Impurity-crystal interactions, effect

It is also worthwhile to note that it is also possible to establish a close relation between the crystal field effects, covalent effects (overlap between the wave functions of an impurity ion and ligands) and electron-phonon interaction and JT effects [52-54]. It was shown in these works that it is possible to distinguish and analyze separately different contributions (arising from the point charge and exchange interactions) to the vibronic effects. 

Redistribution of the phonon density of states due to local deformations caused by an introduction of an impurity Ln ion is of primary importance for electron-phonon interaction effects. In particular, for Pr CsCdBr3 the effective electron-phonon coupling is strongly suppressed due to a local increase of elastic forces in the activated crystal and the corresponding enhancement of correlation between displacement of the impurity Ln ion and its neighbors. When dealing with nanostructured materials, it is important to take into account... 

Optical spectroscopy has been employed for a long time as an effective tool to investigate molecular dynamics and relaxations in molecular impurity crystals. The spectroscopic methods work effectively in crystals because impurity bands have a well-resolved structure. Optical bands of such type can be treated with the help of the comprehensively developed theory for homogeneous optical band shape. The theory enables one to get information concerning both the electron-phonon interaction and the molecular dynamics in crystals. 

Very generally, point defects distort locally and induce electronic perturbations in the crystal these effects lead to elastic and electronic interactions between them, as well as with other defects (dislocations, grain boundaries, etc.). The complex defects so formed may introduce more distortion into the lattice than simple point defects, and therefore have greater effects on the mechanical properties. With increasing temperature, point defects become more and more randomly positioned, and complex defects (e.g. divacancies, vacancy-impurity or interstitial-impurity bound pairs) dissociate. 

Sauer et al. [185] derived a weak quadmpole interaction from the asymmetry of a poorly resolved Zeeman split spectmm of in W metal versus a Ta metal absorber. They also ascribed the unexpected weak quadmpole effect to deviations from cubic symmetry at the source or absorber atom arising from either interstitial impurities or crystal defects. 

These effects have proved important in improving the methods available for resolution of enantiomers by crystallization (267). Furthermore, by studies of the morphological changes induced, one may determine the faces at which the impurities are dominantly attached (270,271). Then, in suitable systems, it is possible to determine the absolute configuration of a polar crystal if one knows that of the impurity (272), or to determine that of the impurity if one knows the structure of the centrosymmetric crystal with which it interacts (270). 

The usefulness of quadrupolar effects on the nuclear magnetic resonance c I 7 yi nuclei in the defect solid state arises from the fact that point defects, dislocations, etc., give rise to electric field gradients, which in cubic ciystals produce a large effect on the nuclear resonance line. In noncubic crystals defects of course produce an effect, but it may be masked by the already present quadrupole interaction. Considerable experimental data have been obtained by Reif (96,97) on the NMR of nuclei in doped, cubic, polycrystalline solids. The effect of defect-producing impurities is quite... 

Since the edge free energies, y, are different for the vapor and solution phases, and particularly for solute-solvent interaction energies, the same crystal species will exhibit different Tracht and Habitus in different ambient phases and different solvents. If impurities are present in the system, this affects y and the advancing rates of steps. There are two opposite cases in impurity effects, and, depending on the interface state, some will promote growth, whereas others will suppress growth. 

Compared with the momentum of impinging atoms or ions, we may safely neglect the momentum transferred by the absorbed photons and thus we can neglect direct knock-on effects in photochemistry. The strong interaction between photons and the electronic system of the crystal leads to an excitation of the electrons by photon absorption as the primary effect. This excitation causes either the formation of a localized exciton or an (e +h ) defect pair. Non-localized electron defects can be described by planar waves which may be scattered, trapped, etc. Their behavior has been explained with the electron theory of solids [A.H. Wilson (1953)]. Electrons which are trapped by their interaction with impurities or which are self-trapped by interaction with phonons may be localized for a long time (in terms of the reciprocal Debye frequency) before they leave their potential minimum in a hopping type of process activated by thermal fluctuations. 

Thus, one may summarize the physical picture of the relaxation dynamics in KTN crystal-doped with Cu+ ions in the following way In the paraelectric phase, as the ferroelectric phase transition is approached, the Nb5+ ions form dipolar clusters around the randomly distributed Cu+ impurity ions. The interaction between these clusters gives rise to a cooperative behavior according to the AG theory of glass-forming liquids. At the ferroelectric phase transition the cooperative relaxation of the Cu+ ions is effectively frozen. ... 

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