1,720,966 research outputs found
Stereoactive lone-pair behaviour of Pb2+ in the crystal structure of bideauxite: Pb2+2Ag+Cl3F(OH)
No full textThe crystal structure of bideauxite, Pb2(2+) Ag+ Cl3 F (OH), cubic, a 14.1273(6) Å, V 2819.2(4) Å3, Fdbar3m, Z = 16, D-calc = 6.26 g.cm-3, has been solved by direct methods and refined to an R index of 2.8% for 324 observed (5 sigma) reflections measured with MoK-alpha X-radiation. There is one unique Ag site surrounded by an octahedral array of CP anions, and one Pb site surrounded by nine anions with a very asymmetrical distribution of bonds and bond-lengths characteristic of stereoactive lone-pair behavior of Pb2+. Four (AgCl6) octahedra link by sharing corners to form an [Ag4Cl18] cluster, and these clusters link by sharing corners to form a three-dimensional chequerboard arrangement, with [Pb4 (OH)(2) F2 Cl18] clusters filling the interstices. The structure of bideauxite is not related to those of the paragenetically related minerals boleite and pseudoboleite
Vlasovite, Na2Zr(Si4O11), from the Kipawa alkaline complex, Quebec, Canada: crystal-structure refinement and polarized infrared spectroscopy
No full textThe crystal structure of vlasovite, Na-2 Zr (Si4O11), from the Kipawa alkaline complex, Quebec, Canada, monoclinic, a 11.0390(5), b 10.0980(3), c 8.5677(4) angstrom, beta 100.313(1)degrees, V 939.6(1) angstrom(3), space group 4,72/c. Z = 4, D (calc.) = 3.008 g.cm(-3), was refined to all R, index of 1.9% based oil 1352 observed [F-o > 4 sigma F] unique reflections measured with MoK alpha X-radiation on a Broker P4 diffractometer equipped with a 4K CCD detector. An electron-microprobe analysis gave the composition Na-1.98 Zr-1.00 (Si4O11) based on 11 O apfit. In the crystal structure, there ire two tetrahedrally coordinated Si sites with a grand distance of 1.612 angstrom, and one octahedrally coordinated Zr site with = 2.085 angstrom. There are two Na sites: the Na(1) site is [7]-coordinated with = 2.620 angstrom, The Na(2) site is split into two subsites, Na(2A) and Na(2B), with a separation of 0.829 angstrom, and the symmetry-related Na(2B) sites are separated by 1.525 angstrom. Na(2A) is octahedrally coordinated, and No(2B) is [5]-coordinated, and their occupancies are 0.826(8) and 0.092(4) Na, respectively. Thus the Na(1) and aggregate Na(2) sites are each fully Occupied. In the crystal structure of vlasovite, four-membered rings of (SiO4) tetrahedra link together to form an [Si4O11](6-) chain. Chains of tetrahedra and Zr octahedra link together to form a heteropolyhedral framework with channels extending along [001]. Three channels contain the Na atoms, which are bonded to O atoms of the octahedron-tetrahedron framework. Vlasovite is not significantly damaged by exposure to X-rays. Infrared spectro;copy shows that vlasovite can be free of H2O or can contain variable amounts of H2O; single crystals can be anhydrous in so ne regions and hydrous in other regions. Moreover, the H2O content and its spectral response are unaffected by exposure of crystals to X-rays
Spectroscopy and X-ray structure refinement of sekaninaite from Dolnı ́ Bory (Czech Republic).
No full text""The crystal chemistry of sekaninaite from Dolní Bory, Czech Republic, was characterized by a multi-method approach. Particular emphasis was put on the characterization of the channel constituents (i.e. H2O and CO2). Electron microprobe analysis shows the sample to be close to the Fe endmember [XFe = Fe\\\/(Fe+Mg) = 94%) with significant Mn (1.48 wt.%) present; laser ablation mass-spectrometry showed the presence of 0.42 wt.% Li2O. H2O and CO2 contents (1.48 and 0.17 wt.%, respectively) were determined via secondary-ion mass-spectrometry. Sample homogeneity was checked by Fourier-transform infrared (FTIR) imaging using a microscope equipped with a focal plane array detector. Single-crystal FTIR spectroscopy confirmed the presence of two types of H2O groups in different orientations (with prevalence of the type II orientation), and that CO2 is oriented preferentially normal to the crystallographic c axis. Using the Beer-Lambert relation, integrated molar coefficients, εi, were calculated for both types of H2O (εi H2O[I] = 6000±2000; εi H2O[II] = 13000±1000) and for CO2 (εi CO2 = 2000±1000)."
Mössbauer research at University Roma Tre
No full textThe principal goal of the Rome group is studying crystalline materials, especially minerals: MS information (Fe oxidation state, Fe3+/Fetot ratios, Fe ions ordination number, Fe atoms aggregation state) is used together with stmchlral and chemical data collected
on the same single-phase material to retrieve the topochemical
model and to explain the physical properties.
Outstanding materials shldied so far are: nahlral and synthetic spinels, natural borosilicates (tourmaline and
axinite), natural amphiboles (especially fibres), natural and synthetic zeolites, but also obsidian, pumice, day and other heterogeneous materials
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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