Misplaced atoms

The second type of point defect is called an impurity. Impurities can occur in two ways as an interstitial impurity, in which an atom occupies an interstitial site (see Figures 1.21, 1.22, and 1.29) or when an impurity atom replaces an atom in the perfect lattice (see Figure 1.29). In the first instance, either the same atom as in the lattice, or an impurity atom, can occupy an interstitial site, causing considerable lattice strain as the atomic planes distort slightly to accommodate the misplaced atom. The amount of strain created depends on how large the atom is relative to lattice atoms. It... 

As one of the alloy species may not be imaged, either because it is completely field evaporated or from the lack of field ionization or both, it is possible to distinguish atoms of different alloy species from the field ion image alone without relying on atom-probe analysis. In fact this method can identify constituent atoms in an ordered alloy without the aiming error of the atom-probe. Misplaced atoms in an ordered alloy, i.e. atoms... 

Misplaced atoms types of atoms found at a site normally occupied by other types. This defect is only possible in covalent ceramics, however, where the atoms are not charged. 

Figure 6.1 Various types of defects typically found in ceramics. Misplaced atoms can only occur in covalent ceramics due to charge considerations.

Stoichiometric reaction is one in which no mass is transferred across the crystal boundaries. The three most common stoichiometric defects are Schottky defects, Frenkel defects, and antistructure disorder or misplaced atoms. 

Antistructure disorder or misplaced atoms. These are sites where one type of atom is found at a site normally occupied by another. This defect does not occur in ionic ceramics, but it has been postulated to occur in covalent ceramics like SiC. The notation for such a defect would be Si or C j, and the corresponding defect reaction is... 

Misplaced atoms If an atom is present on a crystal site that should be occupied by a different atom, that atom is a misplaced atom and may be called an antisite defect. Antisite defects usually form in covalent ceramics such as AIN and SiC, but can also occur in complex oxides that have several different types of cation, for example, spinels and garnets. (We do not expect to see cations on anion sites and vice versa.)... 

Point defect or zero-dimensional defect. This kind of defects include both the possible existence of vacancies and substituted impurity atoms on their sites of crystal lattice structure and include misplaced parts of atoms with each other in solid compound of AB, namely, A atom occupies the B atomic site, while inversely B atom occupies A, or to say there are misplaced atoms or variable valence ions on the sublattice sites. The interstitial atoms sited in the interstitials of lattice structure are also parts of those point defects. It can further be divided into Schottky defects and Frenkel defect. The former means a metal atomic defect and the original metal atoms are transformed to the metal surface and the latter is composed of an atomic defect and an interstitial atom, as presented in Fig. 3.23. It could be imagined that the existence of inner defects would bring the distortions of lattice, as shown in Fig. 3.24. The issue of point defect is the major subject and key problem for the studies of solid chemistry. 

Atoms of one type are sometimes found at a site normally occupied by other types of atoms. These are called misplaced atoms. This type of point defect is possible only in covalent ceramics, where the atoms are not charged. It cannot happen in ionic solids because, if it happens, like charges will become adjacent, and the structure will not be stable. In Figure 10.2, it is shown that the lattice atoms M and O have interchanged their positions. Thus, they have formed a pair of misplaced atoms. 

It is noted thus that the majority of heterogenous reactions lead to surface nucleations indeed, only some polymorphic transformations, that is, those not requiring any other reactant and not producing s phase, which would occur starting from defects of Frenkel or from defects of misplaced atoms, would be likely to lead to nucleation in the bulk. 

Define the following point defects and identify them as atomic defects or electronic defects vacancy, interstitial, substitutional impurity, misplaced atoms, electron, hole, dopant. 

Figure 9.8 shows a two-dimensional representation of a crystal lattice with some common types of atomic point defects. A vacancy occurs when an atom is absent from a lattice site that is normally occupied. An interstitial occurs when an atom sits in a place in the crystal that is not a distinct lattice site, but rather in between lattice sites. Figure 9.8 shows two types of interstitials. A self-interstitial contains an atom of the same type that makes up the host crystal, while an impurity interstitial consists of a foreign atom. A substitutional impurity occurs when a foreign atom occupies a lattice site normally housed by a host atom. In compound solids, such as AB, we can have misplaced atoms, where species A sits in a B site or vice versa. 

We can examine point defects, defects that occur at single atomic site, by applying the principles of chemical reaction equilibrium from this chapter. Atomic point defects include vacancies, interstitials, substitutional impurities, and misplaced atoms. Electronic point defects include mobile electrons and holes. From this approach, we can study carrier concentrations in semiconductors and see the effect of gas partial pressure on defect concentrations at equilibrium. The Brouwer diagram is a particularly useful tool in seeing the effect of gas partial pressure on defect concentration over many orders of magnitude. 

---