Substitution defects

Note that, in general, anions are larger in size than cations due to the extra electrons present in the former. A hexagonal lattice is shown in 3.1.6. with both Frenkel and Schottky defects, as well as substitutional defects. Thus, if a cation is missing (cation vacancy) in the cation sublattice, a like anion will be missing in the anion sub-lattice. This is known as a Schottky defect (after the first investigator to note its existence). 

In the case of the Frenkel defect, the "square" represents where the cation was supposed to reside in the lattice before it moved to its interstitial position in the cation sub-lattice. Additionally, "Anti-Frenkel" defects can exist in the anion sub-lattice. The substitutional defects axe shown as the same size as the cation or anion it displaced. Note that if they were not, the lattice structure would be disrupted from regularity at the points of ins tlon of the foreign ion. 

For nuclei that have perfect cubic site symmetry (e.g., those in an ideal rock salt, diamond, or ZB lattice) the EFG is zero by symmetry. However, defects, either charged or uncharged, can lead to non-zero EFG values in nominally cubic lattices. The gradient resulting from a defect having a point charge (e.g., a substitutional defect not isovalent with the host lattice) is not simply the quantity calculated from simple electrostatics, however. It is effectively amplified by factors up to 100 or more by the Sternheimer antishielding factor [25],... 

The color of mbies (as well as the action of mby lasers) is thus totally dependent upon substitutional defects. 

A point defect is a localized defect that consists of a mistake at a single atom site in a solid. The simplest point defects that can occur in pure crystals are missing atoms, called vacancies, or atoms displaced from the correct site into positions not normally occupied in the crystal, called self-interstitials. Additionally atoms of an impurity can occupy a normal atom site to form substitutional defects or can occupy a normally vacant position in the crystal structure to form an interstitial. Other point defects can be characterized in pure compounds that contain more than one atom. The best known of these are Frenkel defects, Schottky defects, and antisite defects. 

The microstructure of commercial varistors is extremely complex, and commercial preparations also contain other dopants, mainly oxides of cobalt, manganese, chromium, and antimony, that are used to fine tune the varistor characteristics. The transition-metal dopants are chemically similar to Zn2+ and mainly form substitutional defects within the ZnO grains, such as CoZn, that modify the n-type behavior of the grain interior. (See also Chapter 8 for further discussion of the electronic... 

Figure 5.11 Diffusion mechanisms (a) exchange (e) and ring (r) diffusion (b) kick-out diffusion, leading to (c) a substitutional defect and a self-interstitial.

Substitution-defect pigments, 79 405 Substitutive bromination, 4 343-344 Substoichiometric sulfur burners, 23 660 Substrate additions, in microbial transformations, 76 411—412 Substrate materials... 

ZrSi04. Based on the mechanism of color formation, zircon pigments can be divided into two categories, substitution-defect pigments and inclusion pigments. 

Effects of ionic substitutions, defects and variation in polymorph... 

Figure 2.25. Illustration of the difference between (a) interstitial and (b) substitutional defects in crystalline solids.

Dollase, D, W. (1980). Optimum distance model of relaxation around substitutional defects. Phys. Chem. Mineral. 6, 295-304. 

While the effect of cation impurities on the surface chemistry of MgO has been investigated in detail, very little is known about anion substitution. Defect formation and excitation energies for S and Se -doped bulk MgO have been calculated [182,183] but there are no data for the surface. In the bulk it has been estimated that the presence of S or Se impurities result in a outward relaxation of the Mg neighbors of 6% and 8%, respectively [182]. A recent report of the 0 "-S exchange reaction on MgO has been reported [184]. The reaction involves adsorption of CS2 on MgO powders and the subsequent exchange reaction with formation of COS and of S ions probably located at the low coordinated sites. It has been found that the basicity of the MgO surface doped with sulfur ions is drastically modified with respect to that of pure MgO [184]. 

In this context, Sayle et al. have used atomistic simulation methods to investigate the interaction of ceria with impurities, particularly rhodium, palladium and platinum. The energetics of the most common valence states for the metal atoms were investigated, as well as the variation of the energy of the impurities with depth below the surface and the tendency of defects to segregate to the surfaces. Fig. 8.9. shows the substitutional defect formation energies as a function of the distance from the (111) and (110) surfaces of Ce02 for C e " ", PcP+ and Sayle etal. found that... 

In this diagram, we see three types of point defects. In addition to the vacancy, we also see two types of substitutional defects. Both are direct substitutions in the "lattice", or arrangement of the atoms. One is a smaller atom, while the other is larger than the atoms comprising the lattice. Note the difference, due to size of the impurity, upon the ordering... 

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