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  • 548  (13)
  • English  (13)
  • Russian
  • 2020-2022  (13)
  • 1940-1944
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  • English  (13)
  • Russian
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  • 1
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    Crystal structure of a calcium(II)–pyrroloquinoline quinone (PQQ) complex outside a protein environment (2021)
    5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-04-14
    Description: Pyrroloquinoline quinone (PQQ) is an important cofactor of calcium‐ and lanthanide‐dependent alcohol dehydrogenases, and has been known for over 30 years. Crystal structures of Ca–MDH enzymes (MDH is methanol dehydrogenase) have been known for some time; however, crystal structures of PQQ with biorelevant metal ions have been lacking in the literature for decades. We report here the first crystal structure analysis of a Ca–PQQ complex outside the protein environment, namely, poly[[undecaaquabis(μ‐4,5‐dioxo‐4,5‐dihydro‐1H‐pyrrolo[2,3‐f]quinoline‐2,7,9‐tricarboxylato)tricalcium(II)] dihydrate], {[Ca3(C14H3N2O8)2(H2O)11]·2H2O}n. The complex crystallized as Ca3PQQ2·13H2O with Ca2+ in three different positions and PQQ3−, including an extensive hydrogen‐bond network. Similarities and differences to the recently reported structure with biorelevant europium (Eu2PQQ2) are discussed.
    Description: Pyrroloquinoline quinone (PQQ) is an important cofactor of calcium‐ and lanthanide‐dependent alcohol dehydrogenases. The crystal structure of a Ca–PQQ complex (Ca3PQQ2·13H2O) is reported for the first time outside a protein environment. image
    Description: research
    Keywords: 548 ; pyrroloquinoline quinone ; calcium ; PQQ ; methanol dehydrogenase ; crystal structure ; FID-GEO-DE-7
    Language: English
    Type: article , publishedVersion
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  • 2
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    Crystallization of metastable monoclinic carnallite, KCl·MgCl2·6H2O: missing structural link in the carnallite family (2021)
    Details
    Publication Date: 2021-08-10
    Description: During evaporation of natural and synthetic K–Mg–Cl brines, the formation of almost square plate-like crystals of potassium carnallite (potassium chloride magnesium dichloride hexahydrate) was observed. A single-crystal structure analysis revealed a monoclinic cell [a = 9.251 (2), b = 9.516 (2), c = 13.217 (4) Å, β = 90.06 (2)° and space group C2/c]. The structure is isomorphous with other carnallite-type compounds, such as NH4Cl·MgCl2·6H2O. Until now, natural and synthetic carnallite, KCl·MgCl2·6H2O, was only known in its orthorhombic form [a = 16.0780 (3), b = 22.3850 (5), c = 9.5422 (2) Å and space group Pnna].
    Keywords: 548 ; crystallization ; metastable ; monoclinic ; potassium carnallite ; crystal structure ; potash ; fertilizer
    Language: English
    Type: article
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  • 3
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    Crystal structure and characterization of magnesium carbonate chloride heptahydrate (2021)
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    Publication Date: 2021-07-23
    Description: MgCO3·MgCl2·7H2O is the only known neutral magnesium carbonate containing chloride ions at ambient conditions. According to the literature, only small and twinned crystals of this double salt could be synthesised in a concentrated solution of MgCl2. For the crystal structure solution, single-crystal diffraction was carried out at a synchrotron radiation source. The monoclinic crystal structure (space group Cc) exhibits double chains of MgO octahedra linked by corners, connected by carbonate units and water molecules. The chloride ions are positioned between these double chains parallel to the (100) plane. Dry MgCO3·MgCl2·7H2O decomposes in the air to chlorartinite, Mg2(OH)Cl(CO3)·nH2O (n = 2 or 3). This work includes an extensive characterization of the title compound by powder X-ray diffraction, thermal analysis, SEM and vibrational spectroscopy.
    Keywords: 548 ; magnesium ; carbonate ; chloride ; hydrate ; synchrotron ; twinning ; crystal structure
    Language: English
    Type: article
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  • 4
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    Relativistic correction of atomic scattering factors for high-energy electron diffraction (2021)
    Details
    Publication Date: 2021-07-24
    Description: Relativistic electron diffraction depends on linear and quadratic terms in the electric potential, the latter being neglected in the frequently used relativistically corrected Schrödinger equation. The quadratic electric potential term modifies atomic scattering amplitudes in particular for large-angle scattering and backscattering. The respective correction increases with increasing scattering angle, increasing atomic number and increasing kinetic energy. Conventional tabulations for electron scattering and its large-angle extrapolations can be amended in closed form by a universal correction based on the screened Coulomb potential squared.
