Interstitial aggregation

Figure 3.16 Edge dislocation climb (a) an edge dislocation before climb, (b) the start of climb by vacancy aggregation on the dislocation, and (c) the start of climb by interstitial aggregation on the dislocation.

The fluoride ion interstitials again lead to an increase in ionic conductivity. At lower temperatures this increase is modest because the interstitials aggregate into clusters, thus impeding ionic diffusion. At higher temperatures the clusters tend to dissociate, resulting in a substantial increase in conductivity. 

The existence of free interstitial point defects forming the complements to the vacancy centers is generally not observed following irradiation at room temperature. At these temperatures the interstitials cluster together to form interstitial aggregates and dislocation loops. However, lattice disorder can slow down or prevent the aggregation process due to interstitial trapping. 

If the excess of lanthanide is sufficiently great, overloading of the transport system occurs and colloidal aggregates of large size are formed by hydrolysis. The interstitial or intracavitary formation of immobilized lanthanide colloids labeled with relatively short-lived radioisotopes was the basis for the attempted use of radioactive lanthanides as internal sources of therapeutic radiation (Kyker, 1962a, 1962b). 

Figure 3.14 Formation of dislocation loops (a) the aggregation of vacancies onto a single plane, (b) collapse of the plane to form a dislocation loop, and (c) aggregation of interstitials to form a dislocation loop.

An exactly similar situation can be envisaged if the crystal contains a high population of interstitial point defects. Should these aggregate onto a single plane, a dislocation loop will once again form (Fig. 3.14c). 

The aggregation of vacancies or interstitials into dislocation loops will depend critically upon the nature of the crystal structure. Thus, ionic crystals such as sodium chloride, NaCl, or moderately ionic crystals such as corundum, AI2O3, or rutile, TiC>2, will show different propensities to form dislocation loops, and the most favorable planes will depend upon chemical bonding considerations. 

The most stable cluster consists of an aggregation of four cation vacancies in a tetrahedral geometry surrounding an Fe3+ ion, called a 4 1 cluster. Cations in the sodium chloride structure normally occupy octahedral sites in which each metal is coordinated to six nonmetal atoms. The central Fe3+ ion in the 4 1 cluster is displaced into a normally unoccupied tetrahedral site in which the cation is coordinated to four oxygen ions. Because tetrahedral sites in the sodium chloride structure are normally empty, the Fe3+ is in an interstitial site. Equation (4.1) can now be written correctly as... 

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A unique situation exists in the case of diamonds, where the detailed spectroscopic descriptions of the centers are detected, but models are only proposed for a few of these. From more then 100 detected centers models are only determined for seven, mainly based on EPR interpretations. The model includes identification of the impurity, vacancy, interstitial atom, their aggregations and their crystallochemical position together with quantum-chemical and spectroscopic description. 

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