Search results for "defect"
showing 10 items of 879 documents
First-principles and semiempirical calculations forFcenters inKNbO3
1997
The linear muffin-tin-orbital method combined with density functional theory (local approximation) and the semiempirical method of the intermediate neglect of the differential overlap (INDO) based on the Hartree-Fock formalism are used for the study of the $F$ centers (O vacancy with two electrons) in cubic and orthorhombic ferroelectric KNbO$_3$ crystals. Calculations for 39-atom supercells show that the two electrons are considerably delocalized even in the ground state of the defect. Their wave functions extend over the two Nb atoms closest to the O vacancy and over other nearby atoms. Thus, the $F$ center in KNbO$_3$ resembles electron defects in the partially-covalent SiO$_2$ crystal (…
Annealing of Radiation Defects in X-Irradiated LiBaF3
2002
AbstractResults of application of the glow rate technique GRT for analysis of the activation energy of thermostimulated annealing of X-ray created F-type color centers in LiBaF3 crystals, pure and containing hetero-valence oxygen centers are presented. It is shown that depending on impurity composition in crystal two alternative mechanisms are involved in annealing of color centers. It is proposed that either the anion vacancy governed migration of F- centers resulting in recombination with complementary defects, or the thermal delocalization of radiation created fluorine (Fi) interstitials captured by anti-structure defects followed by recombination with all kinds of complementary F- type …
<title>High-excitation-density luminescence as probe of mixed silver halides</title>
1992
Two kinds of intrinsic luminescence in mixed AgBr1-xClx (1 io. Dependence of the parameters of this process on crystal composition has been investigated in terms of spatially well correlated Frenkel defect recombinations. The main effect found was the linear increase of the Agio migration energy with the crystal composition x. The second kind of luminescence arising due to exciton molecules has been shown to be sensitive to the solid solution composition and the quality of a crystal. It is shown that this luminescence correlates with the optical losses of the fibers studied.
Average Structure vs. Real Structure: Molecular Dynamics Studies of Silica
2003
The microscopic structure of a crystal and thermal fluctuations of the atoms constituting the crystal are intimately connected with the macroscopic elastic properties including mechanical stability. In some cases, however, the picture is more complex than that which is drawn in text books on solid state physics. (i) The instantaneous microscopic structure can deviate in a non-Gaussian way from the average structure even when domain disorder and/or crystal defects are absent. Quasi harmonic approximations may then turn out to be meaningless. (ii) The crystal is subject to external pressures that are sufficiently large in order to render the definition of elastic constants non unique. These t…
High-Fluence Implantation of Erbium into Silicon-Germanium Alloys: Structural and Thermal Properties
2000
AbstractHigh-quality crystalline Si1-xGex (x=0.10 and 0.25) alloys were implanted with 70 keV Er+ ions at temperatures of 350°C and 550°C to a fluence of 1015 cm−2. In-situ Rutherford backscattering/channeling (RBS) analysis supplemented with transmission electron microscopy (TEM) showed that as-implanted alloys were in form of ternary solid solutions with a peak Er concentration of 1 at.% without any trace of Er-Si or Er-Ge precipitation.In the samples implanted at 350°C Er atoms were found to be distributed randomly in the amorphous host matrix. Post-implantation annealing at different temperatures up to 600° showed that the solid phase epitaxial regrowth of the damaged layers strongly de…
Coin metal adsorption on defective MgO(001) surface: ab initio study
2005
First principles slab calculations have been performed for Ag and Cu adsorption on periodically distributed point defects (a single O2– or Mg2+ vacancy per 2×2 surface supercell) on the non-polar MgO(001) substrate. Using the procedure implemented in the CRYSTAL-03 code, both Fs and Vs centers were modeled by retaining in the vacancy the basis set of the missing O2– and Mg2+ ions, respectively, with the local relaxation of the nearest substrate ions. Adsorption of metal atom over the Fs center (2.4 eV vs. 2.1 eV per Cu and Ag adatoms, respectively) is much stronger as compared with regular O2− sites (0.6 eV vs. 0.4 eV, respectively). The Fs center donates a substantial charge towards Ag and…
Light Sensitive Lattice Defects In BaTiO 3 Containing Fe
1989
Electron spin resonance spectra, which can consistently be explained by the models Fe4+ - Vo and Fe5+ - vBa, have been observed with BaTiO3 containing Fe. Also Fe3+ and - Fe3+ - Vo have been identified. All these centers as well as several unidentified ones are observed to change their charge states under illumination. They thus are possibly in involved in photorefractive processes in BaTiO3 containing Fe.
Computer Simulations of I-Center Annealing in KCl and KBr Crystals. Theoretical Interpretation of Thermostimulated Experiments
1995
Results of computer simulations of the kinetics of correlated annealing of pairs of close α–I and F–I centers in KCI and KBr crystals, enhanced by I-center diffusion and Coulomb or elastic attractions, respectively, are presented. Special attention is paid to the conditions under which multi-stage annealing stages arise as it has been observed experimentally more than once. Our general conclusions are: (i) a weak elastic interaction affects the recombination kinetics and the survival probability even for relatively well-separated F–I pairs, the more so is true for the case of Coulomb attraction between charged α–I pairs; (ii) the multi-step (kink) structure arises only for close (typically,…
Dense Bodies of Human Cytomegalovirus Induce both Humoral and Cellular Immune Responses in the Absence of Viral Gene Expression
2000
ABSTRACTInfection of fibroblast cell cultures with human cytomegalovirus (HCMV) leads to the production of significant amounts of defective enveloped particles, termed dense bodies (DB). These noninfectious structures contain major antigenic determinants which are responsible for induction of both the humoral and the cellular immune response against HCMV. We tested the hypothesis that, by virtue of their unique antigenic and structural properties, DB could induce a significant immune response in the absence of infectious virus. Mice were immunized with gradient-purified DB, which were either left untreated or subjected to sequential rounds of sonication and freeze-thawing to prevent cellula…
Genomic instability induced by α-pinene in Chinese hamster cell line.
2012
Here, we report the effects of exposure of mammalian cells to α-pinene, a bicyclic monoterpene used in insecticides, solvents and perfumes. Morphological analysis, performed in V79-Cl3 cells exposed for 1 h to increasing concentrations (25 up to 50 μM) of α-pinene, indicated a statistically significant increase in micronucleated and multinucleated cell frequencies; apoptotic cells were seen at 40 and 50 μM. This monoterpene caused genomic instability by interfering with mitotic process; in fact, 50% of cells (versus 19% of control cells) showed irregular mitosis with multipolar or incorrectly localised spindles. Cytogenetic analysis demonstrated high-frequency hypodiploid metaphases as well…