Crystallization principles and techniques

We have described for a number of molecular single crystals the basic principles and techniques which were involved in the optimization of their nonlinear efficiency and crystalline quality. Certainly, other types of molecules or material processing techniques can be used. In particular, it could be worthwile to try and by-pass the difficulty of growing large single crystals for applications which are less demanding in terms of optical quality parametric oscillation. 

The techniques for growth of oxide single crystals of appropriate size and perfection advanced rapidly in the decades following World War II. The principles and techniques are covered in a number of monographs on crystal growth and are only briefly described below ... 

In Part VII, Greg Rutledge discusses the modeling and simulation of polymer crystals. He uses this as an excellent opportunity to introduce principles and techniques of solid-state physics useful in the study of polymers. The mathematical description of polymer helices and the calculation of X-ray diffraction patterns from crystals are explained. Both optimization (energy minimization, lattice dynamics) and sampling (MC, MD) methods for the simulation of polymer crystals are then discussed. Applications are presented from the calculation of thermal expansion, elastic coefficients, and even activation energies and rate constants for defect migration by TST methods. 

This paper has described in very general terms the unique problems encountered in the determination of the molecular packing in polymeric crystals. Standard crystallographic procedures lose their utility when dealing with such problems, and additional information, such as stereochemical constraints and packing energies, should be brought to bear in the problem. Finally, a method of structural refinement which is especially suited to polymeric structures should be employed in place of classical least squares methods. A subsequent paper will deal with the application of these principles and techniques to the determination of the crystal structures of several crystalline polyethers. 

General Principles and Techniques Involved in Crystallization Rate Studies... 

Some of the major areas of activity in this field have been the application of the method to more complex materials, molecular dynamics, [28] and the treatment of excited states. [29] We will deal with some of the new materials in the next section. Two major goals of the molecular dynamics calculations are to determine crystal structures from first principles and to include finite temperature effects. By combining molecular dynamics techniques and ah initio pseudopotentials within the local density approximation, it becomes possible to consider complex, large, and disordered solids. 

Convergent (or focused) beam electron diffraction (CBED) is particularly attractive for determining local crystal structures and space groups in three dimensions (Steeds et al 1979, Tanaka et al 1985). In a modern TEM, CBED is now routinely available. In this technique, two principles of TEM electron diffraction are employed departure from Friedel s law and the formation of extinction bands within refiections that are forbiddden by space groups. 

The conventional tartaric stabilization techniques applied in the wine industry are based on two opposite principles. One is aimed at heightening HT- and T2- precipitation by reducing wine temperature and leads to the so-called cold stabilization technique. To accelerate nucleation, wines are seeded with exogenous KHT crystals, cooled and kept at —4°C for 4-8 days... 

---