Physics
Substitutional Defect
A substitutional defect occurs when an atom in a crystal lattice is replaced by a different type of atom. This can lead to changes in the material's properties, such as conductivity or strength. Substitutional defects are important in understanding the behavior of materials and can be intentionally introduced to modify their characteristics.
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9 Key excerpts on "Substitutional Defect"
eBook - PDFIntroduction to Crystal Growth
Principles and Practice
- H.L. Bhat(Author)
- 2014(Publication Date)
- CRC Press(Publisher)
Both of these defects can be created by thermodynamic fluctuations or other effects resulting in the dislodging of an atom from its original site, which goes to either the surface or an interstitial site. Substitutional Defect Another type of point defect is an impurity atom, an atom of an abnormal type at a normal lattice site. Such an atom is called a Substitutional Defect. The impurity might be an atom of a different chemical element or a differ-ent isotope of the normal element or even an atom of the normal element in a different state of ionization or magnetization. Any of these circumstances is a deviation from the ideal and hence results in a defect. The substitutional atoms are usually close in size (within ~15%) to the atom in the bulk. All these defects are schematically shown in Figure 4.1. Study of ionic conductivity is helpful in understanding the nature of the aforementioned lattice defects, particularly in ionic crystals. Color Centers Finally, we may note that more complicated point defects may be built up by the combinations of some of these simpler ones. The most famous example of this is the color center, which is a defect responsible for the coloration in many crystals. Pure alkali halide crystals are transparent throughout the visible region of the spectrum. But they can be colored in a number of ways such as (1) introduction of chemical impurities such as transition elements, (2) introduction of excess neutral Na in NaCl crystal (crystal turns yellow), (3) introduction of excess neutral K in KCl (crystal turns magenta), and (4) irradiation with x-rays, γ rays, neutrons, or electrons. The most common of the color centers is the F center. The name F center comes from the German word Farbe , which means “color.” The F center has been identified by electron paramagnetic resonance (EPR) as an elec-tron bound to a negative ion vacancy.
- Robert J. Naumann(Author)
- 2008(Publication Date)
- CRC Press(Publisher)
8 Defects in Crystals As we shall see, nature does not allow a crystal to be perfect, but even if it did, the performance of a perfect crystal would not necessarily be improved. In fact the defect structure very much determines the mechanical, electrical, thermal, optical, and magnetic properties of a material and our ability to understand the role of defects and to be able to control their formation is key to the development of useful materials. 8.1 What Are Defects? Generally speaking, defects are disruptions in the long-range order of the crystal that was discussed in Chapter 4. Such disruptions can range from an atom out of its place to gross defects such as voids or inclusions in the crystal. The mechanical properties of a material are largely in fl uenced by its defect structure and such defects are often engineered into the material to improve its properties. On the other hand, certain types of defects degrade the electronic and optical performance of materials used for these purposes and are to be avoided. Therefore, it is important to develop an understanding of how various types of defects arise in crystal and how to control them. The defects we will be concerned with can be classi fi ed into four categories: point defects, line defects, surface defects, and volume defects. The formation of these defects and their relation to the mechanical properties of the material are treated in the following sections. 8.2 Point Defects As the name implies, point defects involve atoms that are missing, out of place, impurities that were purposely added or those that crept in. 8.2.1 Vacancy Defects No crystal is perfect. No matter which solidi fi cation process is used or how careful one is in controlling the process, every crystal will have, at the very least, point defects known as vacancies. Vacancies arise spontaneously to minimize the free energy at the local temperature.
eBook - PDF- Patrick M. Woodward, Pavel Karen, John S. O. Evans, Thomas Vogt(Authors)
- 2021(Publication Date)
- Cambridge University Press(Publisher)
At the local level, this order can be perturbed by three different types of point defects; vacancies, interstitials, and substitutional disorder. These are shown schemat- ically in Figure 2.1. A vacancy occurs when an atom is missing from a site in the structure as shown in Figure 2.1, left. An interstitial defect occurs when an extra atom sits in a site that would not normally be occupied. An interstitial site can be occupied either by an atom of the same type that 54 makes up the structure, or by an impurity atom. It is common, for example, for small non- metallic elements such as C, N, O, and H to occupy a fraction of interstitial positions in the structures of transition metals. Such materials can be of great technological importance: C interstitials in iron greatly increase its mechanical strength; and Pd can store around 0.6% by mass of H interstitials, which is of interest for hydrogen storage (see Box 2.1). The third common type of defect is substitutional disorder where a foreign atom adopts a site in the structure of a pure element. At low levels, foreign aliovalent 1 atoms are frequently referred to as dopants and can significantly alter chemical (Chapter 3), electronic (Chapters 6 and 10), and optical (Chapter 7) properties of a material. Doping silicon with low levels of Al or P, for example, leads to the formation of p- or n-type semiconductors, respectively. At higher levels of doping, one typically refers to solid-solution or alloy formation. This process can again be used to tailor a material’s properties. Real materials exist with any one of these basic types of defects or with various combinations of them. 2.2 Intrinsic Point Defects in Compounds Similar defects to those depicted in Figure 2.1 for elements can occur in ionic compounds, 2 though with the additional constraint that one must maintain overall electrical neutrality in the crystal.
