Vacancy clusters evidence

A closer inspection of the ball radii in Figs. 13a-d reveals that the positive charges of the vanadium ions closest to each oxygen vacancy are smaller than corresponding values of the cluster without the vacancy. This is also clear from a comparison of the charge values q(V) given in Tables 1 and 4 and evidences chemical reduction of the metal sites. A more detailed description of the effect becomes possible by (P)DOS and orbital analyses of the vacancy clusters. As an example. Fig. 14 shows (P)DOS curves obtained for the V10O31H12 based vacan-... 

Valo et found strong evidence for vacancy clusters in irradiated model alloys. They used both positron hfetime and PALA to study ternary alloys irradiated to 4.6 x lO n/cm. The lifetime measurements indicated... 

In addition to studying model alloys, Valo etalso examined various steels irradiated to a fluence of 2.5 x 10 n/cml In contrast to the model alloys, no evidence was found for vacancy clusters. No simple explanation for the difference in irradiation response was suggested however, it was postulated that higher alloying and impurity concentrations in the RPV model alloys may be important. 

Post-irradiation annealing results [55] suggest that microvoids or vacancy clusters are responsible for about 20-30% of the embrittlement effect in commercial RPV steels. Although such defects were detected in irradiated model alloys, there is no solid evidence for their detection in commercial RPV steels. 

The evidence in favor of identification of anomalous muonium with the bond-center position can be considered convincing. The so-called vacancy-associated model (Sahoo et al., 1985, 1989), which had been proposed mainly on the basis of hyperfine calculations for clusters, shows distinct disagreement with the experimental results, which clearly establish that there are two equivalent Si neighbors along (111). 

In this subsection we examine the mechanism of the very fast diffusion. In the bulk medium the vacancies and interstitial site play a primary role in accelerating the diffusion. However, these diffusion mechanisms are not relevant in microclusters. It is well known that the vacancies created inside the cluster are immediately pushed to the surface. Indeed in our simulation the creation of vacancies inside the cluster is a very rare event even at the temperature close to the melting temperature. Moreover, we cannot find any evidence that the interstitial deformation takes place inside the cluster, and therefore neither of them is responsible for the rapid diffusion into the cluster. The key feature of the cluster that distinguishes the cluster from the bulk medium is that it is surrounded by the surface beyond which no atoms exist. In other words, the outside of the cluster is occupied by vacancies. As a result, the atoms on the surface move very actively along the surface. Such an active movement along the surface will be responsible for the rapid diffusion in the radial direction of the cluster. We focus our attention to the details of the active diffusive motion along the surface of the cluster, and we present a direct evidence that the surface activity controls the radial diffusion. A direct measure of the surface motion is the diffusion rate of the surface atoms... 

In this chapter, we have concentrated on MgO, one of the most studied and better understood oxide materials. We have shown that even on such a simple nontransition metal oxide about a dozen of different surface defect centers have been identified and described in the literature. Each of these centers has a somewhat different behavior toward adsorbed metal atoms. It becomes immediately clear that the precise assignment of the defect sites involved in the interaction, nucleation, and growth of the cluster is a formidable task. Nevertheless, thanks to the combined use of theory and experiment, the progress in this direction has been particularly significant and promising. For instance, a lot of evidence has been accumulated that points toward the role of the oxygen vacancies, the F centers. At the moment, these sites seem the most likely sites for nucleation and growth of small metal clusters. 

In this chapter, we have discussed recent theoretical and experimental studies that provide evidence for the important role of surface defects, such as oxygen vacancies, in the metal-oxide bonding. The cases of defect sites, in both MgO and SiC>2 surfaces, clearly show not only the fundamental role played by these sites in both stabilisation and nucleation but also their ability to change the electronic and magnetic properties of the metal atoms. The understanding of the metal-oxide interface and of the properties of deposited metal clusters also requires a deeper knowledge of the nature, concentration and mechanisms of formation and conversion of the defect sites of the oxide surface. 

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