Structural analysis crystal-phase structures

It is now well-established that for atomic fluids, far from the critical point, the atomic organisation is dictated by the repulsive forces while the longer range attractive forces serve to maintain the high density [34]. The investigation of systems of hard spheres can therefore be used as simple models for atomic systems they also serve as a basis for a thermodynamic perturbation analysis to introduce the attractive forces in a van der Waals-like approach [35]. In consequence it is to be expected that the anisotropic repulsive forces would be responsible for the structure of liquid crystal phases and numerous simulation studies of hard objects have been undertaken to explore this possibility [36]. 

Powder X-ray diffraction has verified that the isomerization occurs via a crys-tal-to-crystal reaction process, and that the diffraction profiles of the crystals after photoirradiation consist of overlapped patterns of diffraction due to the crystals of 10 and 11. This indicates that the crystal domains of each isomer exist simultaneously in the crystals accompanied by crystal phase separation during the photoisomerization. Single crystal structure analysis has disclosed that the crystals of 11 as the photoproduct have a symmetry different from that of the starting crystals of 10 (Fig. 14). 

There are two approaches to the solution of the phase problem that have remained in favor. The first is based on the tremendously important discovery or Patterson in the 1930s ihal the Fourier summation of Eq. 3. with (he experimentally known quantities F2 (htl> replacing F(hkl) leads nol to a map of scattering density, but to a map of all interatomic vectors. The second approach involves the use of so-called direct methods developed principally by Karie and Hauptman of the U.S. Naval Research Laboratory and which led to the award of the 1985 Nobel Prize in Chemistry. Building upon earlier proposals that (he relative intensities of the spots in a diffraction pattern contain information about a crystal phase. Hauptman and Karie developed a mathematical means of extracting the information. A fundamental proposition of (heir direct method is that if thrice intense spots in the pattern have positions whose coordinates add up to zero, their relative phases will cancel out. Compulations done with many triads of spots yield probable phases for a significant number of diffracted waves and further mathematical analysis leads lo a likely solution for the structure of the molecule as a whole. 

The recent advent of infrared microspectroscopy (10) suggested the possibility that infrared absorbance data could serve as the basis for analysis. It seemed unlikely that the absorbance of vater in a liquid crystal phase vould be anisotropic to the same degree as is refractive index. Further, infrared data could possibly provide qualitative information as to phase structure - a kind of information not contained in refractive index data (11). 

The consequences of product accumulation on the structural and chemical properties of a reacting crystal have been examined by several authors. These can be analyzed from a perspective that focuses on molecular changes or from a crystal phase point of view. A molecular analysis focuses on local structural properties that affect the reaction mechanism [55,57,58]. A solid-phase analysis focuses on how a crystalline ensemble reacts to the changes in composition that occur as the reaction proceeds [59-61]. However, as suggested in Scheme 5, a separation... 

Thus, the results of studies on sorption properties of adsorbents synthesized at elevated temperatures are affected by some factors whose action, as shown by the analysis, is directed towards reducing parameters of the structure of adsorbents produced. It is more difficult to answer the question about the contribution of each of them to this process and to estimate their effects. The difficulty consists in the fact that the effects of these factors are simultaneous and it is impossible to neglect at least one of them. It is only possible to suggest that the following two factors are the most effective decrease of association in solution that is responsible for formation of fine disperse particles and destabilization of their aggregative state, which facilitates formation of the crystal phase. It is a joint action of these factors that largely determines properties of the materials synthesized. 

Characterization. Actinide metal samples for the determination of properties related to bonding have to be characterized for chemical purity and phase homogeneity. Purity is checked by chemical or physical analysis, crystal structure is determined by X-ray or neutron diffraction techniques phase heterogeneities can be observed by metallography. 

XRD studies show that synthesized composites do not contain any crystal phase, just an amorphous phase. Optical absorption measurements prove that synthesized nanocomposites are containing Ti02 and Ti phases. For comparative analysis the pure Ti containing thin film was deposited onto the cold substrate (77 K) and onto the substrate at room temperature. The same result was obtained XRD analysis shows that the synthesized films only contain the amorphous phase. Kinetics of the electrical resistance increase with the air exposure of Ti/PPX nanocomposites (after synthesis under vacuum) is similar to that of the Al/PPX ones. For a metal content below the percolation threshold the metal particles became insulator within several seconds, whereas for the samples beyond the threshold the observed resistance increase is per cents within several hours. DTA analysis revealed that the heating of amorphous Ti02 nanoparticles up to a temperature of 480°C leads to a phase transformation to anatase, whereas heating up to 580°C results in the anatase transformation to the mtile structure. 

From the chemical point of view it seems to be of particular interest that further (metastable) tellurium subhalides can be obtained by hydrothermal syntheses in acid solutions. In the hydrothermal work on the crystal growth of arsenic telluro-iodides a pure binary tellurium subiodide with the composition Tcsl was obtained as well as a ternary phase with only small amounts of arsenic (AsT Ij The composition Tejl was checked by chemical as well as energy-dispersive X-ray analysis. Crystals of the new tellurium subiodide are shown in Fig. 43 a crystal structure analysis failed because the quality of the crystals was not suitable for single crystal investigation. During annealing of the subiodide a stable phase combination a-Tel -H Te was formed which indicated the metastable character of TesL... 

Organometallic and Coordination Compounds. - Crystallization of ferrocene and ruthenocene substituted in the 1- and T-positions by two nitronyl nitroxide radicals gave the new crystal phases p-1 (besides the known phase a-1, a-2, and a-2 whose structures were determined by X-ray analysis and were investigated by C and H NMR spectroscopy with MAS NMR. The solid state Sn CP MAS NMR spectra of a series of triaryltin chlorides of the form ArsSnCl have been acquired. The indirect spin-spin coupling constants (J( Sn- C1)), quadrupolar-dipolar shifts (6( Sn- Cl)), and the Sn chemical shift tensors were extracted. " Powders of the zinc and copper(II) dimethyl-(MDtc), diethyl-(EDtc), and morpholinedithiocarbamate (MfDtc) complexes quantitatively absorb hexamethyleneimine (Hmi) to produce the adducts... 

The crystal-liquid crystal phase transition behavior and the membrane structure in the artificial amphiphile/ water system were investigated on the basis of the thermal analysis, and wide- and small-angle x-ray analysis. 

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