Volume defects

Point defects are changes at atomistic levels, while line and volume defects are changes in stacking of planes or groups of atoms (molecules) m the structure. Note that the curangement (structure) of the individual atoms (ions) are not affected, only the method in which the structure units are assembled. Let us now examine each of these three types of defects in more detail, starting with the one-dimensional lattice defect amd then with the multi-dimensional defects. We will find that specific types have been found to be associated with each t3rpe of dimensional defect which have specific effects upon the stability of the solid structure. 

On the left, in 3.1.1., are the two types of point defects which involve the lattice itself, while the others involve impurity atoms. Indeed, there do not seem to any more than these four, and indubitably, no others have been observed. Note that we are limiting our defect family to point defects in the lattice and are ignoring line and volume defects of the lattice. These four point defects, given above, are illustrated in the following diagram, given as 3.1.2. on the next page. 

The volume defect is somewhat more difficult to visualize in two dimensions. Let us suppose that a line defect has appeared while the crystal structure was forming. This would be a situation similar to that already shown in 3.1.3. where aline defect was shown. The compression-tension area of the defect has a definitive effect upon the growing crystal and causes it to deform around the line defect. This is shown in the following diagram ... 

This type of volume defect in the crystal is known as a "screw dislocation", so-called because of its topography. Note that the spiral dislocation of the growing lattice deposits around the Une defect at right angles to the line defect. 

Note that "b" in this diagram is the same as that in 3.1.8. Because edge and volume defects propagate throughout the lattice, they affect the physical properties of the solid, whereas it is the point defects that affect the chemical properties of the solid. These latter properties include electrical and resistive, optical and reactivity properties of solids. Thus, we can now classify directs in solids as ... 

The existence of various temperature intervals characterized by predominant manifestation of one of above interactions can be detected from thermal desorption spectra. For instance, the thermal desorption spectrum obtained in [71] for a cleaved ZnO (1010) monocrystal following its interaction with oxygen (Fig. 1.4) indicates the availability of such typical temperature intervals as interval of physical adsorption (a), chemisorption (b), interval of formation of surface defects (c) and, finally, the domain of formation of volume defects (d). 

At temperatures of the order 700 - 900 K the surface point defects play the dominant role in controlling the various eledrophysical parameters of adsorbent on the content of ambient medium [32]. As it has been mentioned in section 1.6, these defects are being formed in the temperature domain in which the respective concentration of volume defects is very small. In fact, cooling an adsorbent down to room temperature results violation of uniform distribution due to redistribution of defects. The availability of non-homogeneous defect distribution led to creation of a new model of depleted surface layer based on the phenomenon of oxidation of surface defects [182] which is an alternative to existing model of the Shottky barrier [183]. 

Fig. 2.11. Geometric and electron structure of the surface of ZnO(lOlO) [33] Vq is the surface-adjacent defect Fq nd O are the volume defects which are...

The topic of defects in semiconductors encompasses point, line, planar and volume defects. Point defects include those defects occupying, or sharing, a single lattice site these would include substitutional impurities... 

Three-dimensional (volume) defects—point defect clusters, voids, precipitates. 

Figure 1.1 Defects in crystalline solids (a) point defects (interstitials) (b) a linear defect (edge dislocation) (c) a planar defect (antiphase boundary) (d) a volume defect (precipitate) (e) unit cell (filled) of a structure containing point defects (vacancies) and (/) unit cell (filled) of a defect-free structure containing ordered vacancies. ...

In a similar fashion, the line and planar defects described above are all, strictly speaking, volume defects. For the sake of convenience it is often easiest to ignore this point of view, but it is of importance in real structures, and dislocation tangles, for instance, which certainly affect the mechanical properties of crystals, should be viewed in terms of volume defects. 

Apart from these, there are volume defects that cannot conveniently be described in any other terms. The most important of these consist of regions of an impurity phase—precipitates—in the matrix of a material (Fig. 3.39). Precipitates form in a variety of circumstances. Phases that are stable at high temperatures may not be stable at low temperatures, and decreasing the temperature slowly will frequently lead to the formation of precipitates of a new crystal structure within the matrix of the old. Glasses, for example, are inherently unstable, and a glass may slowly recrystallize. In this case precipitates of crystalline material will appear in the noncrystalline matrix. 

Even when the composition range of a nonstoichiometric phase remains small, complex defect structures can occur. Both atomistic simulations and quantum mechanical calculations suggest that point defects tend to cluster. In many systems isolated point defects have been replaced by aggregates of point defects with a well-defined structure. These materials therefore contain a population of volume defects. 

Three-dimensional volume defects such as amorphous regions, pores, other phases and metastable regions. 

Lanthanum laurate La(Ci2H25COO)3 exemplifies ionic metal mesogenes, which are known to form thermotropic and liotropic liquid crystals [1], Anisotropy and high molecular mobility are essential properties of liquid crystals, which ensure the fast rate of information processing systems on their base. Molecular mobility is also known to depend on the presence of the free-volume defects. These are intermolecular cavities (nanovoids) or atomic-size vacancies. 

Lanthanum laurates in crystalline (powder) and liquid-crystalline states, as well as after dissolution of Cgo fullerenes were studied by means of the positron annihilation technique which is extremely sensitive to the free-volume defects. 

As most experimenters want to know not the absolute value of the linewidth but how it changes as a function of physical parameters, these ratios have been taken up as the simplest way of describing the linewidth. C/T is called the S (sharpness) parameter, and (A+E)/T is W, the wing parameter. As we can induce from Figure 3.7, S and W should be sensitive to changes in the momentum density of lower- and higher-momentum electrons, respectively. Positron annihilation in open volume defects thus typically leads to an increase in 5 and a decrease in W. 

Ordered arra3rs are not completely uniform but have defects, as do crystals, as shown in Figure 11.16. These defects fall into four main categories (1) point defects or vacancies (i.e., places where a particle is missing) (2) line defects or dislocations, (3) planar defects (i.e., grain boimdaiies), and (4) volume defects like cracks. The point defects are... 

The concept of a defect has undergone considerable evolution over the course of the last century. The simplest notion of a defect is a mistake at normal atom site in a solid. These stmcturally simple defects are called point defects. Not long after the recognition of point defects, the concept of linear defects, dislocations, was invoked to explain the mechanical properties of metals. In later years, it became apparent that planar defects, including surfaces, and volume defects such as rods, tubes, or precipitates, also have important roles to play in influencing the physical and chemical properties of the host matrix. More recently, it has become apparent that interactions between point defects are of considerable importance, and the simple model of isolated point defects is often inadequate with... 

Tensile strength and elasticity modulus increase with decreasing fiber diameter, reflecting the decrease in the probability of volume defects per unit length (see Table 5.2-7). 

Volume fraction is frequently used to define the composition of mixed solvent systems, or to express the solubility of one solvent in another. However, since the volumes of solutions exhibit a dependence on temperature, the expression of concentrations in terms of volume fraction requires a simultaneous specification of the temperature. In addition, since volume defects may occur during the mixing of the solvents, and since these will alter the final obtained volume, defining the solubility of a solution in terms of volume fraction can lead to inaccuracies that can be avoided through the use of other concentration parameters. 

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