scholarly journals The Crystal Structure of Defect KBB’O6 Pyrochlores (B,B’: Nb,W,Sb,Te) Revisited from Neutron Diffraction Data

Crystals ◽  
2018 ◽  
Vol 8 (10) ◽  
pp. 368 ◽  
Author(s):  
Sergio Mayer ◽  
Horacio Falcón ◽  
María Fernández-Díaz ◽  
José Alonso
Keyword(s):  
Pyrochlore Structure ◽  
Structural Features ◽  
Neutron Powder Diffraction ◽  
The Other ◽  
Group Symmetry ◽  
Neutron Diffraction Data ◽  
Fourier Synthesis ◽  
Crystallization Water ◽  
Defect Pyrochlore ◽  
Difference Fourier

Three defect pyrochlores KNbWO6·xH2O, KNbTeO6 and KSbWO6 were synthesized by solid state reaction at 750 °C, from stoichiometric mixtures of K2C2O4, Sb2O3, Nb2O5, WO3 and 20% excess TeO2. A neutron powder diffraction (NPD) data analysis allowed unveiling some structural features. They are all defined in the cubic F d 3 ¯ m space group symmetry, with α = 10.5068(1) Å, 10.2466(1) Å and 10.2377(1) Å, respectively. Difference Fourier synthesis for KNbWO6·xH2O clearly showed the presence of crystallization water, with extra O’ oxygen and H+ atoms that were located from NPD data. These O’ oxygen atoms are placed at 32e Wyckoff sites, conforming a K2O’ sublattice interpenetrated with the covalent framework constituted by (Nb,W)O6 octahedra. The H+ ions coordinate the O’ atoms at partially occupied 96g Wyckoff sites while K+ ions shift also along 32e sites, but closer to the 16c special site (0,0,0). By contrast, extra H2O molecules are absent in the other two pyrochlores: in KNbTeO6 and KSbWO6 K+ ions are shifted along 32e (x,x,x) sites further away from the origin than for the previous material, and the higher covalency of the octahedral network determines more compact structures, with shorter B–O distances and narrower B–O–B angles in the proposed AB2O6 defect pyrochlore structure.

Crystal chemistry and location of hydrogen atoms in prehnite

2008 ◽  
Vol 72 (6) ◽  
pp. 1163-1179 ◽  
Author(s):  
T. A. Detrie ◽  
N. L. Ross ◽  
R. J. Angel ◽  
M. D. Welch
Keyword(s):  
Single Crystal ◽  
Powder Diffraction ◽  
Rietveld Analysis ◽  
Neutron Powder Diffraction ◽  
X Ray Diffraction ◽  
Fourier Synthesis ◽  
Hydrogen Atoms ◽  
Space Groups ◽  
Neutron Powder Diffraction Data ◽  
Difference Fourier

AbstractThe structure of prehnite Ca2Al(AlSi3O10)(OH)2, including H positions, has been determined by a combination of single-crystal X-ray diffraction and neutron powder diffraction on four natural samples. The symmetry of the average structure with Al/Si disordered at the T2 siteis Pncm. However, for four of the crystals studied, numerous violations of the n- and c-glide reflection conditions indicate lower symmetry corresponding to space groups P2cm and P2/n and Al-Si ordered structures, possibly as domains of different symmetries and ordering within a single crystal. Time-of-flight neutron powder diffraction was carried out on a sample from Mali at 293 K and 2 K. The structure was refined in space group Pncm by Rietveld analysis. Although it was not possible to locate the missing H using the 293 K neutron data, these data were used to refine the H position located approximately by single-crystal XRD and to refine Uiso. For the 2 K neutron powder diffraction data, H was located directly by difference-Fourier synthesis and its refined position found to be in close agreement with that obtained by the combined XRD/neutron 293 K dataset.

Neutron powder diffraction study of α-Ti(HPO4)2.H2O and α-Hf(HPO4)2.H2O; H-atom positions

1996 ◽  
Vol 52 (5) ◽  
pp. 896-898 ◽  
Author(s):  
M. A. Salvadó ◽  
P. Pertierra ◽  
S. García-Granda ◽  
J. R. García ◽  
J. Rodríguez ◽  
...  
Keyword(s):  
Hydrogen Bonding ◽  
Neutron Diffraction ◽  
Powder Diffraction ◽  
Neutron Powder Diffraction ◽  
Water Molecules ◽  
Neutron Diffraction Data ◽  
Fourier Synthesis ◽  
Intense Source ◽  
Bonding Scheme

The complete crystal structures, including H-atom positions, of α-Ti(HPO4)2.H2O (α-TiP) and α-Hf(HPO4)2.H2O (αHfP) were determined by Rietveld refinement and Fourier synthesis, using constant-wavelength neutron diffraction data. This work is one of few recent examples of the determination of H atoms using neutron powder diffraction with an intense source. The orientation of water molecules in the cavities of α-TiP is similar to that previously found for α-ZrP. For α-HfP, although the hydrogen-bonding scheme is identical, the orientation of water molecules in similar cavities was found to be different.

