Phase identification defects

Liquid crystal phases possess characteristic textures when viewed in polarized light under a microscope. These textures, which can often be used to identify phases, result from defects in tire stmcture. Compendia of micrographs showing typical textures exist to facilitate phase identifications [37, 38]. These monographs also discuss tire origins of defect stmctures in some detail. 

Besides phase identification XRD is also widely used for strain and particle size determination in thin films. Both produce peak broadenings, but they are distinguishable. Compared to TEM, XRD has poor area resolution capability, although by using synchrotron radiation beam diameters of a few pm can be obtained. Defect topography in epitaxial films can be determined at this resolution. 

Between crossed polars these defects appear as dark lines or brushes with curved or irregular shapes that correspond to extinction positions of the director and molecular long axes. Thus, the director can be either parallel or perpendicular to the polarizer and analyzer. The brushes tend to cover the specimen in rather a continuous way, indicating the liquid-like nature of the mesophase. The points where the brushes meet are called singularities in the texture (see Figure 3A). For nematic phases two forms of schlieren defect are found, one where two brushes meet at a point and one where four brushes meet. All tilted smectic phases (C, I, F, and ferrielectric C), except for the antiferroelectric phase, exhibit four brush singularities. Therefore, this provides a simple way of distinguishing between smectic and nematic phases. It should be noted that phases such as smectics A and B(hexatic) and crystal phases B(crystal), E, G, H, J, and K do not exhibit schlieren textures and so this narrows down the possibilities for phase identification. 

The major area of application for solids and liquids is chemical fingerprinting and the identification of unknown compounds. For solids, Raman is also used for phase identification, following amorphous/crystalline transitions, measurement of stress and strain, and, in the microscope mode, the detection and analysis of defects, including particles during wafer processing. 

Liquid crystal materials may exhibit birefringence (discussed in Section 2.5), so polarized optical microscopy is an ideal technique for visualization of liquid crystal textures. Samples are prepared as a thin film ( 2-20 pm thick) between glass plates. By observing the interesting defect textures that may form in each phase, it is often possible to make accurate phase identifications by microscopy alone, even though the microscope is unable to resolve the actual molecular packing structure. 

The nature of the clustered phase is not well understood. One possible interpretation is that a cluster is composed of a relaxed divacancy whose inner surface is dressed with hydrogens however there is no direct NMR data which supports this identification (Reimer and Petrich, 1988). Alternatively, it has been suggested that the broad component of the NMR spectrum arises from hydrogen atoms lined up alone microtubular structural defects (Chenevas-Paule and Bourret, 1983). 

Electron diffraction performed with a parallel incident beam, i.e. Selected-Area Electron Diffraction is used to obtain good electron micrographs. The two-beam condition allows the observation of defects. SAED is also used in High-Resolution Electron Microscopy (HREM) to set a crystal to a zone axis so that the atomic columns are vertical in the microscope. SAED is very useful for the identification of phases and the... 

Regardless of whether the non-imaging of a species is due to preferential field evaporation or to preferential field ionization, the distinguisha-bility of alloy components in ordered alloys makes much easier the identification of lattice defects and of all types of domains, such as orientational and translational domains, and the discernment of order-disorder phase boundaries in ordered alloys, as well as facilitating the study of clustering and order-disorder phase transformation, etc.88 In most cases, image interpretations become self-obvious. For example in PtCo, which has the LI 0 structure, a Co layer can be distinguished from a... 

Techniques of transmission electron microscopy have proved valuable in many areas of solid state science. Use of electron diffraction permits identification of crystal types, determination of unit cell sizes and characterization of crystal defects in the phases. Measurement of Energy Dispersive X-ray (EDS) line intensity allows calculation of the elemental composition of the phases. It is difficult to overestimate the value of such applications to metallic alloys, ceramic materials and electron-device alloys (T-4V Applications to coal and other fuels are far fewer, but the studies also show promise, both in characterization of mineral phases and in determination of organic constituents (5-9. This paper reports measurements on a particular feature of coal, the spatial variation of the organic sulfur concentration. 

Photophysics in the condensed phase is much more involved than in solutions. In fact, there is much debate on the subject and the nature of the primary excitations are not easily assigned. Due to new mechanisms not yet elucidated or identified, singlet, triplet and charged states are all possible candidates for observation at short times, and there are experimental claims and evidence for each of them [97-103]. Defects and impurities may mask the identification of molecular states, which can also be substantially modified by intermolecular interactions [61,104,105]. 

Thus, these mesostructures are predominantly lamellar, and identified as conventional (parabolic) lamellar phases, although they may in fact be hyperbolic. Indeed, unless v/al is exactly unity, a planar interface (lamellar mesophase) incurs a bending energy cost hyperbolic sponge monolayers or bilayers or mesh monolayer mesophases are favoured if v/al differs from unity. It is likely then that many "lamellar"" phases in fact adopt a hyperbolic geometry. Careful neutron-scattering studies of a lamellar phase have revealed the presence of a large number of hyperbolic "defects" (pores within the bilayers) in one case [16]. (An example of this mis-identification of hyperbolic phases in block copolymers is discussed in section 4.10.)... 

It is clear that electron microscopy is not the most favourable technique for structure determination of new (superconducting) phases X-ray diffraction and particularly neutron diffraction do a far better job in the ab initio structure determination. Electron microscopy and electron diffraction are extremely powerful however to determine the local structure i.e. to detect deviations from the average structure, as determined by X-rays or neutrons. In this way several new phases have been first identified by electron microscopy some of them have been later made into bulk superconductors. In other cases the identification of isolated defects in an existing material have inspired chemists to produce new superconducting materials this was, for example, the case for the occurrence of double HgO layers in a one-layer Hg-1223 superconductor. 

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