The process of crystal growth

The number (concentration) of nuclei that are formed has an important effect on the course of the subsequent crystallization. To obtain a crystal suitable for X-ray diffraction studies, conditions should be such that only a few nuclei are formed. Failure to obtain crystals at all may indicate a difficulty in obtaining stable nuclei. On the other hand, if too many nuclei are formed, masses of microcrystals, unsuitable for singlecrystal X-ray diffraction studies, result. 

We noted the importance of obtaining an appropriate number of nuclei for suitable crystallization. The fate of dislocations upon which growth occurs is important. High-quality crystals are obtained when the rate of deposition of material from solution and the healing of defects 

Sometimes different compounds give apparently identical crystals. Isomorphism is the similarity of crystal shape, unit cell dimensions, and structure between substances of nearly, but not completely, identical chemical composition. It is derived from the Greek words - isos meaning equal and morphe for form or shape. The arrangements of atoms in the isomorphous crystals are identical, but the identity of one or more atoms in this arrangement has been changed. For example, sulfur in a sulfate may often be replaced by selenium, to give an isomorphous selenate. Ideally, isomorphous compounds are so closely similar in composition that 

Isomorphous replacement is now employed in the determination of the structures of biological macromolecules. These molecules crystallize with 50% or more of the crystal volume filled with solvent molecules. Murray Vernon King, working with David Harker, conceived the idea of soaking protein crystals in solutions of compounds containing a heavy atom. These heavy-atom compounds are diffused into the crystals through the solvent channels and settle on preferred sites on protein molecules. The diffraction patterns of the unperturbed crystal (described as native ) and the heavy-atom derivative are then compared in such a way that an electron-density map for the protein results. The method of isomorphous replacement, and the manner by which it is used to derive relative phases, are described in detail in Chapter 8. 

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When a precipitate separates from a solution, it is not always perfectly pure it may contain varying amounts of impurities dependent upon the nature of the precipitate and the conditions of precipitation. The contamination of the precipitate by substances which are normally soluble in the mother liquor is termed co-precipitation. We must distinguish between two important types of co-precipitation. The first is concerned with adsorption at the surface of the particles exposed to the solution, and the second relates to the occlusion of foreign substances during the process of crystal growth from the primary particles. 

On crystal faces bounding a polyhedral crystal, step patterns resembling the contour lines on a topographic map or striations in one direction are observable depending on the nature of the face. These show the process of crystal growth or dissolution at an atomic level, and are referred to as the surface morphology or surface microtopography. 

The processes of crystal growth are divided into the following three stages. 

These intergrowth relations are formed through the processes of crystal growth, phase transformation or decomposition associated with a decrease in temperature and pressure, or metasomatism due to the supply of new components from outside. 

The process of crystal growth is thought to occur in three steps ... 

In addition to the edge dislocation there is also another type, known as a screw dislocation, which plays an important part in the process of crystal growth. 

This low density of surface defects is the major factor that determines the record performance of single-crystal OFETs and enables exploration of the fundamental limits of charge carrier transport in organic materials, hi addition, these devices provide an efficient tool for studying the polaron-defect interactions. This section focuses on defects that can be formed in the process of crystal growth, OFET fabrication, and as a result of the interaction with ambient environment. 

Once stable nuclei have been formed within the supersaturated solution they begin to grow into crystals of finite size. Mullin [1972] draws attention to three theories that have been used to explain the process of crystal growth. They include surface energy effects, the presence of an adsorption layer and theories based on diffusion. 

Very few crystals are perfect. Indeed, in many cases they are not required to be, since lattice imperfections and other defects can confer some important chemical and mechanical properties on crystalline materials. Surface defects can also greatly influence the process of crystal growth. There are three main types of lattice imperfection point (zero-dimensional, line (one-dimensional) and surface (two-dimensional). 

The former case is usually associated with the use of evaporators for crystallization objectives. Crystal growth on the heat transfer surface competes with the process of crystal growth on the greater deposition area of the suspended crystals. Supersaturation with respect to the heat transfer surface which has e higher temperature than the bulk solution is lower for normal solubility salts and higher for inverse solubility salts. 

One of the most important features of such a facility is the presence of a toroidal feeder. It allows for diverse chemical procedures of the treatment and an additional purification of the melt both prior to the growth procedure and in the process of crystal growth. 

The authors wish to acknowledge their indebtedness to Dr. J. E. Keem of Troy, MI, who was instrumental in the initial installation and operation of the equipment to the late Dr. H. Harrison, and to Drs. D. Buttrey and R. Aragon (presently at the University of Delaware), all of whom developed and perfected the procedures and instituted many improvements to J. W. Koenitzer, who was always willing to assist with the process of crystal growth and to help in innumerable repairs and to Mr. C. Hager, who assisted in the microprobe analysis. 

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