1,720,962 research outputs found
Static positional disorder in ulvöspinel: A single-crystal neutron diffraction study
No full textA single-crystal neutron diffraction study of a synthetic ulvospinel sample of composition Fe0.403+Fe1.802+Ti0.80O4 was performed to investigate the static positional disorder at the octahedrally coordinated M site. Anisotropic structural refinement was performed in the space group Fd (3) over barm against neutron Laue diffraction data collected at 298 K from two millimetric-sized crystals. Initial structure refinements were conducted with Fe and Ti sharing the M site (at 1/2, 1/2, 1/2), and their partial site occupancy was refined. The tetrahedrally coordinated T site (at 1/8, 1/8, 1/8) was modeled as fully occupied by Fe. For both crystals, the final R-1 index was about 3% for 9 refined parameters and 129 unique reflections, with no significant residuals. As the atomic displacement factors obtained were anomalously high, according to the previous experimental findings, F-obs- and (F-obs-F-cal-difference Fourier maps of the nuclear density were generated. Fourier maps showed a significant minimum located out-of-center of the M site, and indicating a displacement of the Ti4+ from the center of the octahedron. A further test refinement was successfully conducted with two mutually exclusive sites: Ti-M out-of-center (at 0.49, 0.49, 0.49) and Fe-M on the center (at 1/2, 1/2, 1/2). The resulting displacement of Ti from the octahedral center appears to be shorter than 0.15 angstrom. Using bond-valence theory, the out-of-center distortion of Ti-M(4+) is interpreted as a result of intrinsic distortions in the ulvospinel structure. The potential implication of the octahedral distortion on the behavior of ulvospinel at non-ambient conditions is discussed
Blue spinel crystals in the MgAl2O4-CoAl2O4 series: Part I. Flux growth and chemical characterization
No full textNatural blue Co-bearing spinel crystals are rare and actively sought as gemstones, while synthetic blue Co-bearing spinel powders are largely used as ceramic pigments. High-quality spinel single crystals with compositions closely corresponding to the solid-solution series spinel sensu stricto (MgAl 2O 4)-cobalt spinel (CoAl 2O 4) were produced by flux growth method, with Na 2B 4O 7 as flux. Lowcooling rates (2 °C/h) and linear temperature profiles were applied in the thermal interval 1200-800 °C, followed by rapid cooling. Thermal runs were performed in reducing atmosphere (fO 2 10 -8-10 -15 bars) created by a continuous flow of a CO 2:H2 mix with a ratio of 100:4 (cm 3/min). Ten experiments were successfully carried out and hundreds of inclusion-free gem-quality single crystals (up to 1 mm large) were produced in each of them, sometimes together with crusty aggregates and microcrystalline powder. Selected crystals were investigated by SEM/EDS X-ray mapping to check for compositional homogeneity and by electron-microprobe analysis to obtain the chemical formula. Crystals were found to be chemically homogeneous and entirely representing the MgAl 2O 4-CoAl 2O 4 solid-solution series, with the latter component ranging from 7 to 100%. With increasing Co 2+ contents, the crystals vary in color from light blue to intensely dark blue in daylight. The unit-cell parameter a increases from 8.084 to 8.105 Å along the solid-solution series, and the observed increase is determined more by the inversion degree than by the variation in Co contents. The composition of crystal products does not correspond to the composition of the starting oxide mixture, being cobalt enriched in the crystals. A tentative explanation of this behavior is suggested by considering possible ionic potential as well crystal field stabilization effects
Cation ordering over short range and long range scales in the MgAl2O4-CuAl2O4 series
Full text linkA multi-analytical approach using electron microprobe analysis, X‐ray structural refinement, and optical absorption spectroscopy was applied to characterize short-range and long-range structures of synthetic spinel single crystals along the MgAl2O4-CuAl2O4 solid-solution series. Site populations, derived from the results of site-scattering refinement and stereochemical analysis, show that the tetrahedrally
coordinated site (T) is mainly populated by Mg and Cu2+, while the octahedrally coordinated site (M) is dominated by Al. Crystals also show a significant degree of inversion, i.e., occurrence of Al at T counterbalanced by occurrence of divalent cations at M, which increases slightly from 0.24 to 0.29 for the highest Cu2+ contents.
Short-range information derived from optical spectra suggests that the local TCu2+-O distances remain constant at increasing Cu2+ content, whereas local MCu2+-O distances are ca. 0.02 Å shorter in Cu-poor MgAl2O4 spinels as compared to MCu2+-O distances in end-member CuAl2O4. The observed splitting of an absorption band, at ca. 7000 cm–1, caused by electron transitions in TCu2+ as well as the anomalous broadness of an absorption band, at ca. 13 500 cm–1, caused by electron transitions in MCu2+ indicates the occurrence of local Jahn-Teller distortions at T and M. Long-range information, however, shows no violation of Fd3m symmetry. From refinements of our single-crystal XRD data we could for the first time derive for a cubic spinel phase a MCu2+-O distance of 2.080 Å and a TCu2+-O of 1.960 Å. The very limited variations in the unit-cell parameter a from 8.079 to 8.087 Å are mainly related to the disordering of Al. Because of the very similar size of Cu2+ and Mg at the T and M sites, the spinel structure responds to the Cu2+ → Mg substitution by increasing cation disordering in such a manner that mean M-O distances remain constant and the mean T-O distances decrease slightly. This results in increasing length of shared octahedral edges and thereby increase of the octahedral cation-cation repulsion. In line with other studies, the importance of steric factors for controlling the cation distributions in the spinel structure is demonstrated to be valid also in the MgAl2O4-CuAl2O4 solid-solution series
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Full text linkWe 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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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