Interstitial impurity defects

Intrinsic defects (or native or simply defects ) are imperfections in tire crystal itself, such as a vacancy (a missing host atom), a self-interstitial (an extra host atom in an otherwise perfect crystalline environment), an anti-site defect (in an AB compound, tliis means an atom of type A at a B site or vice versa) or any combination of such defects. Extrinsic defects (or impurities) are atoms different from host atoms, trapped in tire crystal. Some impurities are intentionally introduced because tliey provide charge carriers, reduce tlieir lifetime, prevent tire propagation of dislocations or are otlierwise needed or useful, but most impurities and defects are not desired and must be eliminated or at least controlled. 

The presence of defects and impurities is unavoidable. They are created during tire growtli or penetrate into tlie material during tlie processing. For example, in a crystal grown from tire melt, impurities come from tire cmcible and tire ambient, and are present in tire source material. Depending on factors such as tire pressure, tire pull rate and temperature gradients, tire crystal may be rich in vacancies or self-interstitials (and tlieir precipitates). 

Materials that contain defects and impurities can exhibit some of the most scientifically interesting and economically important phenomena known. The nature of disorder in solids is a vast subject and so our discussion will necessarily be limited. The smallest degree of disorder that can be introduced into a perfect crystal is a point defect. Three common types of point defect are vacancies, interstitials and substitutionals. Vacancies form when an atom is missing from its expected lattice site. A common example is the Schottky defect, which is typically formed when one cation and one anion are removed from fhe bulk and placed on the surface. Schottky defects are common in the alkali halides. Interstitials are due to the presence of an atom in a location that is usually unoccupied. A... 

Figure 1 A dilute alloy system, showing a substitutional impurity, an interstitial impurity and an electromigration defect, and its reference system, the unperturbed host system. Some charge transfer effects are shown. Lattice distortion effects are omitted.

It is for this reason that compounds containing impurities sometimes have quite different chemical reactivities than the purest ones. That also has an effect upon the chemical reactivity of the solid. However, the interstitial impurity does not affect the lattice ordering at all. Now, let us look at another type of defect in the solid. Let us consider the heterogeneous lattice... 

Sauer et al. [185] derived a weak quadmpole interaction from the asymmetry of a poorly resolved Zeeman split spectmm of in W metal versus a Ta metal absorber. They also ascribed the unexpected weak quadmpole effect to deviations from cubic symmetry at the source or absorber atom arising from either interstitial impurities or crystal defects. 

Fig. 2-26. Localized electron levels of lattice defects and impurities in metal oxides Mi = interstitial metal ion Vm = metal ion vacancy V = oxide ion vacancy D = donor impurity A = acceptor impurity.

The second type of point defect is called an impurity. Impurities can occur in two ways as an interstitial impurity, in which an atom occupies an interstitial site (see Figures 1.21, 1.22, and 1.29) or when an impurity atom replaces an atom in the perfect lattice (see Figure 1.29). In the first instance, either the same atom as in the lattice, or an impurity atom, can occupy an interstitial site, causing considerable lattice strain as the atomic planes distort slightly to accommodate the misplaced atom. The amount of strain created depends on how large the atom is relative to lattice atoms. It... 

The lattice defects are classified as (i) point defects, such as vacancies, interstitial atoms, substitutional impurity atoms, and interstitial impurity atoms, (ii) line defects, such as edge, screw, and mixed dislocations, and (iii) planar defects, such as stacking faults, twin planes, and grain boundaries. 

The simplest lattice defects as far as FIM observations are concerned are point defects, such as vacancies, self-interstitials and substitutional as well as interstitial impurity atoms. Vacancies invariably show up as dark spots in the field ion images. Other point defects may appear as either bright image spots or vacancies in the image. Thus these defects can be identified from field ion images of high index planes where all the atoms in a plane are fully resolved. 

Figure 1 Idealised representations of point defects in a monatomic crystal, (a) vacancy, (b) (self) interstitial, (c) substimtional impurity or dopant, (d) interstitial impurity or dopant...

Point defects occur where an atom is missing, or is replaced by an impurity atom or is in an irregular place in the structural lattice. Point defects include selfinterstitial atoms and interstitial impurity atoms in a random arrangement. 

Previous studies on phonon confinement in nanocrystals did not account for possible contributions from lattice defects [100-102]. However, the parameter L in (12.3) represents the coherence length and is, therefore, a measure of the distance between dislocation, vacancies, interstitials, impurities, and other defects within the crystal lattice. The assumption that L represents the crystal size is only valid for defect-free crystals, where the surface is considered to limit the propagation of the phonons. This assumption does not hold for imperfect crystals produced by... 

In an ideal situation dislocation lines would penetrate the whole crystal. In reality they mostly extend from one grain boundary to another one or they are pinned by impurities. If the lines form a closed circle inside the crystal, they are called loops. Summarizing, one may say that dislocations can arise from vacancy clusters as well as from interstitial clusters due to their pressure on the lattice. Very often they are the final products of an annealing procedure. Dislocations already existing interact with point defects and impurities acting as traps or sinks. 

Frenkel defects and impurity ions can diffuse through the silver halide lattice by a number of mechanisms. Silver ions can diffuse by a vacancy mechanism or by replacement processes such as collinear or noncollinear interstitialcy jump mechanisms [18]. The collinear interstitial mechanism is one in which an interstitial silver ion moves in a [111] direction, forcing an adjacent lattice silver ion into an interstitial position and replacing it The enthalpies and entropies derived from temperature-dependent ionic conductivity measurements for these processes are included in Table 4. The collinear interstitial mechanism is the most facile process at room temperature, but the other mechanisms are thought to contribute at higher temperatures. 

The reduction in scattering yield associated with channeling can be applied to determine the lattice site position of impurity atoms and defects in the crystal (Fig. 8.3). An impurity on a lattice site has a reduction in scattering yield equal to that of the bulk crystal interstitial impurities or atoms located more than 0.1 A from a lattice site are exposed to the flux of channeled ions. Consequently, the backscattering yield from such nonsubstitutional atoms does not exhibit the same decrease as that of the host crystal. 

Point vacancies, interstitials, impurity atoms, antisite defects ... 

Almost all of the semiconductor clusters prepared so far show red-shifted broad band luminescence (e.g., Figure 13). These luminescences cannot be attributed to states that are populated directly by optical absorption. Rather, they are usually associated with defects such as vacancies, interstitials, impurities, and so on (Figure 10). These defect states have low oscillator strength and therefore are not observable in the absorption spectrum. Very few definitive facts are known about the nature of these defects, except that they are located mostly on the surfaces of the cluster. 

The intrinsic defects found in crystals Include vacancies. Interstitials, impurities and impurity compensations, reverse order, and combinations such as V- S and I- S, etc. Their numbers are well described by ... 

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