Point defects atomic

In a perfect crystal, all atoms would be on their correct lattice positions in the structure. This situation can only exist at the absolute zero of temperature, 0 K. Above 0 K, defects occur in the structure. These defects may be extended defects such as dislocations. The strength of a material depends very much on the presence (or absence) of extended defects, such as dislocations and grain boundaries, but the discussion of this type of phenomenon lies very much in the realm of materials science and will not be discussed in this book. Defects can also occur at isolated atomic positions these are known as point defects, and can be due to the presence of a foreign atom at a particular site or to a vacancy where normally one would expect an atom. Point defects can have significant effects on the chemical and physical properties of the solid. The beautiful colours of many gemstones are due to impurity atoms in the crystal structure. Ionic solids are able to conduct electricity by a mechanism which is due to the movement of fo/ 5 through vacant ion sites within the lattice. (This is in contrast to the electronic conductivity that we explored in the previous chapter, which depends on the movement of electrons.)... 

We wish here to obtain the thermodynamic equations defining the liquidus surface of a solid solution, (At BB)2, ). It is assumed that the A and atoms occupy the sites of one sublattice of the structure and the C atoms the sites of a second sublattice. For the specific systems considered here Sb and play the role of C in the general formula above. It is also assumed that the composition variable is confined to values near unity so that the site fractions of atomic point defects is always small compared to unity. This apparently is the case for the solid solutions in the two systems considered. Then it can be shown theoretically (Brebrick, 1979), as well as experimentally for (Hgj CdJ2-yTe)l(s) (Schwartz et al, 1981 Tung et al., 1981b), that the sum of the chemical potentials of A and C and that of and C in the solid are independent of the composition variable y ... 

The interaction energy between a solute particle (impurity atom, point defect) and an edge dislocation (screw dislocations do not interact, to first order) is... 

Electronic point defects, displaced electrons, almost always exist in connection with atomic point defects. A purely electronic defect, the so-called self-trapped electron trapped by induced polarization in a solid, has been suggested by Landau 29) but never found. If an incoming quantum imparts enough energy to an electron of one of the atoms of a solid, the electron will be freed from the atom and can wander through the solid. If it is not to be recaptured by the radiation-produced positive ion, it must be trapped at some other point in the solid, one with an effective positive charge. This will almost always be an atomic defect, specifically a negative ion vacancy or an impurity of suitable electron affinity relative to that of the host solid. When an electron is thus removed from an atom, the vacancy in the electronic structure is termed a positive hole. Such a hole has mobility like that of an electron... 

There are three circumstances which make a geometrical reason for an altered catalytic activity probable. If the substrate is a metal with a clean surface, any change upon irradiation must be attributed to atomic point defects or dislocations since electronic defects are excluded by the conductivity of metals. Since dislocations are produced or destroyed by radiation only under special circumstances, the normal explanation for a metal is vacancies, subsurface interstitials, or multiple defects. If, with any nonmetallic type of solid, a catalytic activity is introduced only or especially by heavy-particle bombardment and if the induced activity is little changed by annealing at low temperature, then the arrangement of the atoms rather than the presence or absence of electrons must be important. Finally, if the induced catalytic effect depends... 

An excellent example of the attribution of a chemisorption to a specific atomic point defect is afforded by the Harwell work on magnesium oxide and nickel oxide (37, 38). The work was undertaken to discover whether the specific electronic nature of an oxide was the determining factor in chemisorption, or whether more general structural features were important, and the choice of these oxides, isomorphous but different electronically, was dictated by this intention. Likewise, neutron bombardment was chosen in order to emphasize structural defects and determine whether vacancies and interstitials, which would be similar in the two oxides, would lead to similar changes in adsorption, or whether the electronic differences in the host lattices would impose differences in adsorptive behavior. 

As the understanding of atomic point defects in metals increases, it may be desirable to look more closely at some of the effects on catalysis with the hope of assigning the catalytic activities to specific arrangements of atoms. The results to date confirm the supposition that atomic defects in metals should infiuence catalysis, but, for lack of defect identification, do not suggest specific models for the substrate-catalyst interaction. 

A solid containing interstitial atom point defects will have a theoretical density ... 

The catalytic activity of cerium oxide depends on its particle size and surface area. As oxygen vacancy atomic point defects are formed more easily at the surface than in the bulk, high-surface-area materials will have a substantially higher catalytic activity [290]. The activation temperature of carbon combustion is reduced from approximately 700 °C for a micron-sized material to 300 °C, if the surface area of the material is increased by a factor of 20 [291]. 

Experiments which involved the simultaneous introduction of 1 0 and provided comparative values of the bulk self-diffusivities. It was found that Fe diffused slower than O in this system thus showing that the concentrations of atomic point defects in the Fe sub-lattice were lower than the concentrations of atomic point defects in the O sub-lattice. 

This chapter discusses the types of crystal structure and atomic point defect that are foimd in ceramic materials and, in addition, some of their mechanical characteristics. AppUcations and fabrication techniques for this class of materials are treated in the next chapter. 

Imperfections in With regard to atomic point defects, interstitials and vacancies for each anion and Ceramics cation type are possible (Figure 12.18). 

Inasmuch as electrical charges are associated with atomic point defects in ceramic materials, defects sometimes occur in pairs (e.g., Frenkel and Schottky) in order to maintain charge neutrality. 

All physical properties depend on defect concentration but some are far more sensitive than others. Electrical measurements are most often used in studying point defects. Transport properties tend to be more sensitive than equilibrium properties since defects often control diffusion or charge transport. Thus electrical conductivity is capable of detecting electrons, holes, and atomic point defects down to 10 /cm, far better than can be done by density measurements. In the latter, atomic defect concentrations of can be detected by compar-... 

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. 

Figure 9.8 Atomic point defects in a monatomic crystal lattice.

In addition to atomic point defects, semiconductor materials can also have electronic point defects. These defects provide mobile charge carriers that move about the crystal lattice. They provide the basis for many useful applications. In fact, the entire microelectronics industry is based on being able to control electronic defects in these materials. 

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. 

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