Crystallization Issues

Lewis, N. (2000). Shedding some light on crystallization issues. Lecture transcript from the first international symposium on aspects of polymorphism and crystallization—chemical development issues. Org. Process Res. Dev., 4, 407-12. [237]... 

Other experimentation on crystallization issues is beyond the scope of this discussion. 

In addition to these standard crystallization issues, evaporative crystallizers must have adequate circulation at the wall surface and maintain sufficient temperature difference between the jacket fluid and batch fluid for heat transfer to achieve satisfactory distillation rates. This issue can be complicated by restrictions on wall temperature imposed by the common temperature instability of organic compounds. These problems can be severe when the crystallizing material forms a crust on the vessel wall, where it can decompose as well as reduce heat flux. 

Watkin DJ, Prout CK, Carruthers JR and Betteridge PW (1996) CRYSTALS Issue 10. Oxford Chemical Crystallography Laboratory, University of Oxford. 

Since solids do not exist as truly infinite systems, there are issues related to their temiination (i.e. surfaces). However, in most cases, the existence of a surface does not strongly affect the properties of the crystal as a whole. The number of atoms in the interior of a cluster scale as the cube of the size of the specimen while the number of surface atoms scale as the square of the size of the specimen. For a sample of macroscopic size, the number of interior atoms vastly exceeds the number of atoms at the surface. On the other hand, there are interesting properties of the surface of condensed matter systems that have no analogue in atomic or molecular systems. For example, electronic states can exist that trap electrons at the interface between a solid and the vacuum [1]. 

Issues associated with order occupy a large area of study for crystalline matter [1, 7, 8]. For nearly perfect crystals, one can have systems with defects such as point defects and extended defects such as dislocations and grain... 

Unlike melting and the solid-solid phase transitions discussed in the next section, these phase changes are not reversible processes they occur because the crystal stmcture of the nanocrystal is metastable. For example, titania made in the nanophase always adopts the anatase stmcture. At higher temperatures the material spontaneously transfonns to the mtile bulk stable phase [211, 212 and 213]. The role of grain size in these metastable-stable transitions is not well established the issue is complicated by the fact that the transition is accompanied by grain growth which clouds the inteiyDretation of size-dependent data [214, 215 and 216]. In situ TEM studies, however, indicate that the surface chemistry of the nanocrystals play a cmcial role in the transition temperatures [217, 218]. 

A description of Pasteur s work as part of a broader discussion concerning crystal structure can be found in the article Molecules Crys tals and Chirality in the July 1997 issue of the ioc/rna/ of Chemical Education pp 800-806... 

R. O. Gmbel, ed.. Metallurgy of Elemental and Compound Semiconductors, Interscience Pubhshers, New York, 1961. Discusses eady work on semiconductor dendrites and other methods of growing shaped crystals. The special issue of / Cyst. Growth (Sept. 1980) is devoted to shaped crystal growth. 

The degree of surface cleanliness or even ordering can be determined by REELS, especially from the intense VEELS signals. The relative intensity of the surface and bulk plasmon peaks is often more sensitive to surface contamination than AES, especially for elements like Al, which have intense plasmon peaks. Semiconductor surfaces often have surface states due to dangling bonds that are unique to each crystal orientation, which have been used in the case of Si and GaAs to follow in situ the formation of metal contacts and to resolve such issues as Fermi-level pinning and its role in Schottky barrier heights. 

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