Search results for "Vacancy defect"
showing 10 items of 178 documents
Ab initio modeling of oxygen impurity atom incorporation into uranium mononitride surface and subsurface vacancies
2011
The incorporation of oxygen atoms has been simulated into either nitrogen or uranium vacancy at the UN(001) surface, sub-surface or central layers. For calculations on the corresponding slab models both the relativistic pseudopotentials and the method of projector augmented-waves (PAW) as implemented in the VASP computer code have been used. The energies of O atom incorporation and solution within the defective UN surface have been calculated and discussed. For different configurations of oxygen ions at vacancies within the UN(001) slab, the calculated density of states and electronic charge re-distribution was analyzed. Considerable energetic preference of O atom incorporation into the N-v…
First principles simulations of F centers in cubic SrTiO 3
2005
Atomic and electronic structure of regular and O-deficient SrTiO3 have been studied. Several types of first principles atomistic simulations: Hartree-Fock method, Density Functional Theory, and hybrid HF-DFT functionals, have been applied to periodic models that consider supercells of different sizes (ranging between 40 and 240 atoms). We confirm the ionic character of the Sr-O bonds and the high covalency of the Ti-O2 substructure. For the stoichiometric cubic crystal; the lattice constant and bulk modulus correctly reproduce the experimental data whereas the band gap is only properly obtained by the B3PW functional. The relaxed geometry around the F center shows a large expansion of the t…
Positron Annihilation in Polycrystalline Metals Deformed by Uniaxial Tension
2005
Angular distributions of the positron annihilation quanta were measured for polycrystalline samples deformed by uniaxial tension up to difierent deformation degrees. The S parameter as a function of the W parameter was determined. The data obtained for samples elongated up to difierent elongation degrees indicate that in the proportionality and limited proportionality regions the changes in the physical properties of samples are governed mainly by generation of vacancies and formation and kinetics of transformations of vacancy clusters occurring flrst of all on the grains of monocrystallites. In the region of plastic deformations the dominant defects are dislocations and vacancies and their…
Positron trapping rate into vacancy clusters
1979
The trapping rate of positrons into vacancy clusters in metals has been calculated. It increases with the trap size and binding energy and approximately scales with the number of vacancies in small clusters. The phonon-mediated contribution to the trapping rate is small. The temperature dependence of the trapping process is discussed.
Ab initio simulation of yttrium oxide nanocluster formation on fcc Fe lattice
2010
Using results of density functional theory (DFT) calculations the first attempt towards the understanding of Y2O3 particles formation in oxide dispersed strengthened (ODS) ferritic–martensitic steels was performed. The present work includes modeling of single defects (O impurity atom, Fe vacancy and Y substitute atom), interaction between substituted Y atoms, Y–Fe vacancy pairs and oxygen impurity atoms in the iron matrix. The calculations have showed the repulsive interaction between the two Y substitute atoms at any separation distances that might mean that the oxygen atoms or O atoms with vacancies are required to form binding between atoms in the yttrium oxide nanoclusters.
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 …
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…
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,…
Charge distribution and optical properties of and F centres in crystals
1997
Results of quantum chemical calculations for the and F centres in cubic and orthorhombic phases of a perovskite ferroelectric are presented and analysed in the light of existing experimental literature. It is shown that one (two) electrons of the and F centres, respectively, are considerably delocalized, even in the ground state of defects, over the two Nb atoms nearest to the O vacancy, and other close atoms. They resemble more electron defects in partly covalent crystals (the so-called centre) than F-type centres in ionic MgO crystals. We predict two or three absorption bands (depending on the crystalline phase) for each of the defects. The calculated absorption energies for the centre ar…