Defects antisite

We consider an AB alloy which consists of an equal number of A and B sites. For the subsequent analysis, every site is uniquely associated with either an A or a B sublattive. The following is trivially generalised to A iBn alloys. The alloy is not quite stoichiometric, and has the composition A Bj.x, where for the validity of the independent defect approximation we must suppose x to be within a few percent of 0.5. Each site of each sublattice can be occupied by its own atom, an atom of the other kind (an antisite defect) or a vacancy. There are therefore six species for which we define the concentrations on each sublattice ... 

II. Triple defect on A two atoms vacate B sites and occupy new A and B sites, thus creating two B vacancies and an A antisite defect Denote the energy requred by Cia. 

What is the "effective charge on a defect What is an antisite defect ... 

The position of a defect that has been substituted for another atom in the structure is represented by a subscript that is the chemical symbol of the atom normally found at the site occupied by the defect impurity atom. The impurity is given its normal chemical symbol, and the site occupied is written as a subscript, using the chemical symbol for the atom that normally occupies the site. Thus, an Mg atom on a Ni site in NiO would be written as MgNi. The same nomenclature is used if an atom in a crystal occupies the wrong site. For example, antisite defects in GaN would be written as GaN and NGa. 

An antisite defect is an atom on a site normally occupied by a different chemical species that exists in the compound. Antisite defects are a feature of a number of important materials, especially weakly ionic or covalently bonded ones. 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 (Fig. 1.16a), or a B atom on a site normally occupied by an A atom (Fig. 1.16b). 

In metallic and many semiconducting crystals, the valence electrons are delocalized throughout the solid, so that antisite defects are not accompanied by prohibitive energy costs and are rather common. For example, an important defect in the semiconducting material GaAs, which has the zinc blend structure (Supplementary Material SI), is the antisite defect formed when an As atom occupies a Ga site. 

In cases where the antisite defects are balanced, such as a Ga atom on an As site balanced by an As atom on a Ga site, the composition of the compound is unaltered. In cases where this is not so, the composition of the material will drift away from the stoichiometric formula unless a population of compensating defects is also present. For example, the alloy FeAl contains antisite defects consisting of iron atoms on aluminum sites without a balancing population of aluminum atoms on iron sites. The composition will be iron rich unless compensating defects such as A1 interstitials or Fe vacancies are also present in numbers sufficient to restore the stoichiometry. Experiments show that iron vacancies (VFe) are the compensating defects when the composition is maintained at FeAl. 

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. 

To illustrate exactly how these mles work, a number of examples follow. In the first, the formation of antisite defects, a simple example that does not involve changes in atom numbers or charges on defects, is described. Secondly, two reactions involving oxides, nickel oxide and cadmium oxide, both of which are nonstoichio-metric, but for opposite reasons, indicate how to deal with a solid-gas interaction... 

The creation of a complementary pair of antisite defects consisting of an A atom on a B atom site, AB, and a B atom on an A atom site, Ba, can be written in terms of a chemical equation ... 

Antisite defects can be created via the intermediate formation of a Frenkel defect by the following mles ... 

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. 

At all temperatures above 0°K Schottky, Frenkel, and antisite point defects are present in thermodynamic equilibrium, and it will not be possible to remove them by annealing or other thermal treatments. Unfortunately, it is not possible to predict, from knowledge of crystal structure alone, which defect type will be present in any crystal. However, it is possible to say that rather close-packed compounds, such as those with the NaCl structure, tend to contain Schottky defects. The important exceptions are the silver halides. More open structures, on the other hand, will be more receptive to the presence of Frenkel defects. Semiconductor crystals are more amenable to antisite defects. 

An intrinsic defect is one that is in thermodynamic equilibrium in the crystal. This means that a population of these defects cannot be removed by any forms of physical or chemical processing. Schottky, Frenkel, and antisite defects are the best characterized intrinsic defects. A totally defect-free crystal, if warmed to a temperature that allows a certain degree of atom movement, will adjust to allow for the generation of intrinsic defects. The type of intrinsic defects that form will depend upon the relative formation energies of all of the possibilities. The defect with the lowest formation energy will be present in the greatest numbers. This can change with temperature. 

Zinc oxide is normally an w-type semiconductor with a narrow stoichiometry range. For many years it was believed that this electronic behavior was due to the presence of Zn (Zn+) interstitials, but it is now apparent that the defect structure of this simple oxide is more complicated. The main point defects that can be considered to exist are vacancies, V0 and VZn, interstitials, Oj and Zn, and antisite defects, 0Zn and Zno-Each of these can show various charge states and can occupy several different... 

Antisite defects in the pyrochore structure Er2Ti207 were mentioned previously (Section 1.10). These defects also occur in the nonstoichiometric compound Er2.09Ti194O6.952, which is slightly Er203-rich compared to the stoichiometric parent phase. The formation of the antisite pair is now accompanied by the parallel formation of oxygen vacancies ... 

Figure 9.3 Schematic representation of magnetic defects in a ferrimagnetic matrix (a) a pair of vacancies or nonmagnetic impurities similar to a Schottky defect and (b) an antisite defect.

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