Constituent Atoms (constituent + atom)

Distribution by Scientific Domains


Selected Abstracts


Low-energy irradiation effects of gas cluster ion beams

ELECTRONICS & COMMUNICATIONS IN JAPAN, Issue 2 2008
Shingo Houzumi
Abstract A cluster-ion irradiation system with cluster-size selection has been developed to study the effects of the cluster size for surface processes using cluster ions. A permanent magnet with a magnetic field of 1.2 T is installed for size separation of large cluster ions. Trace formations at HOPG surface by the irradiation with size-selected Ar-cluster ions under an acceleration energy of 30 keV were investigated by scanning tunneling microscopy. Generation behavior of the craterlike traces is strongly affected by the number of constituent atoms (cluster size) of the irradiating cluster ion. When the incident cluster ion is composed of 100 to 3000 atoms, craterlike traces are observed on the irradiated surfaces. In contrast, such traces are not observed at all with the irradiation of the cluster ions composed of over 5000 atoms. Such behavior is discussed on the basis of the kinetic energy per constituent atom of the cluster ion. To study GCIB irradiation effects on macromolecules, GCIB was irradiated on DNA molecules absorbed on graphite surface. Using GCIB irradiation, many more DNA molecules were sputtered away compared with the monomer-ion irradiation. © 2008 Wiley Periodicals, Inc. Electron Comm Jpn, 91(2): 40,45, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10031 [source]


Ion beam synthesis of buried Zn-VI quantum dots in SiO2, grazing incidence small-angle X-ray scattering studies

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3-1 2003
I.D. Desnica-Frankovic
Grazing incidence small-angle X-ray scattering was used to study ion-beam synthesized Zn-VI compound-semiconductor quantum dots (QDs), buried in a SiO2 matrix. The ZnTe and ZnS QDs were formed by successive ion implantation of constituent atoms, at high ion doses and subsequent annealing at different temperatures in the 1070,1370 K range. In Zn and Te implanted SiO2, small nano-crystals were formed at higher annealing-temperatures, a bimodal size distribution of nano-particles was observed for both materials, which could be explained by an interplay of Ostwald ripening and enhanced diffusion in the irradiation-damaged region. [source]


Grazing incidence small-angle X-ray scattering studies of the synthesis and growth of CdS quantum dots from constituent atoms in SiO2 matrix

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3-1 2003
U.V. Desnica
Grazing incidence small angle X-ray scattering was applied to study the synthesis and growth of CdS quantum dots (QDs) from Cd and S atoms implanted in SiO2. For a dose of 1017/cm2, the partial synthesis of CdS QDs occurred already during implantation, with only moderate size increase upon subsequent annealing up to Ta=1073 K. The dynamics of QD synthesis and growth were considerably different for just two times lower dose, where synthesis started only if the implanted samples were annealed at Ta = 773 K or higher, with a strong increase of the size of QDs upon annealing at higher Ta. The results suggest that high-dose implantation followed by low-temperature annealing could lead to better defined sizes and narrower size distributions of QDs. [source]


Pseudoatoms and preferred skeletons in crystals

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2007
Angel Vegas
The generalization of the Zintl,Klemm concept provides a universal formulation of a crystal structure in terms of universal building skeletons formed by Klemm's pseudoatoms: atoms that behave structurally according to their formal total electron charge. An important difference in this novel view is that charge is considered to be transferred, in the strict Zintl's sense, from the donor cations to the building skeleton as a whole, not specifically to a given atom or ion. Although application is restricted to (IV),(IV) compounds (group 14 structures), the principle seems to be universal and can be applied to understand, to relate and to predict the structure of complex compounds on the basis of more simple structures, e.g. a given AB skeleton provides the building block for A2B, AB2, ABXmetc. compounds of a very different nature. The application of such a principle only requires information on the constituent atoms and on the existing phases of the p -block elements (observed under ambient and high-pressure and/or high-temperature conditions). The ideas introduced here demonstrate, for the first time, that a generalization of the Zintl,Klemm concept is possible and that such a generalization helps to establish a univocal link between chemical composition (in terms of pseudoatoms) and the crystalline structures observed experimentally. [source]


