| 1. |
- Aleksis, Rihards, et al.
(författare)
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Probing the electronic structure and hydride occupancy in barium titanium oxyhydride through DFT-assisted solid-state NMR
- 2022
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - 1463-9076 .- 1463-9084. ; 24:46, s. 28164-28173
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Tidskriftsartikel (refereegranskat)abstract
- Perovskite-type oxhydrides such as BaTiO3−xHy exhibit mixed hydride ion and electron conduction and are an attractive class of materials for developing energy storage devices. However, the underlying mechanism of electric conductivity and its relation to the composition of the material remains unclear. Here we report detailed insights into the hydride local environment, the electronic structure and hydride conduction dynamics of barium titanium oxyhydride. We demonstrate that DFT-assisted solid-state NMR is an excellent tool for differentiating between the different feasible electronic structures in these solids. Our results indicate that upon reduction of BaTiO3 the introduced electrons are delocalized among all Ti atoms forming a bandstate. Furthermore, each vacated anion site is reoccupied by at most a single hydride, or else remains vacant. This single occupied bandstate structure persists at different hydrogen concentrations (y = 0.13–0.31) and a wide range of temperatures (∼100–300 K).
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| 2. |
- Carvalho, José P., et al.
(författare)
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Half-integer-spin quadrupolar nuclei in magic-angle spinning paramagnetic NMR : The case of NaMnO2
- 2022
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Ingår i: Journal of magnetic resonance. - : Elsevier BV. - 1090-7807 .- 1096-0856. ; 340
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Tidskriftsartikel (refereegranskat)abstract
- A combination of solid-state NMR methods for the extraction of 23Na shift and quadrupolar parameters in the as-synthesized, structurally complex NaMnO2 Na-ion cathode material, under magic-angle spinning (MAS) is presented. We show that the integration of the Magic-Angle Turning experiment with Rotor-Assisted Population transfer (RAPT) can be used both to identify shifts and to extract a range of magnitudes for their quadrupolar couplings. We also demonstrate the applicability of the two-dimensional one pulse (TOP) based double-sheared Satellite Transition Magic-Angle Spinning (TOP-STMAS) showing how it can yield a spectrum with separated shift and second-order quadrupolar anisotropies, which in turn can be used to analyze a quadrupolar lineshape free of anisotropic bulk magnetic susceptibility (ABMS) induced shift dispersion and determine both isotropic shift and quadrupolar products. Combining all these experiments, the shift and quadrupolar parameters for all observed Na environments were extracted and yielded excellent agreement with the density functional theory (DFT) based models that were reported in previous literature. We expect these methods to open the door for new possibilities for solid-state NMR to probe half-integer quadrupolar nuclei in paramagnetic materials and other systems exhibiting large shift dispersion.
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| 3. |
- Papawassiliou, Wassilios, 1991-, et al.
(författare)
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Detection of Weyl fermions and the metal to Weyl-semimetal phase transition in WTe2via broadband high resolution NMR
- 2022
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Ingår i: Physical Review Research. - 2643-1564. ; 4:3
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Tidskriftsartikel (refereegranskat)abstract
- Weyl fermions (WFs) in the type-II Weyl semimetal (WSM) WTe2 are difficult to resolve experimentally because the Weyl bands disperse in an extremely narrow region of the (E−k) space. Here, by using DFT-assisted high-resolution 125Te solid-state nuclear magnetic resonance (ssNMR) in the temperature range 50–700 K, we succeeded in detecting low energy WF excitations and monitoring their evolution with temperature. Remarkably, WFs are observed to emerge at T∼120 K, while at lower temperatures WTe2 behaves as a trivial metal. This intriguing phenomenon is induced by the rapid raise of the Fermi level upon heating, which crosses the Weyl bands only for T>120 K. The abrupt change of the NMR parameters at this temperature is signature of a topological Lifshitz transition instead of a cursive energy-bands crossing.
