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The peculiar crystal-chemistry of phlogopite from metasomatized peridotites: evidence from laboratory and nature
No full textThe 6H-SiC structure model: Further refinement from SCXRD data from a terrestrial moissanite
No full textThe crystal structure of a terrestrial 6H-SiC moissanite has been refined in the P63mc S.G. from area detector single crystal X-ray data, down to an R-index on the observed reflections of 0.0205. The cell parameters refined over all the collected reflections are a = 3.0810(2) and c = 15.1248(10) Å. The average Si-C bond lengths are 1.8898 Å, with average bonds along the stacking direction (1.8993 Å) slightly longer than those along the bilayer (1.8862 Å). The interlayer distances, defined as the distances along [0001] between Si-Si layers, which may occur either in cubic (c) or hexagonal (h) configurations, are maximal at the c-h interface (2.5270 Å) and minimal at the h-c interface (2.5165 Å), entailing that the h-bilayer is not equidistant from either c-bilayers. All the tetrahedral angles are identical within the experimental error and close to the ideal value of 109.47°, but those at the c-h interface, where a significant distortion of 0.15° is recorded. Finally, the anisotropic displacement factors are utterly very small, identical among different atoms within the experimental error, and significantly spherical. It thus appears that the 6H-SiC structure is affected by a slight relaxation along the [0001] stacking direction with respect to the ideal cubic structure, and that the relaxation is mainly accomplished at the c-h interface, i.e., at the twin-like boundary, where a bilayer in cubic configuration links a bilayer in antiparallel, hexagonal configuration. As far as we know this is the first crystal structure refinement of a natural 6H-SiC moissanite. Possible implications on the polytype stability in the light of these results are briefly discussed
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Variations on the Author
Get PDF“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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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
XRDT and TEM study of defects and polytypism in natural moissanite and synthetic SiC crystals
No full textStructural defects and degree of order of natural and synthetic moissanite have been investigated by XRD Topography and TEM. XRDT analyses of synthetic 6H-SiC wafers allowed to study extended defects and to identify and localize coalescence of polytypes. The observed linear defects are microchannels and dislocations. Axial screw dislocations, either parallel or slightly inclined to the c-axis, suggest that the growth mechanism for the bulk crystals was mainly by spiral growth. Moreover, the study of line orientations of the dislocations allow to reconstruct the growth sector evolution of the sample. Therefore, the convex shape of the growing surface is attained by the development of growth sectors (10l) neighbouring growth sector (001). The coalescence of a thin lamella of a different polytype has been also localized and investigated. The contrast analysis and the diffraction pattern of the lamella are consistent with a 15R-SiC crystal. Such coalescence indicates local variations of growth conditions. TEM images and selected area electron diffractions (SAEDs) strongly differentiate natural from synthetic samples. SAED patterns with [010] incidence of natural crystals are consistent with the 6H polytype and do not show streaks along the [001] stacking direction. Synthetic samples are comparatively much more disordered. Conventional images show high density of (001) faults, not observed in natural samples. Consistently, SAED patterns of the [100] zone are streaked along c*. High resolution (HR) imaging shows that synthetic samples mainly consist of (001) stacking sequence described as (32)3. Locally mixed stacking sequence described by notation 23(3233)5, probably referred to a long period polytype, are present
The peculiar crystal-chemistry of phlogopite from metasomatized peridotites: evidence from laboratory and nature
No full textExperimental petrology suggested that phlogopite has a peculiar mineral chemistry at high pressure: excess of Si coupled with a decrease in IVAl and deficiency in K+Na. This K-edenite exchange, □XIISi(K+Na)-1Al-1, where □XII is a vacancy in the interlayer, should imply that phlogopites incorporate significant amount of talc component. At high pressure conditions, however, in a fluid saturated system, a talc component might translate into a 10 Å phase component, being the latter phase the product of the reaction talc + H2O = 10 Å phase occurring at P = 4-5 GPa and at T = 600-700 °C. We aim to study the structure of natural and synthetic phlogopites showing this peculiar mineral chemistry. Samples were analysed by single-crystal X-ray diffraction, powder X-ray diffraction with full profile fitting, EMP, SIMS, TEM and Mössbauer spectroscopy. The natural samples were recovered from spinel and garnet wedge peridotites of the Ulten Zone Eastern Italian Alps, (Italy), whereas synthetic ones were obtained from high pressure synthesis in a K-doped lherzolite system. The single crystal X-ray refinements of natural phlogopites, together with the EMP analysis, showed two coupled substitutions: Tschermak (Si + Mg = IVAl + VIAl) and talc exchanges (□ + Si = K + Al), with no octahedral vacancies. The Rietveld refinement of the synthetic samples confirmed the ipersilicic character and K deficiency found with EMPA and SIMS analysis. The c lattice parameter of both synthetic and natural phlogopites positively correlates with the increase of vacancies at the interlayer site, pointing toward the c lattice parameter of the 10Å phase. The tetrahedral α rotation angle of natural phlogopites was about 9°, lower than that expected by Tschermak substitution. The present data indicate that the 10 Å phase component, instead of the talc component, stabilizes the phlogopite structure by reducing the α rotation under high pressure condition. This increases the pressure conditions needed to reach the upper limit value for the tetrahedral rotation
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