    Keywords: 548 ; electron diffraction ; atomic scattering factors ; relativity theory ; Schrödinger equation
    Language: English
    Type: article
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  • 5
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    Combined X-ray and neutron single-crystal diffraction in diamond anvil cells (2021)
    Details
    Publication Date: 2021-09-27
    Description: It is shown that it is possible to perform combined X-ray and neutron single-crystal studies in the same diamond anvil cell (DAC). A modified Merrill–Bassett DAC equipped with an inflatable membrane filled with He gas has been developed. It can be used on laboratory X-ray and synchrotron diffractometers as well as on neutron instruments. The data processing procedures and a joint structural refinement of the high-pressure synchrotron and neutron single-crystal data are presented and discussed for the first time.
    Keywords: 548 ; single-crystal X-ray diffraction ; single-crystal neutron diffraction ; high pressure ; diamond anvil cells
    Language: English
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  • 6
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    Dynamical X-ray diffraction imaging of voids in dislocation-free high-purity germanium single crystals (2021)
    Details
    Publication Date: 2021-09-27
    Description: White-beam X-ray topography has been performed to provide direct evidence of micro-voids in dislocation-free high-purity germanium single crystals. The voids are visible because of a dynamical diffraction contrast. It is shown that voids occur only in dislocation-free parts of the crystal and do not show up in regions with homogeneous and moderate dislocation density. It is further suggested that the voids originate from clustering of vacancies during the growth process. A general method is proposed to verify the presence of voids for any crystalline material of high structural perfection.
    Keywords: 548 ; X-ray topography ; dynamical theory ; high-purity germanium ; vacancies ; voids ; dislocation density ; diffraction contrast
    Language: English
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  • 7
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    X-ray diffraction using focused-ion-beam-prepared single crystals (2021)
    Details
    Publication Date: 2021-10-07
    Description: High-quality single-crystal X-ray diffraction measurements are a prerequisite for obtaining precise and reliable structure data and electron densities. The single crystal should therefore fulfill several conditions, of which a regular defined shape is of particularly high importance for compounds consisting of heavy elements with high X-ray absorption coefficients. The absorption of X-rays passing through a 50 µm-thick LiNbO3 crystal can reduce the transmission of Mo Kα radiation by several tens of percent, which makes an absorption correction of the reflection intensities necessary. In order to reduce ambiguities concerning the shape of a crystal, used for the necessary absorption correction, a method for preparation of regularly shaped single crystals out of large samples is presented and evaluated. This method utilizes a focused ion beam to cut crystals with defined size and shape reproducibly and carefully without splintering. For evaluation, a single-crystal X-ray diffraction study using a laboratory diffractometer is presented, comparing differently prepared LiNbO3 crystals originating from the same macroscopic crystal plate. Results of the data reduction, structure refinement and electron density reconstruction indicate qualitatively similar values for all prepared crystals. Thus, the different preparation techniques have a smaller impact than expected. However, the atomic coordinates, electron densities and atomic charges are supposed to be more reliable since the focused-ion-beam-prepared crystal exhibits the smallest extinction influences. This preparation technique is especially recommended for susceptible samples, for cases where a minimal invasive preparation procedure is needed, and for the preparation of crystals from specific areas, complex material architectures and materials that cannot be prepared with common methods (breaking or grinding).
    Keywords: 548 ; X-ray diffraction ; sample preparation ; focused ion beams
    Language: English
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  • 8
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    Isotopy classes for 3-periodic net embeddings (2021)
    Details
    Publication Date: 2021-10-15
    Description: Entangled embedded periodic nets and crystal frameworks are defined, along with their dimension type, homogeneity type, adjacency depth and periodic isotopy type. Periodic isotopy classifications are obtained for various families of embedded nets with small quotient graphs. The 25 periodic isotopy classes of depth-1 embedded nets with a single-vertex quotient graph are enumerated. Additionally, a classification is given of embeddings of n-fold copies of pcu with all connected components in a parallel orientation and n vertices in a repeat unit, as well as demonstrations of their maximal symmetry periodic isotopes. The methodology of linear graph knots on the flat 3-torus [0,1)3 is introduced. These graph knots, with linear edges, are spatial embeddings of the labelled quotient graphs of an embedded net which are associated with its periodicity bases.
    Keywords: 548 ; periodic nets ; embedded nets ; coordination polymers ; isotopy types ; crystallographic frameworks
    Language: English
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  • 9
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    Optimized reconstruction of the crystallographic orientation density function based on a reduced set of orientations (2021)
    Details
    Publication Date: 2021-10-12
    Description: Crystallographic textures, as they develop for example during cold forming, can have a significant influence on the mechanical properties of metals, such as plastic anisotropy. Textures are typically characterized by a non-uniform distribution of crystallographic orientations that can be measured by diffraction experiments like electron backscatter diffraction (EBSD). Such experimental data usually contain a large number of data points, which must be significantly reduced to be used for numerical modeling. However, the challenge in such data reduction is to preserve the important characteristics of the experimental data, while reducing the volume and preserving the computational efficiency of the numerical model. For example, in micromechanical modeling, representative volume elements (RVEs) of the real microstructure are generated and the mechanical properties of these RVEs are studied by the crystal plasticity finite element method. In this work, a new method is developed for extracting a reduced set of orientations from EBSD data containing a large number of orientations. This approach is based on the established integer approximation method and it minimizes its shortcomings. Furthermore, the L1 norm is applied as an error function; this is commonly used in texture analysis for quantitative assessment of the degree of approximation and can be used to control the convergence behavior. The method is tested on four experimental data sets to demonstrate its capabilities. This new method for the purposeful reduction of a set of orientations into equally weighted orientations is not only suitable for numerical simulation but also shows improvement in results in comparison with other available methods.
