Defect crystal

Different defects in solids will be described and some examples of super ionic conductors presented. 

A zero-dimensional defect in the lattice creates varying distances between the neighboring atoms, which also generate tensions. 

When talking about crystals, the word defect means that the regular long-range crystal structure is altered. Although this can seem bad, it s sometimes a useful property, especially in the semiconductor industry. 

Defects come in many shapes and sizes. When the regularity of the periodic 

1 Line defect A line defect can occur when one crystal plane doesn t extend uninterrupted all the way through the crystal. It s like having a stack of paper with one sheet slightly shorter than all the others. The paper above and below that shorter sheet experiences a slight strain because of the change. 

Point defect Point defects are important for the semiconductor industry and are classified according to whether they have vacancies, interstitials, or impurities. They re used to dope semiconductors for use in information technology industry. 

Plane defect Plane defect forms a plane that is cleavable because both planes don t match up at the defect location. Imagine a deck of cards where half the deck are sideways. The plane at which the two halves meet is the plane defect. 

One of the characteristics attributed to ionic compounds is solubility in water. In fact, many ionic compounds are not very soluble in water and, indeed, were it not for our familiarity with compounds like sodium chloride, ionic compounds would probably be thought of as insoluble. However, ionic compounds are more soluble in water than they are in less polar compounds such as hydrocarbons. And the converse is generally true for covalent compounds they are usually more soluble in organic solvents than in water. The solubility of some ionic compounds in water can be atttibuted to the interaction of the ions with the water molecules. To understand this interaction, we need to examine the electton density in the water molecule. 

Which atom is more electfonegative— hydrogen or oxygen  

Oxygen is more electtonegative. In fact, it is the second most electfonegative atom in the periodic table (see Appendix 2), being immediately to the left of the most electtonegative atom, fluorine. 

Ionic Compounds, By Qaude H. Yoder Copyright 2006 John WQey Sons, Inc. 

Draw an arrow through the water molecule, using the head of the arrow to show excess electron density and the tail of the arrow to show partial positive charge. 

Substituting the numerical values into this expression we arrive at 

A crystal represents a complex quantum mechanical system with an enormous amount of particles with a strong interaction between them. If all the particles are located in space strictly ordered with the formation of an ideal three-dimensional crystal structure, then such a system possesses minimal free energy. In a real crystal, however, the ideal periodicity is often broken due to inevitable thermal fluctuations some atoms break periodic array, abandon their ideal position and produce a defect. The frequency and amount of fluctuations are defined by the Boltzmann factor, i.e., they depend on temperature and binding strength or, in other words, on the depth of the potential well corresponding to the regular position of atoms. 

The role of defects in crystal properties is very high. On the other hand, new and improved old methods of defect crystal structure studies have appeared. This leads to the fact that, in recent decades, there has been extensive investigation into the physics of solids from the point of view of their deflection from perfection. Natural and new synthetic materials are being investigated. 

Let us distinguish point (zero-dimensional) and extended defects (dislocations). 

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As in crystals, defects in liquid crystals can be classified as point, line or wall defects. Dislocations are a feature of liquid crystal phases where tliere is translational order, since tliese are line defects in tliis lattice order. Unlike crystals, tliere is a type of line defect unique to liquid crystals tenned disclination [39]. A disclination is a discontinuity of orientation of tire director field. 

The atom radius of an element is the shortest distance between like atoms. It is the distance of the centers of the atoms from one another in metallic crystals and for these materials the atom radius is often called the metal radius. Except for the lanthanides (CN = 6), CN = 12 for the elements. The atom radii listed in Table 4.6 are taken mostly from A. Kelly and G. W. Groves, Crystallography and Crystal Defects, Addison-Wesley, Reading, Mass., 1970. 

Unfortunately, both EEEM and EIM microscopes require a conducting sample, usually metaUic, capable of being fashioned into a very tine point. The microscopes are used for study of crystal defects, purity, and, with EIM, the identification of single impurity atoms. 

MgCl2-Supported Catalysts. Examination of polymerizations with TiCl catalysts has estabUshed that only a small percentage of titanium located on lateral faces, edges, and along crystal defects is active (52) (see Titanium and titanium alloys). This led to the recognition that much of the catalyst mass acted only as a support, promoting considerable activity aimed at finding a support for active titanium that would not be detrimental to polymer properties. 