Quo Vadis Tyrannosaurus?: The Future of Dinosaur Studies

1989 ◽  
Vol 2 ◽  
pp. 175-183 ◽  
Author(s):  
Daniel J. Chure
Keyword(s):  
Public Relations ◽  
Earth Science ◽  
Structural Features ◽  
The Other ◽  
Canada Geese ◽  
New Genera ◽  
Research Activity ◽  
Detailed Understanding ◽  
The Public ◽  
Quo Vadis

“Although I work a lot with fossils in my own research on fishes, I do not care to be called a paleontologist; and I am turned off by many aspects of the public-relations hoopla surrounding paleontology, especially dinosaurs…. One could easily argue that the schools' fascination with dinosaurs might also detract from the other aspects of earth science and biological science and, in the end, weaken paleontology's image as an activity for hard-nosed grown-ups.”K.S. Thomson, 1985: p. 73“Let dinosaurs be dinosaurs. Let the Dinosauria stand proudly alone, a Class by itself. They merit it. And let us squarely face the dinosaurness of birds and the birdness of the Dinosauria. When the Canada geese honk their way northward, we can say: “The dinosaurs are migrating, it must be spring!”R.T. Bakker, 1986: p. 462It is a now oft-repeated statement that we are in the Second Golden Age of dinosaur studies. This may at first seem to be yet another overstatement by dinosaur fanatics; in fact, it is substantiated on a number of fronts. Research activity is certainly at an all-time high, with resident dinosaur researchers on every continent (except Antarctica) and dinosaurs known from every continent (including Antarctica). This activity has resulted in a spate of discoveries, including not only new genera and species, but entirely new types of dinosaurs, such as the segnosaurs. Well-known groups are producing surprises, such as armored sauropods and sauropods bearing tail clubs. Good specimens of previously named genera are revealing unsuspected structural features that almost defy explanation, as in the skull of Oviraptor. However, dinosaur studies extend far beyond the traditional emphasis on dinosaur morphology, and encompass paleobiogeography, paleoecology, taphonomy, physiology, tracks, eggs, histology, and extinction, among others. In some cases, several of these studies can be applied to a single taxon or locality to give us a fairly detailed understanding of the paleobiology of some species.

scholarly journals IgA: Structure, Function, and Developability

Antibodies ◽  
2019 ◽  
Vol 8 (4) ◽  
pp. 57 ◽  
Author(s):  
Patrícia de Sousa-Pereira ◽  
Jenny M. Woof
Keyword(s):  
Monoclonal Antibodies ◽  
Structure Function ◽  
Immunoglobulin A ◽  
Structural Features ◽  
The Other ◽  
Immune Defence ◽  
Major Site ◽  
Mucosal Surfaces ◽  
Serum Iga ◽  
Antibody Class

Immunoglobulin A (IgA) plays a key role in defending mucosal surfaces against attack by infectious microorganisms. Such sites present a major site of susceptibility due to their vast surface area and their constant exposure to ingested and inhaled material. The importance of IgA to effective immune defence is signalled by the fact that more IgA is produced than all the other immunoglobulin classes combined. Indeed, IgA is not just the most prevalent antibody class at mucosal sites, but is also present at significant concentrations in serum. The unique structural features of the IgA heavy chain allow IgA to polymerise, resulting in mainly dimeric forms, along with some higher polymers, in secretions. Both serum IgA, which is principally monomeric, and secretory forms of IgA are capable of neutralising and removing pathogens through a range of mechanisms, including triggering the IgA Fc receptor known as FcαRI or CD89 on phagocytes. The effectiveness of these elimination processes is highlighted by the fact that various pathogens have evolved mechanisms to thwart such IgA-mediated clearance. As the structure–function relationships governing the varied capabilities of this immunoglobulin class come into increasingly clear focus, and means to circumvent any inherent limitations are developed, IgA-based monoclonal antibodies are set to emerge as new and potent options in the therapeutic arena.

