Search results for "Vacancy defect"
showing 10 items of 178 documents
The kinetics of F-center aggregation under irradiation: many-particle effects in ionic solids
1994
The accumulation kinetics of primary Frenkel defects created in solids under permanent irradiation is calculated using the microscopic formalism of many-particle densities. It is based on the Kirkwood superposition approximation for three-particle densities as described in our previous paper p. N. Kuzovkov and E. A. Kotomin, Physica Scripta 47, 585 (1993)l. This formalism is generalized in this paper by incorporating the elastic attraction between similar defects (called in ionic solids F-centers) which causes their efficient aggregation. It is shown that the aggregation process starts only if the dose rate and elastic attraction energy exceed certain critical values; it also happpens in th…
Positron Annihilation in Defected Monocrystalline Gold Samples
2005
Angular distribution of the positron annihilation quanta was measured for monocrystalline gold samples, oriented in (110) and (111) directions. The samples were deformed by elongation for different deformation degrees. The S and W parameters as a function of deformation degree of the sample were determined. It was found that the dynamics of the dislocations and vacancy generation during the sliding of some crystallographic planes, depends on the crystallographic direction.
Positron Annihilation in Metals Defected by Action of the Tensile Force
2006
Results of experimental investigations of uniaxially elongated mono- and polycrystalline samples of several metals (Fe, Ta, Pd, Ag, and Au), performed using the positron annihilation methods, are reviewed. The dependences of the S-parameters and positron lifetimes on the relative elongation of the samples were presented. The data obtained for polycrystalline samples indicate that in the proportionality and limited proportionality regions the changes in the physical properties are governed mainly by generation of vacancies and by kinetics of formation and transformations of vacancy clusters occurring flrst of all on the grain boundaries of monocrystallites. In the region of plastic deformati…
Stability of carbon nanotubes under electron irradiation: Role of tube diameter and chirality
2005
As recent experiments demonstrate, the inner shells of multiwalled carbon nanotubes are more sensitive to electron irradiation than the outer shells. To understand the origin of such counterintuitive behavior, we employ a density-functional-theory based tight-binding method and calculate the displacement threshold energies for carbon atoms in single-walled nanotubes with different diameters and chiralities. We show that the displacement energy and the defect production rate strongly depend on the diameter of the nanotube and its chirality, with the displacement energy being lower, but saturating towards the value for graphite when the tube diameter increases. This implies that the threshold…
F centre production in CsI and CsI–Tl crystals under Kr ion irradiation at 15 K
2000
Abstract We present results of simultaneous in situ luminescence and optical absorption studies in scintillator CsI and CsI–Tl crystals, exposed to very dense electronic excitations induced by 86Kr ions (8.63 MeV/amu). Irradiation at 15 K leads to the formation of the prominent F absorption band. In addition, several other features of the broad absorption between exciton and F bands were ascribed to an anion vacancy, α centre (240 nm), self-trapped hole, Vk centre (410 nm) and interstitials, H centres (560 nm). We have found that low doping of thallium (∼1017 cm−3) causes the F centre formation to proceed more rapidly than in pure crystal. On the other hand, we were not able to create any a…
Ab initio simulations of oxygen interaction with surfaces and interfaces in uranium mononitride
2012
Abstract The results of DFT supercell calculations of oxygen behavior upon the UN (0 0 1) and (1 1 0) surfaces as well as at the tilt grain boundary are presented. Oxygen adsorption, migration, incorporation into the surface N vacancies on (0 0 1) and (1 1 0) surfaces have been modeled using 2D slabs of different thicknesses and supercell sizes. The temperature dependences of the N vacancy formation energies and oxygen incorporation energies are calculated. We demonstrate that O atoms easily penetrate into UN surfaces and grain boundaries containing N vacancies, due to negative incorporation energies and a small energy barrier. The Gibbs free energies of N vacancy formation and O atom incor…
First principles modelling of oxygen impurities in UN nuclear fuels
2008
We report results of first principles VASP supercell calculations of O impurity in UN fuels placed either at an interstitial tetrahedral position or as a substitution for a host N ion. In the latter case O perfectly fits into N site producing no lattice distortion. Such the O substitutional impurity only slightly affects the formation energies of U and N vacancies nearby. In both interstitial and substitutional positions O atom attracts the additional electron density and transforms into the negatively charged ion. Oxygen incorporation into pre-existing N vacancy is energetically more favourable than into the interstitial position. The O impurities produce an additional peak at the low ener…
Magnetic resonance investigations of oxygen-related luminescence centres in AlN ceramics
2001
Abstract The structure of oxygen-related luminescence centres in nominally undoped and Y2O3-doped AIN ceramics were investigated by electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and optically-detected EPR. The photoluminescence-detected EPR lines having g values of 1.990 and 2.008 were attributed to neighbouring donor and acceptor pairs causing the recombination luminescence excited in the ultraviolet. The two EPR lines at g = 1.987 and g = 2.003, detected via the recombination luminescence in the afterglow, are thought to be due to a recombination between the same, but more distant donor and acceptor pairs. The donor is supposed to be an electron trapped …
Theoretical and experimental study of primary radiation defects in KNbO3 perovskite crystals
2000
Abstract The results of large-scale computer simulations of point defects – F-type electron centers and hole polarons bound to a potassium vacancy – in perovskite KNbO3 crystals are presented. One-site polarons and two-site (molecular) polarons are expected to coexist, both are characterized by close absorption energies around 1 eV. The transient absorption spectra and decay kinetics observed after ns-pulsed electron beam irradiation have been measured for different KNbO3 crystals. The relaxation kinetics of the optical density vary considerably for different impurity concentrations and sample stoichiometries. Experimental data are discussed in the light of the presented calculations.
Raman spectra of vacancy-containing LiF: Predictions from first principles
2020
Abstract The electronic structure and Raman scattering from face-centered cubic LiF with three types of vacancies — an F-center, an F ′ -center and a vacancy without electrons ( v F ) — has been calculated by means of hybrid exchange–correlation functional within density functional theory. Our modeling predicts the effective charge of 0.87 e , 1.50 e and 0.10 e on the defect, respectively. The F-center induces two in-gap energy levels for two spin states split by 3.4 eV, while the F ′ -center and the v F induce a single in-gap level 3.8 eV and 2.9 eV below the bottom of conduction band, respectively. The calculated Raman spectra are thoroughly discussed and are found to match well the earli…