1,720,979 research outputs found
Monazite structure from dehydrated CaSeO4.2H2O
No full textThe structure of the high-temperature form of CaSeO4, formed by dehydration of the gyspum-type structure dihydrate is presented. The material is equivalent to that described previously as a P21212 1 form, but is however, a monazite with unit cell a = 6.85661(16) Å, b = 7.04962(15) Å, c = 6.68817(15) Å and β = 104.2675(21)° in space group P121/n1. Also presented is evidence for two intermediate trigonal and pseudo-trigonal phases related to the structurally similar minerals rhabdophane, bassanite, and γ-CaSO 4. This result permits closer comparisons between intermediate-temperature structures of the selenate with related sulphates, and orthophosphates with a view to extending structural stability via synthesis of solid-solutions
High-pressure behavior of akermanite and gehlenite and phase stability of the normal structure in melilites
No full textÅkermanite (Ca 2MgSi 207) and gehlenite (Ca 2Al 2Si0 7) have been studied at high pressure by synchrotron radiation powder and single-crystal diffraction up to 30 GPa. At about 2 GPa, the incom-mensurately modulated structure (IC) transforms to a nornal structure (N). The bulk modulus for the N structure, fitted with a Birch Murihagan EoS on powder data, is 93.5(5) GPa. The compressibility is anisotropic, and it is greater along the c axis, in the direction perpendicular to the tetrahedral layers of the structure. Above 15 GPa, a phase transition is observed, marked by a discontinuity in the elastic behavior and a small change in intensity and in the full-width at half maximum (FWHM) of the powder diffraction peaks. The diffraction patterns are indexed with respect to tetragonal cell of the N-melilite structure up to 30 GPa. A hysteresis in the elastic behavior is observed during decompression. In contrast, single-crystal data show a new monoclinic phase appearing above 15 GPa. The unit-cell parameters are a = 8.82(1) Å, b = 7.34(1) Å, c = 9.13(1) Å, β = 115.1(2)°. This unit cell is similar to that of Ca 2ZnGe 1.25Si o.75O 7.reported in the literature. A refinement using the corresponding model in space group P2 1/n fits the single-crystal data with a reasonable R Bragg = 15%, considering that the crystal is twinned and the mosaicity is large. Gehlenite has a higher bulk modulus, 106.1(4) GPa, than does åkermanite. The compressibility is anisotropic, and the behavior is similar to that of åkermanite, but the presence of Al in tetrahedral sites decreases the compressibility parallel to the (001) plane. The structure of gehlenite is stable up to 25 GPa, when a phase transition occurs
Thermal equations of state of dioctahedral micas on the joint muscovite-paragonite
No full textPowder diffraction measurements at simultaneous high pressure and temperature on samples of 2M1 polytype of muscovite (Ms) and paragonite (Pg) were performed at the beamline ID30 of ESRF (Grenoble), using the Paris-Edinburgh cell. The bulk moduli of Ms, calculated from the least-squares fitting of V-P data on each isotherm using a second-order Birch-Murnaghan EoS, were: 57.0(6), 55.1(7) ,51.1(7) and 48.9(5) GPa on the isotherms at 298, 573, and 873 K, respectively. The value of (∂KT/∂T)P was -0.0146(2) GPa K-1. The thermal expansion coefficient α varied from 35.7(3) × 10-6 K-1 at P ambient to 20.1(3) × 10-6 K-1 at P = 4 GPa [(∂α/∂P)T = -3.9(1) × 10-6 GPa-1 K-1]. The corresponding values for Pg on the isotherms at 298. 723 and 823 K were: bulk moduli 59.9(5), 55.7(6) and 53.8(7) GPa, (∂KT/∂T)P -0.0109(1) GPa K-1. The thermal expansion coefficient α varied from 44.1(2) × 10-6 K-1 at P ambient to 32.5(2) × 10-6 K-1 at P = 4 GPa [(∂α/ ∂P)T= -2.9(1) × 10-6 GPa-1 K-1]. Thermoelastic coefficients showed that Pg is stiffer than Ms: Ms softens more rapidly than Pg upon heating; thermal expansion is greater and its variation with pressure is smaller in Pg than in Ms
The crystal structure of barite, BaSO4, at high pressure
No full textThe structure of barite, or baryte, BaSO 4, has been refined by single-crystal methods at room temperature to a maximum of 21.5 GPa in He medium. Lattice parameters have been obtained at select pressures by powder diffraction and complemented by the collection of Raman specta at the highest attained pressure. The results of all analyses correspond to BaSO 4 remaining in the barite-type structure to the highest investigated pressure. Therefore, the search for transitions leading to a post-barite structure, in BaSO 4, must be conducted at pressures higher than those investigated here, or, in "lower pressure" analogs
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
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