Observation of 4f electron transfer from Ce to B6 in the Kondo crystal CeB6 and its mechanism by multi-temperature X-ray diffraction

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 3-2 2002
Kiyoaki Tanaka
Electron density distributions (EDD) in CeB6 were measured by X-ray diffraction at 100, 165, 230 and 298,K. Analysis with a weak-field model, in which the spin-orbit interaction dominates the energy splitting of the 4f levels, revealed that more 4f electrons were donated from Ce to B6 at the lower temperature. Donated electrons localize around the B,B bonds connecting B6 octahedra. The localized electrons and an expansion of the outermost 5p orbitals change the effective atomic potentials and enhance the anharmonic vibration (AHV) of constituent atoms at lower temperature. Enhanced AHV increases the entropy and makes the electron donation inevitable. Changes in crystal structure, EDD, electron configuration and AHV are found to be closely correlated with one another and the mechanism of the electron transfer in the Kondo crystal CeB6 in the studied temperature range was elucidated. This is, to the authors' knowledge, the first multi-temperature measurement of EDD that elucidates a mechanism of change from the temperature dependence of the EDD. Parameters change consistently at all the temperatures except 298,K, at which the excited states ,7 of the Ce 4f states have significant electron population. The thermal excitation to ,7 levels expands the B6 octahedra, since ,7 has main lobes along ,111, or from Ce to the centre of B6 octahedra. The energy gap between the ground state ,8 and ,7 was calculated to be 470,K from the ratio of electron populations of both states. The present experiment opens the door to accurate X-ray EDD analyses of rare earth complexes. [source]


Calcium chloride rhenate(VII) dihydrate

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 9 2007
Urszula Jarek
The crystal structure of calcium chloride rhenate(VII) dihydrate, CaCl(ReO4)·2H2O, investigated at 85,K, consists of calcium cations, chloride anions, rhenate(VII) anions and water molecules. In the nearly tetrahedral rhenate(VII) anion, all constituent atoms lie on special positions of m2m (Re) and m (O) site symmetries. The Cl, anion and water O atom lie on special positions of m2m and 2 site symmetries, respectively. The Ca2+ ion, also on a special position (m2m), is eight-coordinated in a distorted square-antiprismatic coordination mode. The crystal has a layered structure stabilized by Ca,O coordination bonds and O,H...Cl hydrogen bonds. [source]


Van der Waals and Polar Intermolecular Contact Distances: Quantifying Supramolecular Synthons

CHEMISTRY - AN ASIAN JOURNAL, Issue 5 2008
Parthasarathy Ganguly Prof.
Abstract Crystal structures are viewed as being determined by ranges and constraints on interatomic contact distances between neighboring molecules. These distances are considered to arise from environment-dependent atomic sizes, that is, larger sizes for isotropic, van der Waals type contacts and smaller sizes for more-polar, possibly ionic contacts. Although the idea of different, or anisotropic, radii for atoms is not new, we developed a method of obtaining atomic sizes that is based on a theoretical framework. Using different atomic sizes for the same atom in different environments, we were able to rationalize some structural observations and anomalies. For example, benzene with the Pbca structure may be described in terms of two types of C,,,H interactions: a longer contact largely of the van der Waals type, and a shorter, structure-determining type (C,,,,,H,+), which we term "n-polar". Our approach is illustrated with three examples: 1),the equivalence in crystal packing of fluorobenzene, benzonitrile, pyridine N -oxide, and pyridine/HF 1:1 molecular complex, all of which take the not-so-common tetragonal P41212 space group and are practically isomorphous; 2),the similarity of the Pa3 acetylene and Pbca benzene crystal structures; and 3),the equivalence between an increase in pressure and an increase in the "n-polar" contacts in Pbca benzene; in other words, the equivalence between hydrostatic pressure and chemical pressure. In the context of crystal engineering, we describe a method whereby the topological information conveyed in a supramolecular synthon is recast in a more quantitative manner. A particular synthon, and in turn the crystal structure to which it leads, is viable within small ranges of distances of its constituent atoms, and these distances are determined by chemical factors. [source]