    Keywords: 548 ; texture reconstruction ; integer approximation ; crystal plasticity ; micromechanical modeling
    Language: English
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  • 10
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    The modulated low-temperature structure of malayaite, CaSnOSiO4 (2021)
    Details
    Publication Date: 2021-10-13
    Description: The crystal structure of the mineral malayaite has been studied by single-crystal X-ray diffraction at a temperature of 20 K and by calculation of its phonon dispersion using density functional perturbation theory. The X-ray diffraction data show first-order satellite diffraction maxima at positions q = 0.2606 (8)b*, that are absent at room temperature. The computed phonon dispersion indicates unstable modes associated with dynamic displacements of the Ca atoms. The largest-frequency modulus of these phonon instabilities is located close to a wavevector of q = 0.3b*. These results indicate that the malayaite crystal structure is incommensurately modulated by static displacement of the Ca atoms at low temperatures, caused by the softening of an optic phonon with Bg symmetry.
    Keywords: 548 ; modulation ; optic phonon ; titanite ; satellite reflections ; DFPT
    Language: English
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  • 11
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    Multiple Bragg reflection by a thick mosaic crystal. II. Simplified transport equation solved on a grid (2021)
    Details
    Publication Date: 2021-10-13
    Description: The generalized Darwin–Hamilton equations [Wuttke (2014). Acta Cryst. A70, 429–440] describe multiple Bragg reflection from a thick, ideally imperfect crystal. These equations are simplified by making full use of energy conservation, and it is demonstrated that the conventional two-ray Darwin–Hamilton equations are obtained as a first-order approximation. Then an efficient numeric solution method is presented, based on a transfer matrix for discretized directional distribution functions and on spectral collocation in the depth coordinate. Example solutions illustrate the orientational spread of multiply reflected rays and the distortion of rocking curves, especially if the detector only covers a finite solid angle.
    Keywords: 548 ; mosaic crystals ; multiple scattering ; Darwin–Hamilton equations ; spectral collocation
    Language: English
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  • 12
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    A new hydrate of magnesium carbonate, MgCO3·6H2O (2021)
    Details
    Publication Date: 2021-09-24
    Description: During investigations of the formation of hydrated magnesium carbonates, a sample of the previously unknown magnesium carbonate hexahydrate (MgCO3·6H2O) was synthesized in an aqueous solution at 273.15 K. The crystal structure consists of edge-linked isolated pairs of Mg(CO3)(H2O)4 octahedra and noncoordinating water molecules, and exhibits similarities to NiCO3·5.5H2O (hellyerite). The recorded X-ray diffraction pattern and the Raman spectra confirmed the formation of a new phase and its transformation to magnesium carbonate trihydrate (MgCO3·3H2O) at room temperature.
    Keywords: 548 ; magnesium ; carbonate ; low-temperature hydrate ; Raman spectroscopy ; crystal structure
    Language: English
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  • 13
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    speckle-tracking: a software suite for ptychographic X-ray speckle tracking (2021)
    Details
    Publication Date: 2021-12-03
    Description: In recent years, X-ray speckle-tracking techniques have emerged as viable tools for wavefront metrology and sample imaging applications. These methods are based on the measurement of near-field images. Thanks to their simple experimental setup, high angular sensitivity and compatibility with low-coherence sources, these methods have been actively developed for use with synchrotron and laboratory light sources. Not only do speckle-tracking techniques give the potential for high-resolution imaging, but they also provide rapid and robust characterization of aberrations of X-ray optical elements, focal spot profiles, and sample position and transmission properties. In order to realize these capabilities, software implementations are required that are equally rapid and robust. To address this need, a software suite has been developed for the ptychographic X-ray speckle-tracking technique, an X-ray speckle-based method suitable for highly divergent wavefields. The software suite is written in Python 3, with an OpenCL back end for GPU and multi-CPU core processing. It is accessible as a Python module, through the command line or through a graphical user interface, and is available as source code under Version 3 or later of the GNU General Public License.
    Keywords: 548 ; software ; wavefront metrology ; speckle tracking ; ptychography ; X-ray projection imaging
    Language: English
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