The Structural Framework of Pictorial Balance

Perception ◽  
1996 ◽  
Vol 25 (12) ◽  
pp. 1419-1436 ◽  
Author(s):  
Paul Locher ◽  
Sharon Gray ◽  
Calvin Nodine
Keyword(s):  
Visual Arts ◽  
Structural Features ◽  
Original Compositions ◽  
The Other ◽  
Global Integration ◽  
Artistic Style ◽  
Structural Framework ◽  
Museum Professionals ◽  
Balance Structure ◽  
Good Agreement

Two experiments were performed to examine how the subjective balance of a painting is created by its structural features and to determine if balance influences the way people look at paintings. Stimuli consisted of sixteen reproductions of twentieth-century paintings varying in artistic style and a reconstructed less-balanced version of each. Participants in experiment 1 determined the location of the balance center of each composition, assigned ‘weights’ to the pictorial features which contributed to the location of the balance center, and rated the picture for balance. It was found that design and museum professionals and individuals untrained in the visual arts were in good agreement as to the structural framework underlying the balance organization of a painting. For all participants, disruption of the balanced organizations of the original compositions led to reliable shifts in the location of the perceived balance centers of the originals compared with their less-balanced perturbations. Additionally, it was observed that particular features as such were not the origin of the balance phenomenon; rather, judgments concerning the balance structure and its center were dependent on the global integration of information across a wide area of the display field, but especially from its central region. Last, the subtle changes in balance structure between versions resulted in lower ratings of balance being assigned to the less-balanced perturbations by the design professionals only; the other two participant groups evaluated overall balance of the versions as comparable. In experiment 2, eye movements of a different group of untrained individuals were recorded as they performed similar tasks on the art stimuli. It was found that disruption of the balance structure of the original representational but not abstract compositions resulted in different regions of the original and perturbed versions being visually explored. Findings of both experiments are related to theoretical notions of balance.

scholarly journals Setninger I Norsk Og Polsk – Definisjoner Og Inndelinger

2018 ◽  
Vol 25 (1) ◽  
pp. 22-40
Author(s):  
Marta Olga Janik ◽  
Oliwia Szymańska ◽  
Barbara Łukaszewicz
Keyword(s):  
Structural Features ◽  
The Other ◽  
Significant Bias ◽  
Other Hand

Abstract In this article we give a brief summary of how Norwegian and Polish sentences are classified in the widely acknowledged grammar books. Therefore, we review the definitions of sentences in both languages, and compare the various classifications applied in Norwegian and Polish. Additionally, much focus is given to classification of sub clauses, which happen to be differently characterized in the respective languages. We would claim that there is a significant bias regarding features that determine classification of sub clauses in Norwegian and Polish. While in Norwegian a lot of emphasis is put on structural features, focusing on how particular units are organized within a sentence, the Polish classifications seem more semantic-oriented. As far as grammatical terms are concerned, Norwegian is featured by far more notions that might yield intransparency for a Polish learner or grammarian. On the other hand, the Norwegian classifications seem far more transparent. Due to a lack of 1-1 relation between terms used in Norwegian and Polish, we cater for this need by providing terms applicable for both languages. We believe that this may come into useful for all who try to systematize their knowledge about sentences in both languages.

scholarly journals Redetermination of (2,2′-bipyridine-κ2N,N′)dichloridopalladium(II) dichloromethane solvate

2009 ◽  
Vol 65 (6) ◽  
pp. m615-m616 ◽  
Author(s):  
Nam-Ho Kim ◽  
In-Chul Hwang ◽  
Kwang Ha
Keyword(s):  
Hydrogen Bonds ◽  
Structure Refinement ◽  
Chloride Ions ◽  
Fourier Synthesis ◽  
Complex Molecules ◽  
Bipy Ligand ◽  
Π Interactions ◽  
Centroid Distance ◽  
Square Planar ◽  
Difference Fourier

In the title compound, [PdCl2(C10H8N2)]·CH2Cl2, the Pd2+ion is four-coordinated in a slightly distorted square-planar environment by two N atoms of the 2,2′-bipyridine (bipy) ligand and two chloride ions. The compound displays intramolecular C—H...Cl hydrogen bonds and pairs of complex molecules are connected by intermolecular C—H...Cl hydrogen bonds. Intermolecular π–π interactions are present between the pyridine rings of the ligand, the shortest centroid–centroid distance being 4.096 (3) Å. As a result of the electronic nature of the chelate ring, it is possible to create π–π interactions to its symmetry-related counterpart [3.720 (2) Å] and also with a pyridine ring [3.570 (3) Å] of the bipy unit. The present structure is a redetermination of a previous structure [Vicenteet al.(1997). Private communication (refcode PYCXMN02). CCDC, Cambridge, England]. In the new structure refinement all H atoms were located in a difference Fourier synthesis. Their coordinates were refined freely, together with isotropic displacement parameters.

scholarly journals OBSERVATIONS ON THE FINE STRUCTURE OF THE DEVELOPING SPERMATID IN THE DOMESTIC CHICKEN

1962 ◽  
Vol 14 (2) ◽  
pp. 193-205 ◽  
Author(s):  
Toshio Nagano
Keyword(s):  
Plasma Membrane ◽  
Fine Structure ◽  
Cross Section ◽  
Neck Region ◽  
Structural Features ◽  
Dense Granule ◽  
Domestic Chicken ◽  
The Other ◽  
Dense Structure ◽  
Distal Centriole