Dehydrogenation is considered to occur on the corners, edges, and other crystal defect sites on the catalyst where surface vacancies aid in the formation of intermediate species capable of competing for hydrogen with ethylbenzene. The role of the potassium may be viewed as a carrier for the strongly basic hydroxide ion, which is thought to help convert highly aromatic by-products to carbon dioxide. 

Batsanov, S.S., Dynamic Compression of Crystals, Defects and Their Influence on Physiochemical Transformations, Combust. Explos. Shock Waves 23 (1), 78-82(1987). 

Crystal Structure, Crystal Defects and Chemical Reactions... 

Nowick (1996) has outlined the researches done on crystal defects during the period 1949-1959 and called this the "golden age of crystal defects . A recent, very substantial overview (Kraftmakher 1998) admirably surveys the linkage between vacancies in equilibrium and thermophysical properties of metals this paper includes a historical table of 32 key papers, on a wide range of themes and techniques, 1926-1992. 

The early understanding of the geometry and dynamics of dislocations, as well as a detailed discussion of the role of vacancies in diffusion, is to be found in one of the early classics on crystal defects, a hard-to-find book entitled Imperfections in Nearly Perfect Crystals, based on a symposium held in the USA in 1950 (Shockley et al. 1952). Since in 1950, experimental evidence of dislocations was as yet very sparse, more emphasis was placed on a close study of slip lines (W.T. Read, Jr.,... 

Crystal structure, crystal defects and chemical reactions. Most chemical reactions of interest to materials scientists involve at least one reactant in the solid state examples inelude surfaee oxidation, internal oxidation, the photographie process, electrochemieal reaetions in the solid state. All of these are critieally dependent on crystal defects, point defects in particular, and the thermodynamics of these point defeets, especially in ionic compounds, are far more complex than they are in single-component metals. I have spaee only for a superficial overview. 

Transmission electron microscopes (TEM) with their variants (scanning transmission microscopes, analytical microscopes, high-resolution microscopes, high-voltage microscopes) are now crucial tools in the study of materials crystal defects of all kinds, radiation damage, ofif-stoichiometric compounds, features of atomic order, polyphase microstructures, stages in phase transformations, orientation relationships between phases, recrystallisation, local textures, compositions of phases... there is no end to the features that are today studied by TEM. Newbury and Williams (2000) have surveyed the place of the electron microscope as the materials characterisation tool of the millennium . 

In this book, the process of plastic deformation and the related crystal defects liave been discussed repeatedly. In Section 2.1.6, the distinction between continuum... 

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 summary, a hexagonal phase occurs in PE for the following reasons in Cases 2 and 3, because To-, increases (by constraining the melt) beyond 7 . /, and in Cases 4-6 primarily because To-,i, decreases below 7o, (by introduction of crystal defects). 

Effect of Crystal Defects on Corrosion—General Considerations... 

Before considering specific examples it is appropriate to note that there are, in principle, two quite distinct ways in which crystal defects can affect corrosion behaviour. 

Firstly, they might be expected to have an effect when corrosion occurs under conditions of active (film-free) anodic dissolution and is not limited by the diffusion of oxygen or some other species in the environment. However, if the rate of active dissolution is controlled by the rate of oxygen diffusion, or if, in general terms, the rate-controlling process does not take place at the metal surface, the effect of crystal defects might be expected to be minimal. 

Secondly, crystal defects might be expected to affect the corrosion behaviour of metals which owe their corrosion resistance to the presence of thin passive or thick protective films on their surface. The crystal defects and structural features discussed in Section 20.4 might, in principle, be expected to affect the thickness, strength, adhesion, porosity, composition, solubility, etc. of these surface films, and hence, in turn, the corrosion behaviour of the filmed metal surfaces. Clearly, this is the more common situation in practice. 

Finally, it should be noted that in both cases the effect of crystal defects and microstructural features must, in general, be to tend to make the corrosion less uniform and more localised. 

Active Dissolution and Crystal Defects—Energy Considerations... 

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