Point defects structural

Until recently very little was understood as to the factors which determine whether point or extended defects are formed in a non-stoicheiometric phase, although interesting empirical correlations between shear-plane formation and both dielectric and lattice dynamical properties of the defective solid had been noted. Theoretical techniques have, however, provided valuable insight into this problem and into the related one of the relative stabilities of extended and point defect structures. The role of these techniques is emphasized in this article. 

These alternative descriptions of shear-plane formation will be valuable in our discussion of mechanism in Section 4. In the account now presented of the relative stabilities of shear plane and point defect structures we will assume that vacancies are the predominant point defects. However, the arguments we present could be adapted for metal interstitial defect structures. 

This value (and that referred to subsequently when point defect structures are discussed) is the energy per eliminated O " ion. 

Thus to summarize, the extent of cation relaxation around a shear plane has emerged from our analysis as the most decisive factor in stabilizing shear planes with respect to point defect structures. Our discussion now continues with an account of the behaviour of the crystals at low deviations from stoicheiometry where an equilibrium may exist between point and extended defect structures. 

Aim and Background. Investigate the influence of a strong electric field on the electrical, transport and diffusion properties of carbon nanostructures with point defects structure. 

Complete structural characterization of a material involves not only the elemental composition for major components and a study of the crystal structure, but also the impurity content (impurities in solid solution and/or additional phases) and stoichiometry. Noncrystalline materials can display unique behavior, and noncrystalline second phases can alter properties. Both the long-range order and crystal imperfection or defects must be defined. For example, the structural details which influence properties of oxides include the impurity and dopant content, nonstoichiometry, and the oxidation states of cations and anions. These variables also influence the point-defect structure, which in turn influences chemical reactivity, and electrical, magnetic, catalytic, and optical properties. 

The determination of how nonstoichiometry is accommodated (i.e., by what type and amount of defect) is an active research area. Nonstoichiometry can also be accommodated by subtle changes in structure known as extended defects or crystallographic shear. Crystallinity, impurity levels, point-defect structure, and nonstoichiometry are each controlled by or influenced by the preparation method therefore, it is discussed further in Section III. 

Surface properties can differ from the bulk stracturally, both as clean surfaces or because of products formed on reactive surfaces (physisorbed or chemisorbed). The former can experience relaxation, that is, surface reconstruction due to the distortion in bonding for surface atoms which are lacking bonds. Impurity segregation at a surface can further alter properties, as can second phases formed on a surface. The activity of heterogeneous catalysts and corrosion is controlled by such surface properties and by the bulk and surface point-defect structures. 

The possible point-defect structures, migration properties,. . . are so numerous in intermetallic compounds that experiment alone cannot solve the problem. Unfortunately, theory here is still in its infancy. For example, the commonly used Miedema and bond-breaking semiempirical models to estimate point-defect formation energies are quite contradictory. It is only recently, since the 1980s, that more sophisticated theoretical methods have been developed and seem to be able to predict point-defect structures and properties with some accuracy. Great progress can be expected from the combined use of Monte-Carlo and molecular-dynamics simulations (Rey-Losada et al., 1993). 

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