
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Jan 2021
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I15-Extreme Conditions
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Diamond Proposal Number(s):
[4631, 6707]
Abstract: High-pressure studies have been performed on the ε-form of the powerful explosive CL-20. Hydrostatic compression over the pressure range 0–12 GPa has been monitored using synchrotron X-ray powder diffraction. The potential effects of X-ray radiation damage were observed and circumvented through a follow-up compression study over the pressure range 0–7 GPa using neutron powder diffraction. This second study revealed smooth compression behavior, and the absence of any phase transitions. Intermolecular interaction energies as obtained using PIXEL calculations did not show any discontinuity upon the application of pressure. An isothermal equation of state has been determined, and the high-pressure response is supported by dispersion-corrected density functional theory calculations. Inelastic neutron scattering (experimental and simulated) spectra for the ε-form are in excellent agreement.
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Dec 2020
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B07-C-Versatile Soft X-ray beamline: Ambient Pressure XPS and NEXAFS
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Andrea
Resta
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Uta
Hejral
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Sara
Blomberg
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Stefano
Albertin
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Alina
Vlad
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Yves
Garreau
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Corentin
Chatelier
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Federica
Venturini
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Pilar
Ferrer
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Georg
Held
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Dave
Grinter
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Edvin
Lundgren
,
Alessandro
Coati
Diamond Proposal Number(s):
[21936]
Abstract: The reaction of ammonia oxidation over PtRh binary alloy has been studied with a surface science approach by operando techniques such as Near Ambient Pressure X-Ray Photoemission Spectroscopy (NAP-XPS) and Surface X-Ray Diffraction (SXRD) combined with mass spectrometry. The article will explore the surface evolution across five different oxygen to ammonia ratios in the millibar regime for two different temperatures. The presented data-set allows to link variations in the atomic structures measured by diffraction methods and surface species information from NAP-XPS to reaction products in the gas phase. We will show that NO production coincides with significant changes of the surface structure and the formation of a RhO2 surface oxide. It was also observed that the RhO2 surface oxide only fully forms when the nitrogen signal in the N1s has disappeared.
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Sep 2020
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