Interstitial pair

The defects generated in ion—soHd interactions influence the kinetic processes that occur both inside and outside the cascade volume. At times long after the cascade lifetime (t > 10 s), the remaining vacancy—interstitial pairs can contribute to atomic diffusion processes. This process, commonly called radiation enhanced diffusion (RED), can be described by rate equations and an analytical approach (27). Within the cascade itself, under conditions of high defect densities, local energy depositions exceed 1 eV/atom and local kinetic processes can be described on the basis of ahquid-like diffusion formalism (28,29). 

From a thermodynamic point of view the formation of an interstitial pair obeys the chemical equilibrium (Kittel, 1968)... 

Fig. 4.7 is an attempt to illustrate an ionic displacement for an interstitial pair by a VTF mechanism along a macromolecular chain. The... 

Fig. 4.7 A schematic representation of a cationic displacement along a polymeric chain above its Tg. (a) An initial activated step (ft) allows the formation of an interstitial pair, the migration of which (c) and (d) is assisted by local free volume redistribution.

The 2.2 eV in equation 57 may represent the migration enthalpy of the phosphorus-self-interstitial pair (56) with the excess self-interstitials produced by damage annealing. 

A) Shottky defects consist of paired cation and anion vacancies. (B) Frenkel defects consist of ion vacancy and ion interstitial pairs. (C) Anion vacancies may be neutralized by substitution of cations of higher valence. 

The ES-mechanism of Frenkel-pair formation as a result of excitation of Rydberg atomic states was confirmed by recent molecular dynamics calculations [28,29]. After the bubble formation the surrounding ground state atoms appear to have moved to the second shell. It was found that the second-nearest neighboring vacancy-interstitial pairs could create the permanent defects, which remain in the lattice after exciton annihilation (Fig.Sb) [29],... 

Ce enters the CaF2 lattice substitutionally for the host Ca ions. Several site symmetries are possible depending upon the charge compensation. Non-locally compensated Ce + produces a center of 0 symmetry, whereas C4v symmetry results when F" ions in interstitial positions take up nn positions to the impurities along <110> directions. In addition many higher-order cluster configurations of these impurity-interstitial pairs exist (23). 

AGPrenkel represents the Gibbs free energy of formation of the interstitial pair... 

Intrinsic point defects are deviations from the ideal structure caused by displacement or removal of lattice atoms [106,107], Possible intrinsic defects are vacancies, interstitials, and antisites. In ZnO these are denoted as Vzn and Vo, Zn and 0 , and as Zno and Ozn, respectively. There are also combinations of defects like neutral Schottky (cation and anion vacancy) and Frenkel (cation vacancy and cation interstitial) pairs, which are abundant in ionic compounds like alkali-metal halides [106,107], As a rule of thumb, the energy to create a defect depends on the difference in charge between the defect and the lattice site occupied by the defect, e.g., in ZnO a vacancy or an interstitial can carry a charge of 2 while an antisite can have a charge of 4. This makes vacancies and interstitials more likely in polar compounds and antisite defects less important [108-110]. On the contrary, antisite defects are more important in more covalently bonded compounds like the III-V semiconductors (see e.g., [Ill] and references therein). 

We mentioned above the collision cascade producing displaced atoms in a solid target. If we consider a single collision event in a crystalline solid, we can see that this displacement of atoms leads to the preferential formation of point defects. The most important types of point defects are a single vacancy (one atom or ion is missing), a single interstitial (an additional atom), a vacancy-interstitial pair (Frenkel pair). 

Figure 12.8 Binary ionic crystal showing defects that can lead to lattice diffusion, (a) Frenkel defect vacancy-interstitial pair), (b) Schottky defect (anion-cation vacancy). (After Kingery ct a .. 1976.)...

We have presented here results on the acceptor properties of Bes. Beryllium is also known to produce in silicon substitutional-interstitial pairs which are electrically inactive. These pairs can trap an exciton and the absorption of these excitons will be discussed in due time. 

For Si, there are three types of native defects the vacancy, the interstitial, and the interstitialcy. The vacancy, V, is an empty lattice site. Depending on the configuration of the unsatisfied bonds due to the missing atom, a vacancy in Si can be either neutral, negatively or positively charged. A vacancy is also referred to as a Schottky defect. A Si atom residing in the interstices of the Si lattice is defined as a self-interstitial. A Frenkel pair is a vacancy-interstitial pair formed when an atom is displaced from a lattice site to an interstitial site. An interstitialcy... 

In compound crystals, balanced-defect reactions must conserve mass, charge neutrality, and the ratio of the regular lattice sites. In pure compounds, the point defects that form can be classified as either stoichiometric or nonstoichiometric. By definition, stoichiometric defects do not result in a change in chemistry of the crystal. Examples are Schottky (simultaneous formation of vacancies on the cation and anion sublattices) and Frenkel (vacancy-interstitial pair). 

Interstitials If an atom is present on any site that would be unoccupied in a perfect crystal then that atom is an interstitial. A Frenkel defect is a vacancy -i- interstitial pair formed by removing an atom from its site in the crystal structure and putting it into an interstice as illustrated in Figure 11.2. Frenkel defects formed in iodine-containing AgBr are essential to the photographic process. 

The quotient of electrical conductivity a and thermal conductivity X is inversely proportional to the temperature. The ionic conductivity of solids depends on the lattice type and the type of the defects. The conductivity increases with temperature. This property is used to distinguish the ion conductor from the electron conductor. For vacancies and interstitials in the ion lattice, conductivity depends on the formation enthalpy for vacancy-interstitial pairs, Afo v. 

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