Factors influencing crystal structure

Fundamentally, the solvent can influence crystal structure, crystal size, morphology, and purity by modifying solution properties (i.e., density, viscosity, and component diffusivities), solute solubility, as well as the structure of the crystal-liquid interface. The influence of the solvent on the first three factors are well known especially morphology (e.g., Srinivisan et al. 2000 Maruyuma et al. 2000 Wang et al. 1999 Walker 1997 Khoshkhoo et al. 1996 Blagden et al. 1998 Roberts et al. 1994 Geertman et al. 1992 Docherty et al. 1991 Shimon et al. 1990 van der Voort et al. 1990). The solvent properties affecting heat and mass transport... 

Thermal condnctivity is strongly influenced by such factors as crystal structure, atomic weights, grain size, and porosity. Figure 3.6 illustrates how thermal conductivity is affected by differences in atomic weights of constituent atoms. The greater... 

Investigation of the differences in crystal packing between (431) and (426) from comparison of their respective X-ray structures, revealed that (431) was more tightly packed than (442), reflected in their respective melting points of 235 and 170 °C. It was postulated that the absence of in vivo activity for (431) may be explained by the resultant reduction in water solubility and dissolution rate compared with (426). The comparatively high calculated polar surface area of (431) (122.5A ) compared with (426) (89.3 A ) was also proposed as a factor influencing the marked difference in bioavailability between the two related compounds. Compound (426) (SLV-319) is currently being developed with Bristol-Myers Squibb for the potential treatment of obesity and other metabolic disorders. Phase I trials for obesity were started in April 2004. Earlier Phase I clinical trials for the treatment of schizophrenia and psychosis, which commenced in April 2002, appear to have been abandoned. 

Numerous investigators have attempted to control the precursor structure and related solution chemistry effects with varying degrees of success, to influence subsequent processing behavior, such as crystallization tempera-ture.40-42,78,109 110 Particular attention has been given to precursor characteristics such as structural similarity to the desired product and the chemical homogeneity of the precursor species. For multicomponent films, this latter factor is believed to influence the interdiffusional distances associated with the formation of complex crystal structures, such as perovskite compounds. Synthetic approaches have been geared toward the preparation of multimetal species with cation stoichiometry identical to that of the desired crystalline phase.40 42 83 84... 

The diffusion of an impurity atom in a crystal, say K in NaCl, involves other considerations that influence diffusion. In such cases, the probability that the impurity will exchange with the vacancy will depend on factors such as the relative sizes of the impurity compared to the host atoms. In the case of ionic movement, the charge on the diffusing species will also play a part. These factors can also be included in a random-walk analysis by including jump probabilities of the host and impurity atoms and vacancies, all of which are likely to vary from one impurity to another and from one crystal structure to another. All of these alterations can be... 

Factors like the type of catalyst system, catalyst Crystal Structure, molar ratios of the catalyst components and temperature of the catalyst complex influence the shift or migration of the... 

When the two wells are of similar energies, and the crystal structure allows, the above will no longer be the situation. We may then expect a number of consequences There may be a measurable displacement of the hydrogen between the two sites induced by such factors as a change in temperature, application of an electric field, and irradiation with light both tautomers may be present at symmetry-independent sites in the crystal different tautomers may be present in different crystal modifications and the presence of molecular substituents that do not directly affect the properties of the hydrogen bond may influence the tautomerism via the crystal structure. 

Considering the influence of electronic configurations on crystal structures it may be asked, whether certain structure t5rpes are restricted to fluorine compounds of the transition elements. Apart from the structure types distorted by the Jahn-Teller effect such a limitation is not obvious at all. On the contrary quite a number of structure prototypes are represented by compounds of the main group elements. Bonding thus must be similar in both, main group and transition element fluorides, at least as for the factors that influence crystal structmes. 

With regard to visual comparison, it is important to note that the appearance of an experimental powder XRD pattern can be influenced by several factors that are not related to the actual crystal structure of the material, but originate instead from... 

This paper reports in detail our investigation on the factors influencing the formation of MCM-41 in fluoride medium, and the optimization of crystallization parameters of MCM-41. Since it is found in this study that siliceous MCM-41 structure could form in different media like [F] or [Cl] medium, the so-called salt effects were always present in both cases due to the presence of large amount of salts (such as NaBr, NaCl and NaF). However, this study shows that only siliceous MCM-41 synthesized in fluoride medium has higher hydrothermal stability during heating in boiling water. 

In order to optimize the synthesis parameters, a series of experiments were carried out to study in detail the factors influencing the formation of integrated MCM-41 structure in fluoride medium, such as F7Si molar ratio, pH value, Na+/Si molar ratio, CTMABr/Si molar ratio, temperature as well as Al/Si molar ratio. In these experiments, the crystallization was performed at 100 °C using the molar composition of SiC 0.54 NaOH 0.50 CTMABr 0.34 HF 100 H20. 

Heterogeneous photocatalytic reaction products and their production rates depend on the kind of photocatalysts. As noted above, each photocatalyst powder has different properties. In the case of the sonophotocatalytic reaction, products or their yields also depend on the kind of photocatalysts. The effect of surface area on product ratio was discussed in section (12.2.1C). The influence of surface area on product ratio was noted, but factors other than surface area must be introduced to explain the difference in product ratios. It is known that there are several crystal structures of Ti02. The major structures are anatase and rutile. 2... 

The next steps toward a deeper understanding of the catalytic processes were possible through researches which elucidated the nature of crystal structures and the geometrical positions of electrons, ions, or atom groups and their energetic conditions and that also determined the factors which influence the mobility of these particles within the lattice or at phase boundaries. 

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