1,720,974 research outputs found
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
Strategies for structure solution of small-molecule organics by 3D-ED using a small beam
No full textSnapshots 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
Complete structural studies of long period stacking ordered (LPSO) phases in the Y-Ni-Mg system by 3D electron diffraction
No full textThe crystal structures of three Y-Ni-Mg LPSO phases were directly solved from diffraction data of X-rays
((Mg@Y8Ni6)Mg18, tI66-Nd8+xRu6Mg19-x) and electrons ((Mg@Y8Ni6)16Mg505 and (Mg@Y8Ni6)3Mg154)). The
latter two are modulated structures described in 6D(Fm-3m(α00)000(0α0)000(00α)000, q1 = 0.441(6) a*, q2 =
0.441(6) b*, and q3 = 0.441(6) c*) and 5D (R-3c(αα0)00(-2α,α0)00, q1 = 0.1457(7) a* + 0.1457(7) b*, q2 =
-0.2915(7) a*+ 0.1457(7) b*) superspaces, respectively. The 3D electron diffraction was applied here for the first
time to LPSO compounds, turning out to be the only method successfully overcoming the numerous problems
hampering their complete structure solution.
The structural analysis of these compounds resulted in a generalized description of Y-Ni-Mg LPSO phases in
terms of Mg@Y8Ni6 clusters more or less densely distributed in a Mg matrix, justifying the proposed formulas.
LPSO classification based on the layer stacking modes is proposed, leading to fcc (c-type) and hybrid fcc/hcp
((h)ncc-type) subfamilies, the former being identified in this work and including (Mg@Y8Ni6)Mg18 as well as
(Mg@Y8Ni6)16Mg505.
The inter-clusters coordination in form of distorted anticuboctahedra or cuboctahedra is a further fingerprint
of membership in either family. Increasing the magnesium content, the Mg@Y8Ni6 units tend to form aggregates
at characteristic inter-cluster distances, the distribution of which is at the origin of the structural modulation
Polymorphism in oxyresveratrol studied by 3D ED
Full text linkThe polymorphism of oxyresveratrol, a natural extract widely used in traditional Asian medicine, was investigated by means of the most recent structure characterization techniques. A previously unknown anhydrate oxyresveratrol crystal structure was identified for the first time from a submicrometric polyphasic mixture using 3D electron diffraction (3D ED). Additionally, a new polymorph of the dihydrate form of oxyresveratrol was also discovered and structurally studied. Detailed thermal and calorimetry studies revealed their thermal behavior and dehydration path. DFT calculations were also employed to investigate the stability of the rotational conformers involved in the hydrated phases (in both new and already known phases). This research exemplifies how 3D ED combined with cryo-plunging and routine solid-state analysis can elucidate the polymorphism scenario of a nanocrystalline natural compound
Two New Organic Co-Crystals Based on Acetamidophenol Molecules
Full text linkHerein we present two new organic co-crystals obtained through a simple solution growth process and based on an acetamidophenol molecule, either paracetamol or metacetamol, and on 7,7,8,8-tetracyanoquinodimethane (TCNQ). These co-crystals are part of a family of potential organic charge transfer complexes, where the acetamidophenol molecule behaves as electron donor and TCNQ behaves as electron acceptor. Due to the sub-micron size of the crystalline domains, 3D electron diffraction was employed for the structure characterization of both systems. Paracetamol-TCNQ structure was solved by standard direct methods, while the analysis of metacetamol-TCNQ was complicated by the low resolution of the available diffraction data and by the low symmetry of the system. The structure determination of metacetamol-TCNQ was eventually achieved after merging two data sets and combining direct methods with simulated annealing. Our study reveals that both paracetamol-TCNQ and metacetamol-TCNQ systems crystallize in a 1:1 stoichiometry, assembling in a mixed-stack configuration and adopting a non-centrosymmetric P1 symmetry. It appears that paracetamol and metacetamol do not form a strong structural scaffold based on hydrogen bonding, as previously observed for orthocetamol-TCNQ and orthocetamol-TCNB (1,2,4,5-tetracyanobenzene) co-crystals
Structural characterization of intermediate and metastable phases by electron microscopy
Full text linkStructure characterization of nanocrystalline intermediates and metastable phases is of primary importance for a deep understanding of synthetic processes undergoing solid-to-solid state phase transitions. Understanding the evolution from the first nucleation stage to the final synthetic product supports not only the optimization of existing processes, but might assist in tailoring new synthetic paths. A systematic investigation of intermediates and metastable phases is hampered because it is impossible to produce large crystals and only in few cases a pure synthetic product can be obtained. Structure investigation by X-ray powder diffraction methods is still challenging on nanoscale, especially when the sample is polyphasic. Electron diffraction has the advantage to collect data from single nanoscopic crystals, but is limited by data incompleteness, dynamical effects and fast deterioration of the sample under the electron beam. Automated diffraction tomography (ADT), a recently developed technique, making possible to collect more complete three-dimensional electron diffraction data and to reduce at the same time dynamical scattering and beam damage, thus allowing to investigate even beam sensitive materials (f.e. hydrated phases and organics). At present, ADT is the only technique able to deliver complete