141 research outputs found
Effect of cation vacancies on the crystal structure and luminescent properties of Ca0.85-1.5xGdxEu0.1□0.05+0.5xWO4(0 <= x <= 0.567) scheelite-based red phosphors
Using electron vortex beams to determine chirality of crystals in transmission electron microscopy
We investigate electron vortex beams elastically scattered on chiral
crystals. After deriving a general expression for the scattering amplitude of a
vortex electron, we study its diffraction on point scatterers arranged on a
helix. We derive a relation between the handedness of the helix and the
topological charge of the electron vortex on one hand, and the symmetry of the
Higher Order Laue Zones in the diffraction pattern on the other for
kinematically and dynamically scattered electrons. We then extend this to atoms
arranged on a helix as found in crystals which belong to chiral space groups
and propose a new method to determine the handedness of such crystals by
looking at the symmetry of the diffraction pattern. Contrary to alternative
methods, our technique does not require multiple scattering which makes it
possible to also investigate extremely thin samples in which multiple
scattering is suppressed. In order to verify the model, elastic scattering
simulations are performed and an experimental demonstration on
MnSbO is given where we find the sample to belong to the right
handed variant of its enantiomorphic pair. This demonstrates the usefulness of
electron vortex beams to reveal the chirality of crystals in a transmission
electron microscope and provides the required theoretical basis for further
developments in this field
\require{mhchem}Misfit phase \ce{(BiSe)_{1.10}NbSe2} as the origin of superconductivity in nobium-doped bismuth selenide
\require{mhchem}Topological superconductivity is of great contemporary
interest and has been proposed in doped \ce{Bi2Se3} in which
electron-donating atoms such as Cu, Sr or Nb have been intercalated into the
\ce{Bi2Se3} structure. For \ce{Nb_{x}Bi2Se3}, with , it is assumed in the literature that Nb is inserted in the van
der Waals gap. However, in this work an alternative origin for the
superconductivity in Nb-doped \ce{Bi2Se3} is established. In contrast to
previous reports, it is deduced that Nb intercalation in \ce{Bi2Se3} does not
take place. Instead, the superconducting behaviour in samples of nominal
composition \ce{Nb_{x}Bi2Se3} results from the \ce{(BiSe)_{1.10}NbSe2}
misfit phase that is present in the sample as an impurity phase for small
() and as a main phase for large (). The
structure of this misfit phase is studied in detail using a combination of
X-ray diffraction and transmission electron microscopy techniques
Data Mining and Social Media for promotion and marketing in the Aerospace Industry
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The crystal and defect structures of polar KBiNb2O7
Funding: Experiments at the Diamond Light Source were performed as part of the Block Allocation Group award “Oxford/Warwick Solid State Chemistry BAG to probe composition-structure– property relationships in solids” (CY25166). Experiments at the ISIS pulsed neutron facility were supported by a beam time allocation from the STFC (RB 2000148). SM thanks Somerville College for an Oxford Ryniker Lloyd scholarship. ‘PSH and WZ thank the National Science Foundation (DMR-2002319) and Welch Foundation (Grant E-1457) for support.KBiNb2O7 was prepared from RbBiNb2O7 by a sequence of cation exchange reactions which first convert RbBiNb2O7 to LiBiNb2O7, before KBiNb2O7 is formed by a further K-for-Li cation exchange. A combination of neutron, synchrotron X-ray and electron diffraction data reveal that KBiNb2O7 adopts a polar, layered, perovskite structure (space group A11m) in which the BiNb2O7 layers are stacked in a (0, ½, z) arrangement, with the K+ cations located in half of the available 10-coordinate interlayer cation sites. The inversion symmetry of the phase is broken by a large displacement of the Bi3+ cations parallel to the y-axis. HAADF-STEM images reveal that KBiNb2O7 exhibits frequent stacking faults which convert the (0, ½, z) layer stacking to (½, 0, z) stacking and vice versa, essentially switching the x- and y-axes of the material. By fitting the complex diffraction peak shape of the SXRD data collected from KBiNb2O7 it is estimated that each layer has approximately a 9% chance of being defective-a high level which is attributed to the lack of cooperative NbO6 tilting in the material, which limits the lattice strain associated with each fault.Publisher PDFPeer reviewe
Binary/ternary phases in Cu2ZnGeSe4 thin-film solar cells and their impact on the cell performance
Effect of Zinc Oxide Modification by Indium Oxide on Microstructure, Adsorbed Surface Species, and Sensitivity to CO
Additives in semiconductor metal oxides are commonly used to improve sensing behavior of gas sensors. Due to complicated effects of additives on the materials microstructure, adsorption sites and reactivity to target gases the sensing mechanism with modified metal oxides is a matter of thorough research. Herein, we establish the promoting effect of nanocrystalline zinc oxide modification by 1–7 at.% of indium on the sensitivity to CO gas due to improved nanostructure dispersion and concentration of active sites. The sensing materials were synthesized via an aqueous coprecipitation route. Materials composition, particle size and BET area were evaluated using X-ray diffraction, nitrogen adsorption isotherms, high-resolution electron microscopy techniques and EDX-mapping. Surface species of chemisorbed oxygen, OH-groups, and acid sites were characterized by probe molecule techniques and infrared spectroscopy. It was found that particle size of zinc oxide decreased and the BET area increased with the amount of indium oxide. The additive was observed as amorphous indium oxide segregated on agglomerated ZnO nanocrystals. The measured concentration of surface species was higher on In2O3-modified zinc oxide. With the increase of indium oxide content, the sensor response of ZnO/In2O3 to CO was improved. Using in situ infrared spectroscopy, it was shown that oxidation of CO molecules was enhanced on the modified zinc oxide surface. The effect of modifier was attributed to promotion of surface OH-groups and enhancement of CO oxidation on the segregated indium ions, as suggested by DFT in previous work
Effect of cation vacancies on the crystal structure and luminescent properties of Ca 0.85−1.5x Gd x Eu 0.1 □ 0.05+0.5x WO 4 (0 ≤ x ≤ 0.567) scheelite-based red phosphors
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