Defect misplaced atom

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... 

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. 

There are two major types of point defects atomic defects and electronic defects. An atomic defect occurs when atoms are misplaced from their regular position in the crystal lattice. 

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. 

On the right are the t5rpes of point defects that could occur for the same sized atoms in the lattice. That is, given an array of atoms in a three dimensional lattice, only these two types of lattice point defects could occur where the size of the atoms are the same. The term vacancy is self-explanatory but self-interstitial means that one atom has slipped into a space between the rows of atoms (ions). In a lattice where the atoms are all of the same size, such behavior is energetically very difficult unless a severe disruption of the lattice occurs (usually a "line-defect" results. This behavior is quite common in certain types of homogeneous solids. In a like manner, if the metal-atom were to have become misplaced in the lattice cuid were to have occupied one of the interstitial... 

The creation of antisite defects can occur during crystal growth, when atoms are misplaced on the surface of the growing crystal. Alternatively, they can be created by internal mechanisms once the crystal is formed, provided that sufficient energy is applied to allow for atom movement. 

An antisite defect is an atom on an inappropriate site in a crystal, that is, a site normally occupied by a different chemical species. In a compound of formula AB the antisite defects that can occur are an A atom on a site normally occupied by a B atom, or a B atom on a site normally occupied by an A atom. Antisite defects are not very important in binary ionic compounds, as the misplacement of an ion is energetically costly, and so unfavorable. In ternary ionic compounds, however, such as spinels, AB204, the transfer of A ions to B sites and vice versa, is not... 

Point defects. Point defects (Fig. 5.1) are limited to a single point in the lattice, although the lattice will buckle locally so that the influence of point defects may spread quite far. A Frenkel defect consists of a misplaced interstitial atom and a lattice vacancy (the site the atom should have occupied). For example, silver bromide, which has the NaCl structure, has substantial numbers of Ag+ ions in tetrahedral holes in the ccp Br array, instead of in the expected octahedral holes. Frenkel defects are especially common in salts containing large, polarizable anions like bromide or iodide. 

A convenient description of die crystalline structure of solids is thus seen to consist of successive stages of approximation. First, the mathematically perfect geometrical model is described then departures from diis perfect regularity are permitted. The defonuabilily of solids is allowed for by letting die force constants between adjacent atoms be finite, not infinite. Then, misplacement of atoms is permitted and a variety of crystalline irregularities, called defects, is described. Some of these defects have intrinsic features which affect properties of die crystal other affect the properties by their motion from site to site in the crystal. In spite of their relatively small number, defects are of immense importance. 

The study of the electronic structure of impurities and defects in solids has a long tradition, both because of its own intrinsic theoretical interest and because of the technological importance in improving the performance of solid state devices. Lattice defects can be point defects (such as substitutional or interstital foreign atoms, vacancies, antisite defects in composite lattices), line defects (such as dislocations), planar defects (such as boundaries, adatom surfaces, stacking faults corresponding to misplaced planes of atoms), and so on. 

Point (microscopic) defects in contrast from the macroscopic are compatible with the atomic distances between the neighboring atoms. The initial cause of appearance of the point defects in the first place is the local energy fluctuations, owing to the temperature fluctuations. Point defects can be divided into Frenkel defects and Schottky defects, and these often occur in ionic crystals. The former are due to misplacement of ions and vacancies. Charges are balanced in the whole crystal despite the presence of interstitial or extra ions and vacancies. If an atom leaves its site in the lattice (thereby creating a vacancy) and then moves to the surface of the crystal, it becomes a Schottky defect. On the other hand, an atom that vacates its position in the lattice and transfers to an interstitial position in the crystal is known as a Frenkel defect. The formation of a Frenkel defect therefore produces two defects within the lattice—a vacancy and the interstitial defect—while the formation of a Schottky defect leaves only one defect within the lattice, that is, a vacancy. Aside from the formation of Schottky and Frenkel defects, there is a third mechanism by which an intrinsic point defect may be formed, that is, the movement of a surface atom into an interstitial site. Considering the electroneutrality condition for the stoichiometric solid solution, the ratio of mole parts of the anion and cation vacancies is simply defined by the valence of atoms (ions). Therefore, for solid solution M X, the ratio of the anion vacancies is equal to mJn. 

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