Defect interstitialcy

When Schottky defects are present in a crystal, vacancies occur on both the cation and anion sublattices, allowing both cation and anion vacancy diffusion to occur (Fig. 5.12a). In the case of Frenkel defects interstitial, interstitialcy, and vacancy diffusion can take place in the same crystal with respect to the atoms forming the Frenkel defect population (Fig. 5.12b). 

Figure 5.12 Diffusion in crystals of composition MX containing (a) Schottky and (b) Frenkel defects, schematic V, vacancy, i, interstitial, iy, interstitialcy.

Interstitialcy migration depends on the geometry of the interstitial defect. However, an a priori prediction of interstitial defect geometry is not straightforward in real materials. For an f.c.c. crystal, a variety of conceivable interstitial defect candidates are illustrated in Fig. 8.5. The lowest-energy defect will be stable and predominant. For example, in the f.c.c. metal Cu, the stable configuration is the (100) split-dumbbell configuration in Fig. 8.5d [3]. 

Figure 8.4 Substitutional diffusion by the interstitialcy mechanism, (a) The interstitial defect corresponding to the interstitial atom (3) is separated from a particular substitutional atom B (shaded), (b) The interstitial defect moved adjacent to B when the previously interstitial atom (3) replaced the substitutional atom (2). (2) then became the interstitial atom, (c) Atom (2) has replaced B, and B has become the interstitial atom, (d) B has replaced atom (4). which has become the interstitial atom, (e) The interstitial defect has migrated away from B. As a result. B has completed one nearest-neighbor jump and the interstitial defect has moved at least four times.

Diffusion by the interstitial mechanism and by the interstitialcy mechanism are quite different processes and should not be confused. Diffusion by the vacancy and interstitialcy mechanisms requires the presence of point defects in the system, whereas diffusion by the ring and interstitial mechanisms does not. 

Self-diffusion of Ag cations in the silver halides involves Frenkel defects (equal numbers of vacancies and interstitials as seen in Fig. 8.116). In a manner similar to the Schottky defects, their equilibrium population density appears in the diffusivity. Both types of sites in the Frenkel complex—vacancy and interstitial— may contribute to the diffusion. However, for AgBr, experimental data indicate that cation diffusion by the interstitialcy mechanism is dominant [4]. The cation Frenkel pair formation reaction is... 

The activation energy for self-diffusivity of the Ag cations by the interstitialcy mechanisms is the sum of one-half the Frenkel defect formation enthalpy and the activation enthalpy for migration,... 

Solution. Substitutional atoms of type 1 may diffuse more rapidly than atoms of type 2 if they diffuse independently by the interstitialcy mechanism in Fig. 8.4. To sustain the unequal fluxes, interstitial-atom defects can be created at climbing dislocations acting... 

Point Defect Generation During Phosphorus Diffusion. At Concentrations above the Solid Solubility Limit. The mechanism for the diffusion of phosphorus in silicon is still a subject of interest. Hu et al. (46) reviewed the models of phosphorus diffusion in silicon and proposed a dual va-cancy-interstitialcy mechanism. This mechanism was previously applied by Hu (38) to explain oxidation-enhanced diffusion. Harris and Antoniadis (47) studied silicon self-interstitial supersaturation during phosphorus diffusion and observed an enhanced diffusion of the arsenic buried layer under the phosphorus diffusion layer and a retarded diffusion of the antimony buried layer. From these results they concluded that during the diffusion of predeposited phosphorus, the concentration of silicon self-interstitials was enhanced and the vacancy concentration was reduced. They ruled out the possibility that the increase in the concentration of silicon self-interstitials was due to the oxidation of silicon, which was concurrent with the phosphorus predeposition process. 

Self-diffusion in materials occurs by repeated occupation of defects. Depending on the defects involved one can distinguish between (1) vacancy, (2) interstial, and (3) interstitialcy mechanisms [107], As an example, different diffusion paths for oxygen interstitials are illustrated in Fig. 1.16 [129]. For a detailed description of diffusion paths for oxygen vacancies, zinc vacancies and zinc interstitials the reader is also referred to literature [129,130]. 

Solid-state diffusion, which is involved in the release of oxygen, proceeds generally through the movement of point defects. The vacancy mechanism, the interstitial mechanism, and the interstitialcy mechanism can occur depending on the distortion of the solid lattice and the nature of the diffusing species. When one of the steps 1-5 is the slowest step representing the major resistance, that step is the rate-controlling one, which is not necessarily the chemical reaction (step 3). 

Fig. 7.15 Mechanisms of ionic conduction in crystals with defect structures (a) vacancy (Scholtky defect) mechanism, (b) interstitial (Frenkel defect) mechanism, (c) interstitialcy (concerted Schoilky-Frenkel) mechanism.

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. 

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... 

B is one of the most commonly used p-type dopants in SiC. Diffusion of B in SiC was found to be activated by a kick-out mechanism, in which a B substitutional impurity at a Si lattice site is displaced by a nearby Si interstitial [49]. The displaced B takes an interstitial site that can then diffuse through the crystal. Rurali et al. [49, 50] also determined the lowest energy diffusion path of the interstitial B impurity, going from a trigonal site to the next one via C and Si interstitialcies, with a barrier of 0.65 eV. B interstitials therefore diffuse easily through the SiC crystal until they recombine with an existing vacancy or they reach the surface. The difficulty observed in the experiment is therefore associated with the activation of the B interstitial, which must proceed via a Si self-interstitial, rather than the diffusion process itself. This explains the experimental observations of an enhanced diffusion rate in the presence of intrinsic defects [54]. 

In elemental solids also other mechanisms have been proposed. The crowdion is a variant of the interstitialcy mechanism. In this case it is assumed that an extra atom is crowded into a line of atoms, and that it thereby displaces several atoms along the line from their equilibrium positions. The energy to move such a defect may be small, but it can only move along the hne or along equivalent directions. 

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