three-dimensional structural information from single nanoscopic grains, independently from other surrounding phases. Thus, ADT is an ideal technique for the study of on-going processes where different phases exist at the same time and undergo several structural transitions. In this study ADT was used as the main technique for structural characterization for three different systems and combined subsequently with other techniques, among which high-resolution transmission electron microscopy (HRTEM), cryo-TEM imaging, X-ray powder diffraction (XRPD) and energy disperse X-ray spectroscopy (EDX).rnAs possible laser host materials, i.e. materials with a broad band emission in the near-infrared region, two unknown phases were investigated in the ternary oxide system M2O-Al2O3-WO3 (M = K, Na). Both phases exhibit low purity as well as non-homogeneous size distribution and particle morphology. The structures solved by ADT are also affected by pseudo-symmetry. rnSodium titanate nanotubes and nanowires are both intermediate products in the synthesis of TiO2 nanorods which are used as additives to colloidal TiO2 film for improving efficiency of dye-sensitized solar cells (DSSC). The structural transition from nantubes to nanowires was investigated in a step by step time-resolved study. Nanowires were discovered to consist of a hitherto unknown phase of sodium titanate. This new phase, typically affected by pervasive defects like mutual layer shift, was structurally determined ab-initio on the basis of ADT data. rnThe third system is related with calcium carbonate nucleation and early crystallization. The first part of this study is dedicated to the extensive investigations of calcium carbonate formation in a step by step analysis, up to the appearance of crystalline individua. The second part is dedicated to the structure determination by ADT of the first-to-form anhydrated phase of CaCO3: vaterite. An exhaustive structure analysis of vaterite had previously been hampered by diffuse scattering, extra periodicities and fast deterioration of the material under electron irradiation. rnZum besseren Verständnis von Syntheseprozessen, die über einen fest-fest-Phasenübergang ablaufen, müssen die beteiligten nanokristallinen Zwischenstufen und metastabilen Phasen strukturell charakterisiert werden. Ein tieferer Einblick in diesen Ablauf, von der ersten Nukleationsphase bis hin zum Endprodukt, unterstützt nicht nur die Optimierung bestehender Prozesse, sondern kann auch bei der Entwicklung neuer Synthesewege helfen. Eine systematische Untersuchung der Zwischenprodukte und metastabilen Phasen durch Rötgenbeugungsmethoden ist stark eingeschränkt, da oft keine Einkristalle vorliegen und nur in wenigen Fällen reine Syntheseprodukte erhalten werden können. Eine Strukturaufklärung mit Hilfe der Röntenpulverdiffraktion ist im nanokristallinen Bereich immer noch sehr schwierig, besonders für mehrphasige Materialien. Vorteil der Elektronenbeugung ist es, dass Daten eines einzelnen Nanokristall gesammelt werden können, sie wird aber limitiert durch die geringe erreichbare Datenmenge, dynamische Beugungseffekte und Elektronenstrahlschäden in der Probe. Die automatische Beugungstomographie (ADT), eine kürzlich entwickelte Methode, erlaubt es, wesentlich vollständigere dreidimensionale Daten mit stark reduzierten dynamischen Effekten zu sammeln. Strahlenschäden sind hier so stark verringert, dass sogar strahlempfindliche Materialien, wie Hydratphasen oder organische Materialien, vermessen werden können. Momentan ist ADT die einzige Methode, die vollständige dreidimensionale Strukturinformationen von nanoskopischen Körnern liefert. Deswegen ist ADT die ideale Technik, um Prozessabläufe zu untersuchen, in denen mehrere Phasen gleichzeitig existieren und sich ineinander umwandeln. In dieser Arbeit wurde ADT als Hauptmethode zur strukturellen Charakterisierung dreier verschiedener Systeme verwendet und die Ergebnisse mit anderen Techniken untermauert (Hochauflösende Transmissionselektronenmikroskopie (HRTEM), Cryo-TEM Abbildung, Röntgenpulverdiffraktometrie (XRPD) und energiedispersive Röntgen-spektroskopie (EDX)).rnAls mögliche Laser-Wirtsmaterialien wurden zwei unbekannte Phasen des ternären oxidischen Systems M2O-Al2O3-WO3 (M = K, Na) untersucht. Beide Phasen zeigen eine schlechte Reinheit und haben keine einheitliche Größenverteilung und Partikelmorphologie. Die mit ADT gelösten Strukturen wiesen beide eine Pseudosymmetrie auf. rnNatriumtitanat Nanoröhren und Nanodrähte sind Zwischenprodukte in der Synthese von TiO2 Nanostäbchen. Diese sind Additive in einem kolloidalen TiO2 Film, der als Elektrodenmaterial für Farbstoff Solarzellen (DSSC) genutzt wird. Der strukturelle Übergang von Nanoröhren zu Nanodrähten wurde zeitaufgelöst untersucht. Es wurde festgestellt, dass Nanodrähte aus einer bisher unbekannten Natriumtitant-Phase bestehen. Diese neue Phase, die ab-initio auf der Basis von ADT Daten gelöst wurde, weist typische Defekte wie Schichtversetzungen auf. rnDas dritte System liegt im Bereich der Calciumcarbonat Keimbildung und frühen Kristallisation. Im ersten Teil der Untersuchung wurde eine schrittweise Analyse dieses Vorgangs, bis hin zur Bildung einzelner Kristallite durchgeführt. Im zweiten Teil wurde eine ADT Strukturaufklärung der ersten Anhydratphase von CaCO3, dem Vaterite, durchgeführt. Eine ausführliche Strukturanlayse von Vaterit war bislang durch das Auftreten starker diffuser Streuung, zusätzlicher Periodizitäten und der schnellen Zerstörung des Materials im Elektronenstrahl nicht möglich.r
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
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