Solid-state chemistry crystal defects

So, by the 1990s, Professor Rao had been active in several of the major aspects which, together, were beginning to define materials chemistry crystal defects, phase transitions, novel methods of synthesis. Yet, although he has been president of the Materials Research Society of India, he does not call himself a materials chemist but remains a famous solid-state chemist. As with many new conceptual categories, use of the new terminology has developed sluggishly. 

In this and the following chapter, we will describe the most important simple (binary) crystal structures found in ceramic materials. You need to know the structures we have chosen because many other important materials have the same structures and because much of our discussion of point defects, interfaces, and processing will use these materials as illustrations. Some, namely FeSi, TiOi, CuO, and CU2O, are themselves less important materials and you would not be the only ceramist not to know their structure. We include these oxides in this discussion because each one illustrates a special feature that we find in oxides. These structures are just the tip of the topic known as crystal chemistry (or solid-state chemistry) the mineralogist would have to learn these, those in Chapter 7, and many more by heart. In most examples we will mention some applications of the chosen material. 

Schmaizried, H. (1964) Point defect in ternary ionic crystals, in Progress in Solid State Chemistry.vcA. 2 (ed. H. Reiss), North-Holland Publishing Co., Amsterdam, pp. 265-303. 

Based on these well-known prindples of solid-state chemistry, the early evolution of HMD may be associated with the nudeation stage the diamine volatilization results in creating new defective surfaces in the crystal lattice and in increasing the active centers for the nudeation of the generated polymer phase, which further grows following water formation. 

About five years later (1970) LeRoy organized another conference on the Chemistry of Extended Defects in Non-Metallic Crystals. Everyone came and it set the stage and the agenda for the immensely fruitful synergy of electron microscopy and solid state chemistry so beautifully exemplified by LeRoy s own work over the next three decades. 

Dislocation theory as a portion of the subject of solid-state physics is somewhat beyond the scope of this book, but it is desirable to examine the subject briefly in terms of its implications in surface chemistry. Perhaps the most elementary type of defect is that of an extra or interstitial atom—Frenkel defect [110]—or a missing atom or vacancy—Schottky defect [111]. Such point defects play an important role in the treatment of diffusion and electrical conductivities in solids and the solubility of a salt in the host lattice of another or different valence type [112]. Point defects have a thermodynamic basis for their existence in terms of the energy and entropy of their formation, the situation is similar to the formation of isolated holes and erratic atoms on a surface. Dislocations, on the other hand, may be viewed as an organized concentration of point defects they are lattice defects and play an important role in the mechanism of the plastic deformation of solids. Lattice defects or dislocations are not thermodynamic in the sense of the point defects their formation is intimately connected with the mechanism of nucleation and crystal growth (see Section IX-4), and they constitute an important source of surface imperfection. 

During the course of the last century, it was realized that many properties of solids are controlled not so much by the chemical composition or the chemical bonds linking the constituent atoms in the crystal but by faults or defects in the structure. Over the course of time the subject has, if anything, increased in importance. Indeed, there is no aspect of the physics and chemistry of solids that is not decisively influenced by the defects that occur in the material under consideration. The whole of the modem silicon-based computer industry is founded upon the introduction of precise amounts of specific impurities into extremely pure crystals. Solid-state lasers function because of the activity of impurity atoms. Battery science, solid oxide fuel cells, hydrogen storage, displays, all rest upon an understanding of defects in the solid matrix. 

If the technique of X-ray crystallography had attained sufficient refinement, it would be possible to determine by sin e-crystal analysis the nature of the defects present and their distribution over the possible sites in a structure, even for small defect concentrations. As it is, intensity measurements have been used in a few instances of grossly defective structure. The study of the structure of aggregates of defects is one which is of the utmost importance in the chemistry of the solid state refinement of the technique of X-ray or electron diffraction to the point where this is possible would be a significant advance. 

A.L.G. Rees, Chemistry of the Defect Solid State, Methuen, London, 1954. A. Kelly and G. W. Groves, Crystallography and Crystal Defects, Longman, London, 1970. 

If chemists are to be the atomic-molecular domain custodians of solid state materials science, a serious concern will need to exist for acquiring useful backgrounds in this area. A significant related publication is the recent appearance of F. A. Kroger s second edition of the Chemistry of Imperfect Crystals in three volumes. The defect chemistry concepts from this area need to be incorporated more generally into the new surface science which relates to the environmental... 

In order to be able to calculate the concentrations of point defects at thermodynamic equilibrium, it is necessary to know the change in free energy of the crystal which accompanies the formation of point defects, since the equilibrium is determined by the minimization of the free energy when the pressure, the temperature, and the other independent thermodynamic variables are given. A theoretical calculation of the free energy of formation of defects is still one of the most difficult problems in solid state physics and chemistry. The methods of calculation for each group of materials - metals, covalent crystals, ionic crystals - are all very... 

Charged defects in solids can interact with one another in an analogous way to the interactions between ions (or between ions and electrons) in a solution. In the solid-state situation, the crystal may be viewed as a neutral medium into which the charged defects are dissolved. This similarity between solution chemical interactions and defect interactions in the solid state has resulted in the field of defect chemistry, which provides basic methods for studying the effects of point defects in solids. The methods are normally applicable to fairly low defect concentrations. Generally, a broad distinction is made between intrinsic defects that are thermally gena-ated in pure compounds and extrinsic defects produced by external influences such as impurities and gaseous atmospheres. References 2 and 3 provide a detailed discussion of point defects and defect chemistry in metal oxides. 

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