Search results for " defect"
showing 10 items of 767 documents
Defects in yttrium aluminium perovskite and garnet crystals: atomistic study
2000
Native and impurity point defects in both yttrium aluminium perovskite (YAP) and garnet (YAG) crystals are studied in the framework of the pair-potential approximation coupled with the shell model description of the lattice ions. The calculated formation energies for native defects suggest that the antisite disorder is preferred over the Frenkel and Schottky-like disorder in both YAP and YAG. The calculated values of the distortion caused by the antisite YAl x in the lattice turn out to be in an excellent agreement with the EXAFS measurements. In non-stoichiometric compounds, the calculated reaction energies indicate that excess Y2 O3 or Al2 O3 is most likely to be accommodated by the forma…
Annealing of radiation induced oxygen deficient point defects in amorphous silicon dioxide: evidence for a distribution of the reaction activation en…
2011
The selective annealing of point defects with different activation energies is studied, by performing sequences of thermal treatments on gamma irradiated silica samples in the temperature range 300-450 °C. Our experiments show that the dependence on time of the concentration of two irradiation induced point defects in silica, named ODC(II) (standing for oxygen deficient centre II) and the E(γ)(') centre, at a given temperature depends on the thermal history of the sample for both of the centres studied; moreover in the long time limit this concentration reaches an asymptotic value that depends on the treatment temperature alone. These results suggest the existence of a distribution of the a…
Computed positron lifetimes in vacancies and vacancy-iron clusters in gold
1983
Abstract Annihilation characteristics are calculated for positrons trapped in clean and impurity decorated vacancy clusters in Au. The positron lifetime depends strongly on the structure of the clusters. In a strongly relaxed vacancy cluster, the lifetime can become smaller than the lifetime in a single vacancy. The substitution of some neighbour atoms of a vacancy cluster by Fe atoms has only a minor effect on the positron lifetimes.
Kinetics of coloration of anodic electrochromic films of WO3·H2O
1980
Polycrystalline layers of WO3·H2O are obtained by anodization of tungsten in 1 N H2SO4 at 70° C. The cathodic reduction of these layers in acid solutions causes the formation of blue WO3−x·H2O (0<x⩽:0.12) oxides. The kinetics of coloration are investigated by galvanostatic and potentiostatic techniques. The experimental results are compared with the theoretical data obtained by solving the diffusion equation for a constant flow of oxygen vacancies and for a time-dependent surface vacancy concentration. Except in the initial stage of coloration, the process controlling rate can be ascribed to the diffusion of oxygen vacancies from the oxide-electrolyte interface into the bulk of the layers. …
Synthesis, Characterization, and Catalysis of β3-[(CoIIO4)W11O31(O2)4],10- the First Keggin-Based True Heteropoly Dioxygen (Peroxo) Anion. Spectrosco…
1999
Reactions of hydrogen peroxide with several lacunary polyoxometalates of the 1:11 series, XW11O39m- (X = Co3+, Ga3+, Fe3+, Si4+, and P5+), are reported. Synthetic pathways to new polyoxotungstates incorporating dioxygen moieties (peroxo and/or superoxo) are developed. The key step involves treating lacunary precursors with H2O2 in strongly buffered aqueous solutions. Upon reaction of H2O2 with α-[Co3+W11O39],9- (a) the central tetrahedral Co3+ is reduced to Co2+ and (b) each of the four unshared oxygens surrounding the vacancy are replaced by a peroxide group, yielding salts of the tetraperoxide anion β3-[(Co2+O4)W11O31(O2)4]10- (1). These results are unequivocally established by a combinat…
Diffusion of oxygen in nickel: A variable charge molecular dynamics study
2010
Abstract Variable charge molecular dynamics have been performed to study the diffusion mechanisms of oxygen atoms (O) in nickel (Ni) in the temperature range 950 K–1600 K. It is observed that oxygen does not diffuse via jumps of oxygen through interstitial sites but via a vacancy diffusion mechanism. The oxygen diffusivity can be well described by an Arrhenius law over the temperature range considered. The oxygen diffusion coefficient has been analysed and indicates a value of E a = 1.99 eV for the activation energy and D 0 = 39.2 cm 2 s − 1 for the pre-exponential factor. Our numerical results were compared with a compilation of experimental and theoretical studies, and exhibit a good agre…
Modeling of Point Defects in Corundum Crystals
1994
Several different approaches including Hartree-Fock ab initio cluster calculations, semiempirical INDO calculations, and atom-atom potentials were used for modeling of the spatial and electronic structure as well as migration mechanisms of both intrinsic defects (self-trapped and defect-trapped holes, O and Al vacancies) and impurities (transition-metal ions like Co, Fe, Mg, Mn, Ti). The atomic structure of all hole centers is found to be similar to V[sub K] centers in alkali halides (two-site model); their formation is energetically favorable. The energy required for 60[degree] hole reorientations inside the basic oxygen triangles is found to be similar to both the energy for hops between …
Phonon coupling of non-bridging oxygen hole center with the silica environment: Temperature dependence of the 1.9 eV emission spectra
2008
Abstract We report an experimental study on the shape of the 1.9 eV emission associated with non-bridging oxygen hole centers in silica and its temperature dependence, from 4 up to 300 K, under visible and ultraviolet excitation. Our analysis points out that these defects are coupled with their environment by phonons whose contribution can be described by the single mode of mean frequency between 300–400 cm −1 and Huang–Rhys factor of ∼3. On increasing the temperature, the luminescence intensity undergoes a thermal quenching caused by non-radiative processes, its deviation from a pure Arrhenius law can be accounted for by an uniform distribution of activation energy, from 0.002 to 0.05 eV. …
Temperature dependance of the generation and decay of E’ centers induced in silica by 4.7eV laser radiation
2009
We report a study of the generation of silicon dangling bonds (E' centers) induced in fused silica by 4.7 eV laser irradiation in the 10 200 K the induced defects undergo a post-irradiation decay due to their reaction with mobile H(2). The interplay between generation and annealing gives rise to a bell-shaped temperature dependence of the concentration of induced E' centers, peaking at 250 K
A Contradiction between Pulsed and Steady-State Studies in the Recombination Kinetics of Close Frenkel Defects in KBr and KCl Crystals
1994
Theoretical study of the kinetics of the correlated annealing of pairs of close (geminate) F-H centers in KCl and KBr crystals controlled by their diffusion and elastic attraction shows that the multi-step (kink) decay in defect concentrations observed more than once in thermostimulated experiments takes place only for very close F-H center pairs which are no further than fourth nearest neighbors. On the other hand, it is demonstrated (both theoretically and experimentally) that such F-H center pairs should be destroyed by the tunneling recombination already at time ≤10 -4 s, i.e. much before beginning of the thermostimulated experiments. Possible explanations of this contradiction are sugg…