The kinetic apparatus, the acrosome and associated structures, and the manchette of the spermatid of the domestic chicken have been studied with the electron microscope. The basic structural features of the two centrioles do not change during spermiogenesis, but there is a change in orientation and length. The proximal centriole is situated in a groove at the edge of the nucleus and oriented normal to the long axis of the nucleus and at right angles to the elongate distal centriole. The tail filaments appear to originate from the distal centriole. The plasma membrane is invaginated along the tail filaments. A dense structure which appears at the deep reflection of the plasma membrane is identified as the ring. The fine structure of the ring has no resemblance to that of a centriole and there is no evidence that it is derived from or related to the centrioles. The tail of the spermatid contains nine peripheral pairs and one central pair of tubular filaments. The two members of each pair of peripheral filaments differ in density and in shape: one is dense and circular, and the other is light and semilunar in cross-section. The dense filaments have processes. A manchette consisting of fine tubules appears in the cytoplasm of the older spermatid along the nucleus, neck region, and proximal segment of the tail. The acrosome is spherical in young spermatids and becomes crescentic and, finally, U-shaped as spermiogenesis proceeds. A dense granule is observed in the cytoplasm between acrosome and nucleus. This granule later becomes a dense rod which is interpreted as the perforatorium.

LOW TEMPERATURE NEUTRON DIFFRACTION ON CONGRUENT AND NEAR STOICHIOMETRIC LiNbO3

2012 ◽  
Vol 26 (22) ◽  
pp. 1250142 ◽  
Author(s):  
SHUHUA YAO ◽  
YUANHUA SANG ◽  
DEHONG YU ◽  
MAXIM AVDEEV ◽  
HONG LIU ◽  
...  
Keyword(s):  
Low Temperature ◽  
Low Temperatures ◽  
Structural Features ◽  
Neutron Powder Diffraction ◽  
Structural Behavior ◽  
Li Isotope ◽  
Anisotropic Displacement Parameters ◽  
Li Content ◽  
Large Disorder ◽  
Mg Doped

Neutron powder diffraction has been carried out on a congruent LiNbO 3 sample containing 7 Li isotope ( C 7 LN ) and a near stoichiometric Mg doped LiNbO 3 sample ( Mg : NSLN ) in the temperature range of 4 K and 90 K. Large anisotropic displacement parameters (ADPs) of the Li ions have shown evidence of large disorder along the c-axis for both samples. The results have shown no evidence for the existence of anomalous structural behavior for both samples at low temperatures, although abnormal structural features at 55 K and 100 K for a LiNbO 3 crystal having different Li content as the samples used in the present studies have been observed by Fernandez-Ruiz et al. [Phys. Rev. B72 (2005) 184108].

A functional analysis of flying and walking in pterosaurs

Paleobiology ◽  
1983 ◽  
Vol 9 (3) ◽  
pp. 218-239 ◽  
Author(s):  
Kevin Padian
Keyword(s):  
Functional Analysis ◽  
Shoulder Girdle ◽  
Structural Features ◽  
The Other ◽  
Bipedal Locomotion ◽  
Structural Elements ◽  
Wing Membrane ◽  
Other Hand ◽  
Recovery Stroke

An analysis of the structure and kinematics of the forelimbs and hindlimbs of pterosaurs, and functional analogy with recent and fossil vertebrates, supports a reappraisal of the locomotory abilities of pterosaurs. A hypothesis of structural, aerodynamic, and evolutionary differences distinguishing vertebrate gliders from fliers is proposed; pterosaurs fit all the criteria of fliers but none pertaining to gliders. The kinematics of the reconstructed pterosaur flight stroke reveal a down-and-forward component found also in birds and bats; structural features of the shoulder girdle and sternum unique to pterosaurs may be explained in light of this motion. The recovery stroke of flight was accomplished, in birdlike fashion, by a functional reversal of the action of theM. supracoracoideusby the pronounced enlargement of the acrocoracoid process, which acted as a pulley. The wing membrane was supported and controlled through a system of stiffened, intercalated fibers, which were oriented like the main structural elements in the wings of birds and bats.The hindlimbs of pterosaurs were independent of the wing membrane, and articulated like those of other advanced archosaurs and birds, not like those of bats. The gait was parasagittal and the stance digitigrade. Because of limitations on the motion of the forelimb at the shoulder, pterosaurs could not have walked quadrupedally. However, bipedal locomotion appears to have been normal and quite sufficient in all pterosaurs. There is nothing batlike about pterosaur anatomy; on the other hand, pterosaurs bear close structural resemblances to birds and dinosaurs, to which they are most closely related phylogenetically.

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