1,721,085 research outputs found
Structure characterization of nanocrystalline porous materials by tomographic electron diffraction
No full textSeveral porous phases have been characterized in the last years by tomographic electron diffraction. This paper provides an overview of the analyzed phases, pointing out the difficulties associated with different classes of materials or specific sample characteristics. The two methods for tomographic electron diffraction acquisition, automated diffraction tomography and rotation electron diffraction, are described in detail, as well as data reduction algorithms. Attainments and limits of different structure determination and refinement algorithms are discussed for inorganic, organic and hybrid organic-inorganic materials. Finally, it is shown how tomographic electron diffraction and X-ray powder diffraction data can be combined for a comprehensive characterization of porous materials
Atomic structure solution of the complex quasicrystal approximant Al Rh Ru from electron diffraction data
No full textThe crystal structure of the novel Al77Rh15Ru8 phase (which is an approximant of decagonal quasicrystals) was determined using modern direct methods (MDM) applied to automated electron diffraction tomography (ADT) data. The Al77Rh15Ru8 E-phase is orthorhombic [Pbma, a = 23.40 (5), b = 16.20 (4) and c = 20.00 (5) Å] and has one of the most complicated intermetallic structures solved solely by electron diffraction methods. Its structural model consists of 78 unique atomic positions in the unit cell (19 Rh/Ru and 59 Al). Precession electron diffraction (PED) patterns and high-resolution electron microscopy (HRTEM) images were used for the validation of the proposed atomic model. The structure of the E-phase is described using hierarchical packing of polyhedra and a single type of tiling in the form of a parallelogram. Based on this description, the structure of the E-phase is compared with that of the [epsilon]6-phase formed in Al-Rh-Ru at close composition
Structural insights into M2O–Al2O3–WO3 (M = Na, K) system by electron diffraction tomography
No full textThe M 2O-Al2O3-WO3 (M = alkaline metals) system has attracted the attention of the scientific community because some of its members showed potential applications as single crystalline media for tunable solid-state lasers. These materials behave as promising laser host materials due to their high and continuous transparency in the wide range of the near-IR region. A systematic investigation of these phases is nonetheless hampered because it is impossible to produce large crystals and only in a few cases a pure synthetic product can be achieved. Despite substantial advances in X-ray powder diffraction methods, structure investigation on nanoscale is still challenging, especially when the sample is polycrystalline and the structures are affected by pseudo-symmetry. Electron diffraction has the advantage of collecting data from single nanoscopic crystals, but it is frequently limited by incompleteness and dynamical effects. Automated diffraction tomography (ADT) recently emerged as an alternative approach able to collect more complete three-dimensional electron diffraction data and at the same time to significantly reduce dynamical scattering. ADT data have been shown to be suitable for ab initio structure solution of phases with large cell parameters, and for detecting pseudo-symmetry that was undetected in X-ray powder data. In this work we present the structure investigation of two hitherto undetermined compounds, K5Al(W3O11)2 and NaAl(WO4)2, by a combination of electron diffraction tomography and precession electron diffraction. We also stress how electron diffraction tomography can be used to obtain direct information about symmetry and pseudo-symmetry for nanocrystalline phases, even when available only in polyphasic mixtures. © 2015 International Union of Crystallography
Snapshots of the Formation of NaTi3O6(OH)·2H2O Nanowires: A Time-Resolved XRD/HRTEM Study
No full textLayered titanates are important intermediates during the formation of TiO2-related nanostructures in hot concentrated base solution. Microwave-assisted hydrothermal techniques allow a time-resolved ex-situ analysis of the reaction in one-minute intervals by rapid heating and quenching followed by separation and structure analysis of the intermediates. By a combination of powder X-ray diffraction, high resolution electron microscopy (HRTEM), and selected area electron diffraction (SAED) the individual stages of the reaction could be identified. Sodium titanate nanosheets are formed within several minutes by digesting the crystalline TiO2-P25 precursor in NaOH. These nanosheets with a low sodium content form flake-like aggregates. Subsequently, these nanosheet intermediates form nanoscrolls and/or nanotubes, which transform in the next step to NaTi3O6(OH) ·2H2O nanowires by reaction with NaOH via an oriented attachment of nanotubes from solution. The NaTi3O6(OH) ·2H2O nanowires grow at the expense of the nanotubes and form an ordered structure via dissolution-recrystallization equilibria between solid NaTi3O6(OH)·2H2O and soluble titanate species. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Synthesis of a quenchable high-pressure form of magnetite (h-Fe3O4) with compositionFel(Fe2+0.75Mg0.26)Fe2(Fe3+0.70Cr0.15Al0.11Si0.04)2O4
No full textWe report the synthesis of h-magnetite, ideally h-Fe3O4with considerable amounts of substitutional cations (Cr, Mg, Al, Si) and quenchable to ambient conditions. Two types of experiments were performed at 18 GPa and 1800 °C in a multi-anvil press. In one, we used an oxide mixture with a majoritic stoichiometry Mg1.8Fe1.2(Al1.4Cr0.2Si0.2Mg0.2)Si3O12, with Si and Mg in excess as starting material (MA-367, MA-380). In the second type of experiment (MA-376), we started from an oxide mixture on the composition of the Fe-oxide phase obtained in MA-367. The Fe-oxide phases of both experiments were investigated by electron microprobe and transmission electron microscopy including electron diffraction tomography. Our investigations show that the Fe-oxide phases crystallize in the structure-type of h-magnetite. However, electron diffraction data show that keeping the cell setting from literature, this phase crystallizes in space group Amam and not in space group Bbmm as previously proposed. In the experiment MA-367, the Fe-oxide phase are mutually intergrown with majorite, the major phase of the run products. The formula for h-magnetite in this run was calculated as Fe1(Fe2+ 0.75Mg0.26)Fe2(Fe3+ 0.70 Cr0.15Al0.11Si0.04)2O4. In the experiment on the bulk composition of the Fe-oxide, the main phase was h-magnetite with composition Fe1(Fe2+ 1.02)Fe2(Fe3+ 0.65Cr0.19Al0.13Si0.03)2O4and traces of nearly pure endmember wadsleyite and stishovite. Our results indicate that the substitution of 20 to 30% of Fe (0.7 to 0.9 atoms per formula unit) by smaller cations favored the preservation of the high-pressure form to ambient conditions. We prove that the h-magnetite-type oxide is also stable in chemical systems more complex than Fe-O. Based on our results, which were obtained at 18 GPa and 1800 °C in a system (MA-367) that is closely related to Fe-enriched oceanic lithospheric material, we suggest that a Fe3O4-rich phase may be present in environments connected to deeply subducted slabs and possibly associated with deep carbonatitic melting. Our observations show that Cr strongly partitions in the oxide phase such that the coexisting silicates are depleted in Cr compared to Fe3O4-free assemblages. This may significantly affect the chemical signature of melts produced in the deep mantle
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
Facile Synthesis and Characterization of Functionalized, Monocrystalline Rutile TiO 2 Nanorods
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