Solid-state structures defects

The different responses of the NXL and XL bands would not occur if the NXL material were present as defects within tire XL phase. These data support a two-phase model of solid state structure in semicrystalline PTLE. Of course, the precise location of the two phases is difficult to specify, but for the XL phase we have good data on its initial morphology and subsequent changes based upon microscopy and X-ray diffraction. 

The purpose of this chapter is to explain the difference between point defects and extended defects and the effect these mechanisms have on solid state structure and properties. The principal of solid solutions will also be introduced to show how the solid state scientist can use the size and preferred geometry of cations to synthesize new materials. 

The defects we have discussed in this chapter are largely microscopic and cannot be observed from the macroscopic structure of the materials. However, there are various sorts of macroscopic defect which can be examined using electron microscopy, and which explain certain physical characteristics. For example, metals are generally malleable and ductile but their ordered solid state structure implies that they should be rigid. Sometimes heating metals makes them more brittle in a process known as work hardening . These characteristics indicate that the structures of the metals are not perfect. The malleability of metals is an indication that the structure contains defects which occur in lines and planes, allowing the atoms to slip over each other. As the temperature rises or the metal is worked (as by a blacksmith), the metal becomes harder as the defects are removed. 

Aluminium oxide occurs in two main forms a-alumina corundum) and Y-AI2O3 activated alumina). The solid state structure of 0t-Al2O3 consists of an hep array of ions with cations occupying two-thirds of the octahedral interstitial sites. ot-Alumina is extremely hard and is relatively unreactive (e.g. it is resistant to attack by acids). Its density (4.0gcm ) exceeds that of Y-AI2O3 (3.5gcm ) which has a defect spinel structure (see Box 13.6 and Section 21.10). The a-form is made by dehydrating... 

Packing of spheres Polymorphism Alloys and intermetallic compounds Band theory Semiconductors Sizes of ions Prototype structures Lattice energy Born-Haber cycle Defects in solid state structures... 

Various imperfections in solid-state structures include the Schottky and Frenkel defects. The latter can lead to nonstoichiometric compounds. Edge dislocations can make a metal more malleable. Examples include lead, white tin, and the iron of horseshoes. 

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. 

In this chapter shock modification of powders (their specific area, x-ray diffraction lines, and point defects) measurements via analytical electron microscopy, magnetization and Mossbauer spectroscopy shock activation of catalysis, solution, solid-state chemical reactions, sintering, and structural transformations enhanced solid-state reactivity. 

Marvel et al. described [41] the polymerization of 5,6-dibromocyclohexa-1,3-diene (16) to poly(5,6-dibromo-l,4-cyclohcx-2-ene) 17 followed by a thermally induced, solid state elimination of HBr on the formation of PPP 1. The products, however, display some indications for several types of structural defects (incomplete cyclization, crosslinking). 

The SCF method for molecules has been extended into the Crystal Orbital (CO) method for systems with ID- or 3D- translational periodicityiMi). The CO method is in fact the band theory method of solid state theory applied in the spirit of molecular orbital methods. It is used to obtain the band structure as a means to explain the conductivity in these materials, and we have done so in our study of polyacetylene. There are however some difficulties associated with the use of the CO method to describe impurities or defects in polymers. The periodicity assumed in the CO formalism implies that impurities have the same periodicity. Thus the unit cell on which the translational periodicity is applied must be chosen carefully in such a way that the repeating impurities do not interact. In general this requirement implies that the unit cell be very large, a feature which results in extremely demanding computations and thus hinders the use of the CO method for the study of impurities. 

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