1,721,029 research outputs found
Structural investigation of TS-1: determination of the true nonrandom titanium framework substitution and silicon vacancy distribution from powder neutron diffraction studies using isotopes
No full textThe first complete investigation of the structure of the selective oxidation catalyst TS-I is presented. Constant wavelength powder neutron diffraction data collected on isotopically substituted titanium silicalite (TS-1) samples, with a Si:Ti molar ratio of 39:1, have been studied using a combination of single and multiple data set Rietveld analysis exploiting the scattering length contrast between the different titanium isotopes and silicon. This has allowed both the silicon vacancy and titanium site substitution distributions to be determined, which has not been possible previously. Both distributions are found to be nonrandom with the titanium preferentially substituting on 3 of the 12 crystallographically independent framework sites-T8, T10 and T3 (in order of decreasing titanium content)-and silicon vacancies on 2 framework sites-T1 and T5. This study illustrates the power of isotopic substitution in powder neutron diffraction experiments to yield enhanced structural information in complex systems
Variable temperature powder neutron diffraction study of SmNiO<sup>3</sup> through its M-I transition using a combination of samarium and nickel isotopic substitution
No full textNeutron powder diffraction studies of 154Sm58NiO3, 154Sm60NiO3, and 154Sm62NiO3, at a range of temperatures through the M-I transition at approximately 128° C, have been performed on the new general materials diffractometer (GEM) at ISIS, RAL. With combined data-set Rietveld analysis, using samples containing different nickel isotopes with contrasting scattering lengths, it has been found that extremely high quality structural parameters can be determined, even though total data collection times are more than an order of magnitude shorter than those previously used for this system. Rietveld analysis shows that the evolution of the structural parameters over the temperature range are smooth and that no symmetry change or abrupt structural transition occurs at the M-I transition. This is consistent with evolution of the high-temperature metallic material within the low-temperature insulating phase over the temperature range 108-128° C. The key effects of thermal motion on the M-I transition have been extracted from the data and are discussed
Multiple-data-set Rietveld analysis using isotopes in powder neutron diffraction. I. Accurate determination of the doping level in the ternary system Ni?Mg1??O, 0.005 <? <0.1
No full textThe use of isotopes to extract precise and very accurate structural information in complex Rietveld analysis has been demonstrated by comparison of results obtained for single-data-set analysis with those from multiple-data-set single-crystallographic-model analysis of NixMg1-xO doped with various nickel isotopes. Fractional occupancies of dopants can be accurately determined down to at least the 0.5% doping level, which cannot be obtained through single-sample-data-set refinements because of correlation effects, even when a large contrast exists between the nickel isotope used and magnesium
In-situ neutron powder diffraction and structure determination in controlled humidities
No full textA controlled-humidity sample environment has been constructed,allowing bulk powder samples undergoing humidity-inducedphase transitions and reactions to be studied via in situ neutrondiffraction. Associated developments in data collection andanalysis permit this to be achieved without the use of D2
Cisplatin: polymorphism and structural insights into an important chemotherapeutic drug
No full textThe changing face of cisplatin: The complete crystal structures, including the details of extensive intermolecular hydrogen bonding in two enantiotropic polymorphs of the chemotherapeutic drug cisplatin, have been elucidated. A massive thermal hysteresis effect exists between the two polymorphs, α and β, that are active at ambient temperatures
Crystallography of hydrogen-containing compounds: realizing the potential of neutron powder diffraction
No full textHydrogen forms more compounds than any other element in the Periodic Table, yet methods for accurately, precisely and rapidly determining its position in a crystal structure are not readily available. The latest generation of high-flux neutron powder diffractometers, operating under optimised collection geometries, allow hydrogen positions to be extracted from the diffraction patterns of polycrystalline hydrogenous compounds without resorting to isotopic substitution. Neutron powder diffraction for hydrogenous materials has a wide range of applications within chemistry. These include the study of hydrogen-energy materials, coordination and organometallic compounds, hydrogen-bonded structures and ferroelectrics, geomaterials, zeolites and small molecule organics, such as simple sugars and amino acids. The technique is particularly well suited to parametric studies, for example as a function of temperature or pressure, where changes in hydrogen bonding patterns or decompositions involving hydrogen-containing molecules, such as water, are monitore
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
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