eBook - PDF- Jean-claude Toledano(Author)
- 2011(Publication Date)
- World Scientific(Publisher)
Defects can be classified according to their “dimensionality”. Thus, one can consider point defects (zero-dimensional). For this type of defects, the volume of the perturbed region of a crystal is of the same order of 1 Other important physical properties of solids are also determined by the occurence of defects, such as, for instance, the electrical resistivity of metals. 51 52 Vacancies, an example of point defects in crystals magnitude as the volume of a single atom, or of a few atoms. A simple example is the vacancy consisting in the absence of an atom in a site nor-mally occupied by a constituent of the crystal (Fig. 4.1). Another simple example is the interstitial , i.e. an additional atom located in a normally empty space situated between the constituting-atoms of the structure (as the case of the carbon atoms in the iron structure described in chapter 2, section 4). A solid can also have substitutional impurities . These are atoms occupying a site of the crystal-structure normally occupied by an atom of different chemical nature. More complex point defects also exist, consisting of clusters (groups) of the preceding simple defects. (a) (b) (c) Figure 4.1 Three examples of point defects. (a) Vacancy. (b) Interstitial atom. (c) Substitutional atom. A linear defect (one-dimensional) is a filamentary (thread-shaped) de-fect, such as, for instance, the absence of a row of atoms in a crystal. The section of such a defect has the same order of magnitude as the section of an atom or of a few atoms, while its length is large as compared to the atomic dimensions, and can be as large as the linear size of the macroscopic crystal sample. This class of defects contains the dislocations, which, as mentioned in chapter 1, play a central role in the mechanism of plasticity. Their detailed description and properties will be analyzed in the next three chapters. In anticipation, Fig. 4.2 represents a simple type of dislocation, the so-called straight edge dislocation.
eBook - PDF- Lawrence Murr(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
54 L. E. Murr and Ο. T. Inal electrical conduction, as well as mechanical properties including hardness, strength, ductility, etc. In certain cases, one or many of these properties may be required, while many systems will consist of an integrated system or composite of many materials and properties to facilitate a specified function. In such cases, properties desirable in one portion of the system, which result by the presence of selective imperfections, may be incompatible with those required in another portion. That is, imperfections required to achieve one property may be noticeably detrimental to another. Consequently, some sort of compromise or optimization may be required. This can mean a very delicate variation or control of internal structure, or the number and kind of imper-fections which must be achieved through changes in crystal structure, chemical composition, or both. Crystal defects, or imperfections, in crystalline materials are now well documented (1-4). In general, they are grouped into regimes of zero-dimensional (or point) defects, one-dimensional (or line) defects, two-dimensional (or planar) defects, and three-dimensional (or volume) defects. Vacancies, interstitials, and substitutional impurities constitute the more common point defects (5) while charge balance requirements in ionic solids require pairs of such defects to be formed, or some other charge-compensating mechanism, such as the formation of a color center where an electron is trapped by an anion vacancy (6). Dislocations constitute the more common line defects (2), while interfaces such as grain and phase boundaries, and free surfaces (the solid-vapor or liquid-vapor interfaces) are characteristic of planar imperfections (7,8). Point defect aggregates or other larger voids account for the major types of volume defects (9). The way in which such defects are formed or interact is an impor-tant part of understanding how they will effect residual materi-als properties (3,10).
eBook - PDF- Richard J. Borg, G. J. Dienes(Authors)
- 2012(Publication Date)
- Academic Press(Publisher)
II Point Defects in Elemental Crystalline Substances All real crystals are imperfect containing both line and point defects and this chapter is devoted to their energetics because of their frequent and important role in solid state diffusion. The former, most commonly referred to as dislocations, have a minor influence on the chemistry of solids; although they are dominant in governing the strength properties and are frequently influential in the nucleation of phase transformations; we will consider their influence upon diffusion briefly in a later chapter. A point defect can be defined as any rational lattice point which is not occupied by the proper atom, ion, or molecule necessary to preserve the inherent periodicity of the structure, or the occupancy of a non-rational position by any of the foregoing. Generally, point defects are vacant lattice positions or interstitial atoms, but the term is sometimes enlarged to include trace amounts of impurities. It is, in a sense, the diffusion of point defects through the crystal structure which constitutes the making and breaking of chemical bonds; hence, they are primarily responsible for the occurrence of chemical reactions in solids. In addition, point defects are influential in determining the electrical behavior of extrinsic semiconductors, are respon-sible for color centers, control certain mechanical properties (e.g., second stage creep), and are the net result of radiation damage to solids. Because the laws and relationships governing the geometry, concentration, and mobility of point defects do not depend upon the exact chemical nature of 25 26 II Point Defects in Elemental Crystalline Substances the host crystal, it is convenient to treat the chemistry and physics of defects as separate entities.
