Crystal dynamic effects

The method of strueture analysis developed by the Soviet group was based on the kinematieal approximation that ED intensity is directly related (proportional) to the square of structure factor amplitudes. The same method had also been applied by Cowley in Melbourne for solving a few structures. In 1957 Cowley and Moodie introdueed the -beam dynamical diffraction theory to the seattering of eleetrons by atoms and crystals. This theory provided the basis of multi-sliee ealeulations whieh enabled the simulation of dynamieal intensities of eleetron diffraetion patterns, and later electron microscope images. The theory showed that if dynamical scattering is signifieant, intensities of eleetron diffraetion are usually not related to strueture faetors in a simple way. Sinee that day, the fear of dynamical effects has hampered efforts to analyze struetures by eleetron diffraction. 

The combination of the precession and scanning of the ED pattern reduces the sensitivity of ED intensities to crystal thickness, reduces the effect of Ewald sphere curvature and can also reduce a lot of multiple beam dynamical effects contribution to the reflections (see fig. 3). 

From soluble compounds like FAPPO, crystals of approx. 100-200 A thickness have been grown from thin solutions directly on a carbon coated copper grid (3mm/300 mesh) at room temperature by evaporation. With this method thin crystals with less dynamical effects are usually obtained which he flat on the grid providing a view along the thinnest direction. 

In this work we perform an investigation of cooperative static in the monoclinic phase and dynamic in rhombohedral JT effect of pure LaMn03 using pair interionic potentials in shell model approximation with the direct inclusion of the JT term in crystal energy and dynamic matrix of a crystal. The magnetic and RS properties of the rhombohedral LaMn03 are simulated in the framework of the cooperative dynamical effect approximation. 

The application of a flow field has pronounced effects on the crystallization dynamics and semi-crystalline morphology of polymeric systems. Under quiescent, no-flow conditions the crystallization dynamics is governed by the temperature, T, and spherulites are formed. In the presence of flow, increasing the flow rate can result in an increase of the nuclei density up to factors of 106, but the morphology remains spherulitic [18]. At even higher flow rates the so-called shish-kebab morphology is observed. This results from the de-... 

In the next section the rare-earth compounds that have been studied by optical means under pressure so far will be reviewed. Then, after a brief introduction of the most commonly used high pressure device, the diamond anvil cell, sect. 4 presents a discussion of the pressure-induced changes of the crystal-field levels and their interpretation. In sects. 5 and 6 some aspects of the dynamical effects under pressure are discussed. These include lifetime and intensity measurements, the influence due to excited configurations and charge transfer bands, and the electron-phonon coupling. 

Solvent can affect the electronic structure of the solute and, hence, its magnetic properties either directly (e.g. favouring more polar resonance forms) or indirectly through geometry changes. Furthermore, it can influence the dynamical behaviour of the molecule for example, viscous and/or oriented solvents (such as liquid crystals) can strongly damp the rotational and vibrational motions of the radical. Static aspects will be treated in the following, whereas the last aspect will be tackled in the section devoted to all the dynamical effects. 

Lateral shifts are also due to depth penetration and refer to a shift in the exit location of an X-ray at the crystal surface. This results in a transverse shift at the detector, typically 10%-100% of refractive index corrections [20]. While ray-tracing can adequately describe geometrical effects outside the crystal, the effects of depth penetration and lateral shifts require full dynamical diffraction theory. 

Also, external parameters influencing the crystal field effects (besides the temperature-dependent dynamical behaviour) such as pressure or external electric fields have to be taken into account. 

Of the two main parts A) and B) of crystal field effects, effect B, the influence of lattice dynamics on NQR, can be excluded, at least to a large extent, by an appropriate experimental method, that is, by determining the nuclear quadru-... 

The effect of temperature on the bulk structure can be studied by free energy calculations and by crystal dynamics simulations. Infra-red and Raman spectra, and certain inelastic neutron scattering spectra directly reflect aspects of the lattice dsmamics. Infra-red spectra can be simulated firom the force constant matrix, based on interatomic potential models [94-97]. The matching of simulated mode fiequencies with those measured in Raman or IR spectra can indeed be used to develop, validate or improve the form and parameterization of the interatomic potential functions [97]. 

Three different isothermal crystallization experiments were performed in this work classical static (i.e., quiescent) crystallization in the DSC apparatus, dynamic crystallization with the apparatus described above, and dynamic-static crystallization. Dynamic isothermal crystallization consisted in completely solidifying cocoa butter under a shear in the Couette apparatus. Comparison of shear effect with results from literature was done using the average shear rate y. This experiment did not allow direct measurement of the solid content in the sample. However, characteristic times of crystallization were estimated. The corresponded visually to the cloud point and to an increase of the cocoa butter temperature 1 t) due to latent heat release. The finish time, was evaluated from the temperature evolution in cocoa butter. At tp the temperature Tit) suddenly increases sharply because of the apparition of a coherent crystalline structure in cocoa butter. This induces a loss of contact with the outer wall and a sharp decrease in the heat extraction. 

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