Defect structure of solid

Figure 27. Defective structure of solid trifluoromethane-sulfonic acid hydrate (CF3S0sH H20)4 found using ab initio molecular dynamics (AIMD see Section 2.2.3 for a description of the technique), showing two hydronium ions hydro-gen-bonded to sulfonate groups (as found in the perfect structure) but, more importantly, two shared protons (one between two sulfonate groups and the other as part of a Zundel ion see text). Note that the energy of the defective structure is only --30 kj/mol higher than that of the perfect structure.

Although the electrical properties of materials depend on numerous factors, it is generally recognized that the crystal and defect structures of solids are of key importance in our understanding of conductivity mechanisms. 

The Mdssbauer effect is rapidly assuming an important place as a new technique for investigating chemical problems. It may be used to obtain evidence about the crystallographic and defect structures of solids and their surface properties. But for chemists perhaps more important are the inferences which maybe made about the electronic configurations, and the internal magnetic and electric fields in atoms. 

In the last chapter we examined data for the yield strengths exhibited by materials. But what would we expect From our understanding of the structure of solids and the stiffness of the bonds between the atoms, can we estimate what the yield strength should be A simple calculation (given in the next section) overestimates it grossly. This is because real crystals contain defects, dislocations, which move easily. When they move, the crystal deforms the stress needed to move them is the yield strength. Dislocations are the carriers of deformation, much as electrons are the carriers of charge. 

In the first chapter, we defined the nature of a solid in terms of its building blocks plus its structure and symmetry. In the second chapter, we defined how structures of solids are determined. In this chapter, we will examine how the solid actually occurs in Nature. Consider that a solid is made up of atoms or ions that are held together by covalent/ionic forces. It is axiomatic that atoms cannot be piled together and forced to form a periodic structure without mistakes being made. The 2nd Law of Thermodynamics demands this. Such mistakes seriously affect the overall properties of the solid. Thus, defeets in the lattice are probably the most important aspect of the solid state since it is impossible to avoid defects at the atomistic level. Two factors are involved ... 

We have Investigated the structure of solids In the second chapter and the nature of point defects of the solid in the third chapter. We are now ready to describe how solids react. This will Include the mechanisms Involved when solids form by reaction from constituent compounds. We will also describe some methods of measurement and how one determines extent and rate of the soUd state reaction actually taking place. We will also show how the presence and/or formation of point defects affect reactivity In solid state reactions. They do so, but not In the memner that you might suspect. We will also show how solid state reactions progress, particularly those involving silicates where several different phases appear as a function of both time and relative ratios of reacting components. 

Ueng, H. Y. Hwang, H. L. 1989. The defect structure of copper indium sulfide (CuInS2). Part I. Intrinsic defects. I. Phys. Chem. Solids 50 1297-1305. 

As in the case of varistors, the effect is due to the defect structure of the solid, especially the presence of grain boundaries. There are a considerable number of variables that must be controlled to make a suitable PCT thermistor. 

When the random-walk model is expanded to take into account the real structures of solids, it becomes apparent that diffusion in crystals is dependent upon point defect populations. To give a simple example, imagine a crystal such as that of a metal in which all of the atom sites are occupied. Inherently, diffusion from one normally occupied site to another would be impossible in such a crystal and a random walk cannot occur at all. However, diffusion can occur if a population of defects such as vacancies exists. In this case, atoms can jump from a normal site into a neighboring vacancy and so gradually move through the crystal. Movement of a diffusing atom into a vacant site corresponds to movement of the vacancy in the other direction (Fig. 5.7). In practice, it is often very convenient, in problems where vacancy diffusion occurs, to ignore atom movement and to focus attention upon the diffusion of the vacancies as if they were real particles. This process is therefore frequently referred to as vacancy diffusion... 

This indicates that the defect structure is complex and may vary with degree of doping. Further studies are needed to clarify the defect structure of this notionally simple solid. 

In practice, the defect structure of the materials LiJCo, M)02 and Lix(Ni, M)02 under oxidizing conditions found at cathodes, is complex. For example, oxidation of Fe3+ substituted lithium nickelate, LL(Ni, Fe)02, under cathodic conditions leads to the formation of Fe4+ and Ni4+. Conductivity can then take place by means of rapid charge hopping between Fe3+, Ni3+, Fe4+, and Ni4+, giving average charges of Fe3+S and Ni3+S. These solids are the subject of ongoing research. 

Tsoga A, Naoumidis A, and Stover D. Total electrical conductivity and defect structure of Zr02-Ce02-Y203-Gd203 solid solutions. Solid State Ionics 2000 135 403M09. 

A convenient description of die crystalline structure of solids is thus seen to consist of successive stages of approximation. First, the mathematically perfect geometrical model is described then departures from diis perfect regularity are permitted. The defonuabilily of solids is allowed for by letting die force constants between adjacent atoms be finite, not infinite. Then, misplacement of atoms is permitted and a variety of crystalline irregularities, called defects, is described. Some of these defects have intrinsic features which affect properties of die crystal other affect the properties by their motion from site to site in the crystal. In spite of their relatively small number, defects are of immense importance. 

Site selective laser spectroscopy is a very powerful tool for studying the local environments that are present in samples. We have shown that defect structures in solids are determined by its previous history and that these structures can be measured with site selective spectroscopy. There has been no application of such techniques to geologically important questions since our work has concentrated on understanding fundamental questions about solid state defect chemistry. Our work suggests though that site selective laser spectroscopy could have important application in geological studies if it were used in the hands of people with that background. 

Lattice defects can function both as donors and as acceptors and create free electrons or electron holes. Crystalline surfaces containing unsaturated electron valences act as electron traps and capture free electrons. This leads to changes in binding conditions and in the charge state of e.g. metal ions their ability to polarize O- in a metal oxide decreases. Surface oxidation during the grinding process often causes deep alterations of the surface structure of solids (sulphides, graphite, coal). This usually leads to increases in affinity toward water and in reactivity with the surfactant. 

The idea that the defect structure of a solid reactant affects the rate of decomposition seems to be generally accepted but of all the factors influencing the kinetics, this is the one most difficult to characterize quantitatively. Boldyrev and his many co-workers28 have made considerable progress in elucidating the factors which affect the decomposition rate of solids however, at the level of detail required for the understanding of a particular reaction, it seems difficult to make connections to the standard kinetic equations derived by earlier workers. 

Molecular Structure by Diffraction Methods Vol. 6 Senior Reporters Prof. M. R. TRUTER and Dr. L. E. SUTTON Hardcover 348 pp 0 85186 557 7 Chemical Physics of Solids and their Surfaces Vol. 8 (successor to "Surface and Defect Properties of Solids") Senior Reporters Prof. M. W. ROBERTS and Prof. J. M. THOMAS Hardcover 260 pp 0 85186 740 5... 

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