eBook - PDF- A.K. Macpherson(Author)
- 2012(Publication Date)
- North Holland(Publisher)
CHAPTER 5 CRYSTAL DEFECTS 5 .1 Introduction Crystal defects are i m p o r t a n t in the study of properties of solids for two reasons. T h e first is that m a n y theories have been developed which d e p e n d on the translational symmetry of the crystal. If this condition is removed, then the complexity of the solution is greatly increased. This c a n be seen in the analysis using the h a r m o n i c a p p r o x i m a t i o n . O n c e a n h a r m o n i c terms b e c o m e i m p o r t a n t , then the analysis becomes either very difficult or impos-sible. Imperfections in the structure, whether d u e to a t o m s displaced from their sites or d u e to the inclusion of impurities, have the effect of p r o d u c i n g a n h a r m o n i c disturbances. If the defect density is small, it is often possible to either ignore the effects of the defects o r to treat t h e m as a p e r t u r b a t i o n of the crystal with translational symmetry. T h e second a n d m o s t i m p o r t a n t reason for the i m p o r t a n c e of crystal de-fects in the study of properties of solids is that they greatly influence the calculation of the properties of materials. If a calculation of the tensile strength of a perfect crystal is u n d e r t a k e n , it is found to be very m u c h greater t h a n the experimentally determined value. T h e difference is, in large part, due to the presence of defects. F u r t h e r m o r e the electrical resistivity is in-creased d u e to defects as scattering of electrons from the imperfections is found. If, as in the a b o v e -m e n t i o n e d examples, a small n u m b e r of defects c a n greatly influence a property, this p r o p e r t y is often referred to as a structure-sensitive property. Defects are present in almost all solids a n d are often introduced as impurities, e.g., in steel-making a n d semi-conductors, to obtain particular properties.
eBook - PDFInorganic Chemistry
An Industrial and Environmental Perspective
- Thomas W. Swaddle(Author)
- 1997(Publication Date)
- Academic Press(Publisher)
Chapter 5 The Defect Solid State 5.1 Inevitability of Crystal Defects IN THE PRECEDING chapter, it was tacitly assumed that crystalline solids were perfect, that is, that all of the sites characteristic of a particular struc- ture would be occupied, that the sites that should be vacant in the ideal structure would indeed be unoccupied, and that the atoms or ions making up the lattice were all of the specified kind. In practice, thermodynamics tells us that no crystal can ever be structurally perfect. The equilibrium state of the crystal will be one in which free energy is minimized, and this would seem to favor a perfect crystal lattice since misplaced or foreign ions will lead to a reduced lattice energy and hence to a less negative heat of formation. Disorder at the atomic level, however, will be reflected in a more positive entropy term, and the increase in the product T AS ~ will tend to compensate for the loss of lattice energy due to disorder, with increasing effectiveness as the temperature rises: 1-4 AG ~ = AH ~ - TAS ~ (5.1) Consequently, although hypothetical perfect crystals can be said to exist at the unattainable absolute zero of temperature, real crystals contain defects that increase in number (thermodynamics) and mobility (kinetics~ atoms can move from one site to another on surmounting an Arrhenius-type activation energy barrier) as the temperature rises. Eventually, these de- fects (e.g., thermal vibrations of atoms around their equilibrium positions, dislocations of atoms or planes of atoms from their ideal sites, creation of vacant sites) become severe enough that long-range atomic ordering breaks down, and the crystal melts. Even in the liquid some transient short-range ordering may persist, especially in ionic melts and in hydrogen-bonded liq- uids like water. The local ice like structures that form and decay continually in liquid water have been referred to as flickering clusters. 95
eBook - PDF- Derek Hull, D. J. Bacon(Authors)
- 2001(Publication Date)
- Butterworth-Heinemann(Publisher)
The rate at which a point defect moves from site to site in the lattice is proportional to exp( E m / kT ), where E m is the defect migration energy and is typically 0 : 1 1 : 0 eV. The rate decreases exponentially with Figure 1.10 (a) Vacancy, (b) self-interstitial atom in an (001) plane of a simple cubic lattice. Defects in crystals 9 decreasing temperature and consequently in many metals it is possible to retain a high vacancy concentration at room temperature by rapidly quenching from a high equilibrating temperature. Impurity atoms in a crystal can be considered as extrinsic point defects and they play a very important role in the physical and mechanical properties of all materials. Impurity atoms can take up two different types of site, as illustrated in Fig. 1.11: (a) substitutional , in which an atom of the parent lattice lying in a lattice site is replaced by the impurity atom, and (b) interstitial , in which the impurity atom is at a non-lattice site similar to the self-interstitial atoms referred to above. All the point defects mentioned produce a local distortion in the otherwise perfect lattice. The amount of distortion and hence the amount of additional energy in the lattice due to the defects depends on the amount of `space' between the atoms in the lattice and the `size' of the atoms introduced. The interstice sites between atoms generally have volumes of less than one atomic volume, and the interstitial atoms therefore tend to produce large distortions among the surrounding atoms. This accounts for the relatively large values of E i f referred to above, and can result in crystal volume increases as large as several atomic volumes per interstitial atom. Additional effects are important when the removal or addition of atoms changes the local electric charge in the lattice. This is relatively unimportant in crystals with metallic binding, but can be demonstrated particularly well in crystals in which the binding is ionic.
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