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| 4. |
- Papawassiliou, Wassilios, 1991-
(författare)
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Evaluation of NMR Knight shifts in metallic nanoparticles and topological matter
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Elucidating the surface electron states of transition metal compounds is of primary importance in main heterogeneous catalytic processes, such as the hydrogen and oxygen evolution reactions. Key property in all these processes is the position of the energy of the d-band center relative to the Fermi-level of the catalyst; it must be shifted close to the Fermi level to achieve balance between adsorption and desorption of the catalyst and the adsorbate. Often, these processes involve expensive metals such as Ru or Pt, limiting their applicability. The Nickel Phosphide (NixPy) family has recently emerged as an important catalyst family replacing noble metals; in these systems the surface electronic properties, may be tailored by doping with different transition metals, decreasing size, or by controlling the nanoparticle shape (facet engineering). It is thus crucial to be able to simultaneously monitor the evolution of the morphology as well as the electronic structure of the NP particles while scaling down the size.In most of these materials, surface electron states are extremely sensitive to local disturbances, such as impurities, surface defects, as well as surface termination. In contrast, 3D topological insulators like Bi2Se3, or Bi2Te3, exhibit exceptionally robust metallic surface electron states while the bulk interior is insulating. These extraordinary properties, which become dominant by reducing the system size to the nanometers, have been tied to enhancement of the Seebeck effect, i.e., the conversion of heat into electricity, catalytic activity, and electrochemical performance, the latter of these effects has been pursed in this thesis as well. An important question that has eluded however is the presence of the Dirac electrons themselves and to which extend the Dirac electrons penetrate the nanoparticles, controlling thus the overall electronic properties.In contrast to the TIs, Weyl semimetals (WSMs), another category of topological materials, host protected electron states in the bulk interior. The bulk conduction and valence bands of these systems cross linearly in pairs of conjugate nodal points, the so-called Weyl points, forming characteristic double cones. Remarkably, in specific WSMs, such as the WTe2 and MoTe2, known as type-II WSMs, the Weyl cones are strongly tilted, leading to the formation of electron and hole pockets at the Fermi level, strongly influencing their electronic properties. However, energy bands in these systems are shown to disperse in a very tiny region, rendering standard experimental techniques, such as Angle Resolved Photoemission Spectroscopy obsolete in detecting the Weyl bands. In this thesis all the issues mentioned for each case, were tackled by employing solid-state nuclear magnetic resonance (ssNMR) spectroscopy under various temperatures and magnetic fields, combined with high-resolution transmission electron microscopy and density functional theory calculations.
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| 5. |
- Siebeneichler, Stefanie, et al.
(författare)
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Frustration and 120° Magnetic Ordering in the Layered Triangular Antiferromagnets AFe(PO3F)2 (A = K, (NH4)2Cl, NH4, Rb, and Cs)
- 2022
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 34:17, s. 7982-7994
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Tidskriftsartikel (refereegranskat)abstract
- A new family of oxofluorophoshates, AFe(PO3F)2 (A = K, (NH4)2Cl, NH4, Rb, and Cs), was synthesized via ionothermal methods using PF6 ionic liquids. Single-crystal and powder X-ray diffraction reveal that AFe(PO3F)2 with A = (NH4)2Cl crystallizes in a trigonal structure, while AFe(PO3F)2 with A = NH4, Rb, and Cs crystallizes in a triclinic structure. Dimorphic KFe(PO3F)2 crystallizes in both the trigonal and triclinic forms. The structures of all compounds feature Yavapaiite-like Fe(PO3F)2 slabs, which are characterized by triangular Fe layers, planar in the case of the trigonal structure and undulated in the case of the triclinic one. Magnetization measurements reveal all compounds to order antiferromagnetically at low temperatures. The trigonal phases AFe(PO3F)2 (A = K and (NH4)2Cl) display complex magnetic H–T phase diagrams. The observation of magnetization plateaus at Msat/3 (Msat = saturation magnetization) indicates the existence of the up–up–down (UUD) and V-phases at applied magnetic fields in the magnetically ordered state. Powder neutron diffraction measurements of KFe(PO3F)2 confirm the 120° spin structure at zero fields. Along c, the magnetic moments form a commensurate spiral since the spins in each plane are rotated by 90° with respect to the adjacent one. To our knowledge, this is the first time such a non-centrosymmetric version of the 120° spin structure with a 90° rotation between nearest planes has been reported.
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