Defect foreign atoms

The catalyst particle is usually a complex entity composed of a porous solid, serving as the support for one or more catalytically active phase(s). These may comprise clusters, thin surface mono- or multilayers, or small crystallites. The shape, size and orientation of clusters or crystallites, the extension and arrangement of different crystal faces together with macrodcfects such as steps, kinks, etc., are parameters describing the surface topography. The type of atoms and their mutual positions at the surface of the active phase or of the support, and the type, concentration and mutual positions of point defects (foreign atoms in lattice positions, interstitials, vacancies, dislocations, etc.) define the surface structure. 

An alternative interpretation of electric conduction in solids is based on the fact that charge transfer is possible only in non-ideal lattices. The solid must have crystallographic defects. Foreign atoms as dopants in semiconductors can be considered as a defect itself, but not as the only one. 

Lattice Vacancies and Interstitials Defects such as lattice vacancies and interstitials fall into two main categories intrinsic defects, which are present in pure crystal at thermodynamic equilibrium, and extrinsic defects, which are created when a foreign atom is inserted into the lattice. 

No material is completely pure, and some foreign atoms will invariably be present. If these are undesirable or accidental, they are termed impurities, but if they have been added deliberately, to change the properties of the material on purpose, they are called dopant atoms. Impurities can form point defects when present in low concentrations, the simplest of which are analogs of vacancies and interstitials. For example, an impurity atom A in a crystal of a metal M can occupy atom sites normally occupied by the parent atoms, to form substitutional point defects, written AM, or can occupy interstitial sites, to form interstitial point defects, written Aj (Fig. 1.4). The doping of aluminum into silicon creates substitutional point defects as the aluminum atoms occupy sites normally filled by silicon atoms. In compounds, the impurities can affect one or all sublattices. For instance, natural sodium chloride often contains... 

These novel carbon nanostructures can also be modified by (a) doping, that is the addition of foreign atoms into the carbon nanostructure, (b) by the introduction of structural defects that modify the arrangement of the carbon atoms and (c) by functionalization involving covalent or noncovalent bonding with other molecules. These modifications opened up new perspectives in developing novel composite materials with different matrices (ceramic, polymer and metals). For example, polymer composites containing carbon nanostructures have attracted considerable attention due to... 

States due to different biographical structural defects existing on any real surface and playing the part of local disturbances in the strictly periodic structure of the surface (Sec. IX,A). These include vacant lattice sites in the surface layer of the lattice, atoms or ions of the lattice ejected onto the surface, and foreign atomic inclusions in the surface of the lattice (surface impurities). 

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.)... 

One of the causes of point defects is a temperature increase which results in an increased thermal movement of the atoms which can subsequently lead to the atoms escaping from their place in the lattice. Other causes are the effects of radiation and inbuilt, foreign atoms. In an atomic lattice a vacancy can occur due to the movement of an atom, an absence of an atom or molecule from a point which it would normally occupy in a crystal. In addition to this vacancy an atomic will form elsewhere. This combination of an atomic pair and a vacancy is called the Frenkel defect. In ionic crystals an anion and a cation have to leave the lattice simultaneously due to the charge balance. As a result a vacancy pair remains and this is called the Schottky defect. Both defects can be seen in figure 4.8. 

Crystals contain two major categories of defect point defects and line defects. Point defects occur where atoms are missing (vacancies) or occupy the interstices between normal sites (interstitials) foreign atoms are also point defects. Line defects, or dislocations, are spatially extensive and involve disturbance of the periodicity of the lattice. 

Lattice parameters (LPs) of a semiconductor depend on the following factors [1] (i) chemical composition (including deviation from stoichiometry), (ii) presence of free-charge acting via the deformation potential of the energy-band extremum occupied by this charge, (iii) presence of foreign atoms and defects, (iv) external stresses (for example, exerted on a heteroepitaxial layer by its substrate), and (v) temperature. These factors are not independent [1], For nitrides, studies of such factors are in a state of infancy. 

One of the more recent studies by Zerfoss and Davis (46) reveals that the /3-orthosilicate of calcium can be stabilized by some foreign atoms which form defective solid solutions and by others which physically protect the crystal boundary from initiating the reaction. 

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 Defects and Phase Diagrams. As will become more evident in subsequent parts of this chapter, substitutional impurities are one of the key types of point disorder. These defects correspond to foreign atoms that are taken into the lattice and which occupy sites normally reserved for the host atoms. For example, in the case of fee A1 some small fraction of the host lattice sites can be occupied by Cu... 

An example of the type of data associated with solution hardening it is the mission of our models to explain was shown in fig. 8.2(a). For our present purposes, there are questions to be posed of both a qualitative and quantitative character. On the qualitative side, we would like to know how the presence of foreign atoms dissolved in the matrix can have the effect of strengthening a material. In particular, how can we reconcile what we know about point defects in solids with the elastic model of dislocation-obstacle interaction presented in section 11.6.2. From a more quantitative perspective, we are particularly interested in the question of to what extent the experimental data permit a scaling description of the hardening effect (i.e. r oc c") and in addition, to what extent statistical superposition of the presumed elastic interactions between dislocations and impurities provides for such scaling laws. 

Samples of diamond sometimes also exhibit considerable fluorescence. Especially type I diamonds bearing impurities of nitrogen have a pronounced spectrum with two maxima already known from UV/Vis absorption. (A = 415 nm most likely from transitions without participation of foreign atoms or vacancies, but at defects generated from the breaking of C-C-a-bonds and A = 503 nm from transitions including foreign atoms on lattice positions.)... 

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