161,018 research outputs found
Application of delta recycling to electron automated diffraction tomography data from inorganic crystalline nanovolumes
No full textdelta Recycling is a simple procedure for directly extracting phase information from Patterson-type functions [Rius (2012). Acta Cryst. A68, 399-400]. This new phasing method has a clear theoretical basis and was developed with ideal single-crystal X-ray diffraction data. On the other hand, introduction of the automated diffraction tomography (ADT) technique has represented a significant advance in electron diffraction data collection [Kolb et al. (2007). Ultramicroscopy, 107, 507-513]. When combined with precession electron diffraction, it delivers quasi-kinematical intensity data even for complex inorganic compounds, so that single-crystal diffraction data of nanometric volumes are now available for structure determination by direct methods. To check the tolerance of delta recycling to missing data-collection corrections and to deviations from kinematical behaviour of ADT intensities, delta recycling has been applied to differently shaped nanocrystals of various inorganic materials. The results confirm that it can phase ADT data very efficiently. In some cases even more complete structure models than those derived from conventional direct methods and least-squares refinement have been found. During this study it has been demonstrated that the Wilson-plot scaling procedure is largely insensitive to sample thickness variations and missing absorption corrections affecting electron ADT intensities
Structure Determination of Karibibite by EDT: New Persperctives for Mineralogy and Material Sciences
No full textStructure solution of complex minerals from PD data with EDT-assisted full-symmetry Patterson function direct methods: The decrespignyite-(Y) case
No full textStructural study of decrespignyite-(Y), a complex yttrium rare earth copper carbonate chloride, by three-dimensional electron and synchrotron powder diffraction
No full textThe crystal structure of the mineral decrespignyite-(Y) from the Paratoo copper mine (South Australia) has been obtained by applying d recycling direct methods to 3D electron diffraction (ED) data followed by Rietveld refinements of synchrotron data. The unit cell is a = 8.5462(2), c = 22.731(2) Å and V = 1437.8(2) Å3, and the chemical formula for Z = 1 is (Y10.35REE1.43Ca0.52Cu5.31 σ17.61(CO3)14Cl2.21(OH)16.79• 18.35H2O (REE. rare earth elements). The ED data are compatible with the trigonal P 3m1 space group (no. 164) used for the structure solution (due to the disorder affecting part of the structure, the possibility of a monoclinic unit cell cannot completely be ruled out). The structure shows metal layers perpendicular to [001], with six independent positions for Y, REE and Cu (sites M1 to M4 are full, and sites M5 and M6 are partially vacant), and two other sites, Cu1 and Cu2, partially occupied by Cu. One characteristic of decrespignyite is the existence of hexanuclear (octahedral) oxo-hydroxo yttrium clusters [Y6(μ6-O)(μ3-OH)8O24] (site M1) with the 24 bridging O atoms belonging to two sets of symmetry-independent.CO3/2- ions, with the first set (2×) along a ternary axis giving rise to a layer of hexanuclear clusters and the second set (6×) tilted and connecting the hexanuclear clusters with hetero-tetranuclear ones hosting Cu, Y and REE (M2 and M3 sites). The rest of the crystal structure consists of two consecutive M3 C M4 layers containing the partially occupied M5, M6, and Cu2 sites and additional carbonate anions in between. The resulting structure model is compatible with the chemical analysis of the type material which is poorer in Cu and richer in (REE, Y) than the above-described material
Crystal structure determination of karibibite, an Fe3+ arsenite, using electron diffraction tomography
No full textThe crystal structure of karibibite, Fe3+3 (As3+O2)4(As3+2 O5)(OH), from the Urucum mine (Minas Gerais, Brazil), was solved and refined from electron diffraction tomography data [R1 = 18.8% for F > 4σ(F)] and further confirmed by synchrotron X-ray diffraction and density functional theory (DFT) calculations. The mineral is orthorhombic, space group Pnma and unit-cell parameters (synchrotron X-ray diffraction) are a = 7.2558(3), b = 27.992(1), c = 6.5243 (3) Å, V = 1325.10(8) Å3 , Z = 4. The crystal structure of karibibbite consists of bands of Fe3+O6 octahedra running along a framed by two chains of AsO3 trigonal pyramids at each side, and along c by As2O5 dimers above and below. Each band is composed of ribbons of three edge- sharing Fe3+O6 octahedra, apex-connected with other ribbons in order to form a kinked band running along a. The atoms As(2) and As(3), each showing trigonal pyramidal coordination by O, share the O(4) atom to form a dimer. In turn, dimers are connected by the O(3) atoms, defining a zig-zag chain of overall (As3+O2) n stoichiometry. Each ribbon of (Fe3+O6) octahedra is flanked on both edges by the (As3+O2) n chains. The simultaneous presence of arsenite chains and dimers is previously unknown in compounds with As3+. The lone-electron pairs (4s2 ) of the As(2) and As(3) atoms project into the interlayer located at y = 0 and y = ½, yielding probable weak interactions with the O atoms of the facing (AsO2) chain. The DFT calculations show that the Fe atoms have maximum spin polarization, consistent with the Fe3+ state.Fil: Colombo, Fernando. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Mugnaioli, Enrico. Universita Degli Studi Di Siena; ItaliaFil: Vallcorba, Oriol. Experiments Division - Alba Synchrotron; EspañaFil: García, Alberto. Consejo Superior de Investigaciones Científicas. Instituto de Ciencia de los Materiales de Barcelona; EspañaFil: Goñi, Alejandro. Consejo Superior de Investigaciones Científicas. Instituto de Ciencia de los Materiales de Barcelona; EspañaFil: Rius, Jordi. Consejo Superior de Investigaciones Científicas. Instituto de Ciencia de los Materiales de Barcelona; Españ
Variations on the Author
No full text“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Larry O. Spencer, Conference Author Presentation
No full textGen. Larry O. Spencer, USAF (Ret.), author of Dark Horse: A Journey from the Horseshoe to the Pentago
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Recognizing and understanding silica-polymorph microcrystals in ceramic glazes
No full textDifferent methods have been historically used to identify silica-polymorph microcrystals in various contexts. However, in the existing literature there is a substantial lack of correlation between local probe analytical data and the corresponding images showing crystal morphologies. Here we demonstrate that such correlation is attainable through an approach based on the characterization of samples prepared as polished thin sections combining optical microscope images with local probe techniques (synchrotron through-the-substrate X-ray microdiffraction and confocal micro-Raman). We report the characterization of different morphologies of silica microcrystals embedded in lead glazes from glazed pottery produced in Jouques (France) in the mid-19th century. Among the eight identified morphologies, are worthy of note cristobalite-coated quartz, paramorphic quartz after cristobalite, cristobalite-tridymite intergrowths or cristobalite twinned crystals. Elusive tridymite has been identified and mapped using μ-Raman and locates predominantly at the borders of some morphologies suggesting a growth front of tridymite that turns to cristobalite
- …
