Crystal imperfections Disorder

The effects of lattice disorder defects and crystal imperfection on channeling are used to analyze ion-implanted samples. Host atoms displaced from their lattice sites can interact with the channeled beam, leading to an increase in the scattering yield. 

Structure Used herein to refer to the molecular-level, crystallographic, three-dimensional arrangement of atoms, as controlled by the chemical bonding. Both long-range and short-range order/disorder, including crystal imperfections and defects, must be considered. 

YAG crystal where the inhomogeneous broadening is produced principally by crystal imperfections, b) a mixed crystal of YAG (78%) and YjGajOj, (22%) which produces broadening through compositional disorder and c) absorption broadened by the total disorder inherent in amorphous materials. After Imbusch and Kopelman (1981). 

Raman spectroscopy is primarily a structural characterization tool. The spectrum is more sensitive to the lengths, streng ths, and arrangement of bonds in a material than it is to the chemical composition. The Raman spectmm of crystals likewise responds more to details of defects and disorder than to trace impurities and related chemical imperfections. 

Indeed, in the world of tomorrow we can expect new aspects of polymer solids to extend the conventional and successful structure ideas of this century. These, of course, were the recognition as molecular identities of the chains of repeating chemical monomers. The circumstances of those entities have resulted in interesting concepts of solubilities, viscosity, and other mechanics, and especially thermodynamic limitations m mutual solubility or comparability of polymer mixtures. But we have known for decades that even homogeneous regular chain polymers such as Carothers polyesters and polyamides formed solids with manifold imperfections and irregularities, such as order-disorder crystal configurations.(22,23)... 

The total number of imperfections per unit surface characterizes what may be termed the disorder of the crystal surface. Two parts should be distinguished in the disorder biographical disorder, which is retained at the absolute zero of temperature and depends on the method of preparation of the sample and on its entire preceding history and thermal disorder, which increases on heating and is superposed on the biographical disorder. ... 

The surface imperfections being the centers of localization of free valencies may act as adsorption centers. In this case a real, but homogeneous surface may adsorb as an inhomogeneous one. This is because of thermal disorder on the surface of the crystal. 

In an ideal world, crystals would be perfect or stoichiometric with constant composition. But like people crystals are not exempt from imperfections or defects. Crystals with variable composition are termed non-stoichiometric crystals. The defect chemistry of oxides is enormously complex and is extremely vital to their properties. It has involved extensive research in many laboratories and is providing extraordinary insights into structural variations, the stability of structures and the formation of new structures. Here, we first define order-disorder phenomena that are commonly associated with oxides and describe our current understanding of them. The disorder or non-stoichiometry plays a crucial role in oxide applications including catalysis and it is therefore of paramount importance. 

Perfect crystals do not exist in real life and it has long been known that the physical properties of materials may depend at least as much on some deviations from the perfect periodicity as on the structure itself. Indeed, the structure is always averaged over a large number of unit cells and thus does not show atomic scale defects or disorder. Further, while the introduction of a controlled amount of imperfection in samples tells us very much about the physics of the material, on the contrary, uncontrolled sample imperfections may lead to incorrect or inaccurate deductions. 

Unsubstituted CP crystals all have quite similar chain packings. The crystal sizes are always small (ca. 100 A in all directions) and imperfectly ordered usually, chains are longer than the crystal dimensions. Therefore, the ideal CP is not found in practice chain geometries fluctuate along their length, as well the interchain interactions. This disorder is to be taken seriously in physical study, not as a small perturbation. 

To explain thermal motion and temperature effects, and the diffraction consequences of disorder and imperfections in crystals (see Chapter 13). 

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