Crystal, defect, point experiments

These features make studying the relative stability of the two point defects interesting. Experiment " indicates that, when the crystal is irradiated at low temperature, the hole is trapped at an axial oxygen electron holes at... 

A point defect refers to a localized defect (such as a monovacancy) or impurity (such as interstitial O). This includes any relaxation and/or distortion of the crystal around it. Many point defects are now ratlier well understood, especially in Si, tlranks to a combination of experiments providing infonnation of microscopic nature... 

One type of point defect that cannot be entirely eliminated from a solid compound is the substituted ion or impurity defect. For example, suppose a large crystal contains 1 mole of NaCl that is 99.99 mole percent pure and that the 0.01% impurity is KBr. As a fraction, there is 0.0001 mole of both K+ and Br ions, which is 6.02 X 1019 ions of each type present in the 1 mole of NaCl Although the level of purity of the NaCl is high, there is an enormous number of impurity ions that occupy sites in the lattice. Even if the NaCl were 99.9999 mole percent pure, there would still be 6.02 X 1017 impurity cations and anions in a mole of crystal. In other words, there is a defect, known as a substituted ion or impurity defect, at each point in the crystal where some ion other than Na+ or Cl- resides. Because K+ is larger than Na+ and Br is larger than Cl-, the lattice will experience some strain and distortion at the sites where the larger cations and anions reside. These strain points are frequently reactive sites in a crystal. 

Experiments demonstrate that along crystal imperfections such as dislocations, internal interfaces, and free surfaces, diffusion rates can be orders of magnitude faster than in crystals containing only point defects. These line and planar defects provide short-circuit diffusion paths, analogous to high-conductivity paths in electrical systems. Short-circuit diffusion paths can provide the dominant contribution to diffusion in a crystalline material under conditions described in this chapter. 

A theoretical analysis of the experimental kinetics for Vk centres in KC1-Tl, as well as for self-trapped holes in a-Al203 and Na-salt of DNA, is presented in [55]. The fitting of theory to the experimental curves is shown in Fig. 4.4. Partial agreement of theory and experiment observed in the particular case of Vk centres was attributed to the violation of the continuous approximation in the diffusion description. This point is discussed in detail below in Section 4.3. Note in conclusion that the fact of the observation of prolonged increase in recombination intensity itself demonstrated slow mobility of defects. In the case of pure irradiated crystals, it is a strong... 

These and numerous other experiments prove that in metals the implanted atoms change their position and their surroundings between liquid He temperature and RT and that a considerable reordering of the lattice takes place even at low temperatures. The low-temperature recovery of ion-bombarded compounds is unfortunately completely unknown. Very few experiments at liquid-N temperatures indicate a strong temperature dependence too. A recovery of the majority of the point defects and centers below RT was found experimentally only for ionic crystals irradiated with electrons ... 

Crystal rotation in a thermally asymmetric field. That is, if the crystal is not rotated in a thermally balanced field while it is being pulled, any point on the soUd-Uquidus interface can experience a sinusoidal fluctuations in growth rate. These fluctuations cause growth striations in the crystal and are the source of "lensing" mentioned above. If the fluctuations exceed R, more severe defects occur in the crystal, as we have already shown. 

In the hypothesis of a perfect crystal lattice defined by a single pattern present on all of the lattice s nodes, it is difficnlt to imagine an atom or ion moving aroimd in the structure of the solid. Yet, experience shows scattering of chemical species in most materials. To explain this movement and to conceptualize some heterogenous reactions involving solid compoimds, it was necessary to imagine the existence of point defects in solids. 

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