Indexing unit cell selection

Because of this ambiguity, several other criteria shall be considered before an indexing result is accepted, i.e. a final unit cell selection is made, especially during the ab initio indexing. The somewhat related to one another norms are as follows (all other things are assumed equal) ... 

The arrangement of helices in the solid and liquid crystalline states of poly(a-phenylethyl isocyanide) were determined by X-ray and electron diffraction. Well-defined diffraction patterns were obtained from oriented films using selected area electron diffraction. Intermolecular and intramolecular patterns were calculated from the five Debye-Scherrer rings. All the reflections were indexed in terms of a pseudo-hexagonal triclinic unit cell, with... 

In hexagonal and trigonal crystal systems, the fourth index is usually introduced to address the possibility of three similar choices in selecting the crystallographic basis as illustrated in Figure 7.5 7. In addition to the unit cell based on the vectors a, b and c, two other unit cells, based on the vectors a, -(a + b) and c, and -(a + b), b and c are possible due to the six-fold or the... 

The success of this approach is critically dependent on the selection of the basis set, which generally should contain more Bragg peaks than the number of the unknown unit cell dimensions. Potential caveats include but are not limited to the following the selected basis set contains reciprocal vectors which are collinear or coplanar (see Figure 5.12) there is one or more impurity peak(s) in the basis set or in the list of Bragg peaks included into the consideration during the indexing attempt, and improper selection of the minimum and maximum h, k and / values for index permutations. 

The hydrothermal deactivation of Y zeolite containing 0, 4, 7 and 12 wt.% of REO and its effects on catalytic activity, stability and selectivity were investigated. The Y zeolites were hydrothermally deactivated at 788°C in three consecutive cycles of two hours each. The fresh and deactivated zeolites were characterized by measuring Unit Cell Size (UCS) and surface area. The acidic properties were measured by the Temperature Programmed Desorption (TPD) of ammonia and IR-pyridine desorption. In order to correlate structural, textural and acid properties with catalytic behavior, the zeolites were evaluated in the conversion of cyclohexane. The Hydrogen Transfer Index (HTI) measured as a ratio of paraffins to olefins is a parameter of the selectivity. It was found that the REO was incorporated into zeolite structure up to high concentrations modifying to some extent XRD deflection, the acidic properties and the HTI ratio. After deactivation, the acidity and HTI were diminished and the Lewis/Bronsted acid ratio was modified. HTI decreased as REO concentration increased. 

If the selected cell has M 20 (= M20) > 20 for triclinic crystal systems, or M20 > 30 i.e., M20 > 10) for monoclinic or higher symmetry crystal systems, it will be automatically refined by PI RUM, originally an interactive program, suitably modified to perform the automatic refinement of the unit-cell parameters. If more than 25 observed lines are available, the first 25 lines will be used for finding the cell, while all the lines will be involved in the refinement step. At the end of the PI RUM refinement a statistical study of the index parity of the assigned reflections is performed to detect the presence of doubled axes or of additional lattice points (A-, B-, C-, I-, R- or F-centred cell). If one of the index parity conditions is verified, an additional refinement is performed taking into account this information. 

Recently Fabian and Kalman highlighted certain limitations in the above indices, proposing that the degree of isostructurality of two or more structures be based instead on the extent of their volume overlap.They accordingly defined a "volume isostructurality index" represented by the ratio of the volume overlap of the two compared fragments and their average volumes. This parameter, computed over the entire unit cell, has merit in that it can be used selectively on two or more related inclusion compounds to measure the levels of isostructurality between the entire crystal structures, the hosts alone, or the guests alone. 

For polycrystalline materials, electron methods can be used to supplement and clarify X-ray results. For example, with X-ray powder patterns, it is often difficult or even impossible to index lines in a diffraction pattern if mixtures are present, whereas, using electron diffraction, the problem can be solved by obtaining unit cell data for each phase from selected-area diffraction patterns. Likewise, with complex powders of new materials, the indexing of X-ray patterns is especially difficult if the unit cell is large and of low symmetry. Electron diffraction patterns from small crystals reveal the reciprocal lattice, giving information on the principal axes and crystallographic symmetry. Many new materials are prepared as small samples of tiny crystals suitable only for electrons microscopy, so that electron diffraction is often the best way to obtain crystallographic data on the material. 

By controlling the hydrophobic/hydrophilic equilibrium of the polymers, their selectivity for bacterial cells over red blood cells (RBC) can be improved. This property is related to the effect that the ionic nature and hydrophobic character of these polymers has on cell membrane activity. The length of the alkyl substituents in the polymer repeat units affects antibacterial effectiveness in more hydrophobic polymers (hexyl and higher alkyl chain lengths in the repeat unit), disruption of the membrane integrity occurs more effectively. ROMP is an attractive method of synthesis which is widely used to prepare well-defined polymers with controlled MW and low polydispersity index values (PDl) [27-30]. 

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