Classes, of crystals

Numerous observations of the effect in ionic crystals were carried out by Mineev and Ivanov in the Soviet Union [76M01]. This is a class of crystals in which a number of materials factors can be confidently varied. By choice of crystallographic orientation, various slip directions can be invoked. By choice of various crystals other physical factors such as dielectric constant, ionic radius, and an electronic factor thought to be representative of dielec-... 

Of the three principal classes of crystals, ionic crystals, crystals containing electron-pair bonds (covalent crystals), and metallic crystals, we feel that a good understanding of the first class has resulted from the work done in the last few years. Interionic distances can be reliably predicted with the aid of the tables of ionic radii obtained by Goldschmidt1) by the analysis of the empirical data and by Pauling2) by a treatment based on modem theories of atomic structure. The stability,... 

In crystalline solids, the Raman effect deals with phonons instead of molecular vibration, and it depends upon the crystal symmetry whether a phonon is Raman active or not. For each class of crystal symmetry it is possible to calculate which phonons are Raman active for a given direction of the incident and scattered light with respect to the crystallographic axes of the specimen. A table has been derived (Loudon, 1964, 1965) which presents the form of the scattering tensor for each of the 32 crystal classes, which is particularly useful in the interpretation of the Raman spectra of crystalline samples. 

A further attribute of this host system is that each crystal contains only one enantiomer of the diol, and is therefore an example of the relatively uncommon conglomerate class of crystals 9). 

Preparation and Properties of Barium Salt. The catalyst used to prepare this new class of crystallizing polybutadienes consists of a barium t-butoxide-hydroxide salt in combination with an organolithium ( 8, 9, 10). Rather specific preparative techniques must be used in forming this barium salt, as shown in Figure 1. The use of an amine solvent provided quantitative conversion of the metal to barium salts. 

A plot of them (Figure 5.6) shows that they are proportional to the bond moduli. Thus the bond moduli are fundamental physical parameters which measure shear stiffness, and vice versa. Also, it may be concluded that hardness (and dislocation mobility) depends on the octahedral shear stiffnesses of this class of crystals (see also Gilman, 1973). 

When we consider crystal structures we usually think of the pattern and symmetry of the packing of the atoms, ions, or molecules in building the lattice based on X-ray crystallography. However, detailed descriptions of crystals and their classification are much older. The seven systems of crystals and the 32 classes of crystal symmetry were recognized by 1830. The 14 Bravais Lattices were presented by A. Bravais in 1848. 

A crystal is a well-tailored network or lattice of atoms. The construction of the lattice is directed stringently by the symmetry elements of the crystal. We can choose a central point and consider the periodical stacking around this point. Combinations of symmetry elements limit the arrangements to only 32 patterns—the 32 point groups or the 32 classes of crystals. 

If a molecule or a crystal is chiral, it is necessarily optically active. The converse is, however, not true. There are non-enantiomorphous symmetry classes of crystals that may exhibit optical activity. 

The maximum and the mean value in the distribution for a particular donor/ac-ceptor pair depends on the class of crystal structure surveyed. 

Covalently bonded solids such as quartz, diamond, and graphite form another class of crystals. Quartz is a continuous network of silicon dioxide bonded in a uniform, crystalline arrangement. Sand is a mixture of quartz and other rocks. Glass is solid quartz that has melted and resolidified without the same crystalline uniformity, in the way that melted butter does not re-form the same type of solid when it cools. Glass has been known to form naturally in lightning strikes on sand. 

When one or more models are constructed, they are tested against the experimental diffraction data. Often some of these approaches are combined together but they always stem from the requirement that the generated model must make physical, chemical and crystallographic sense. Thus, their successful utilization requires a certain level of experience and knowledge of how different classes of crystals are built, e.g. what to expect in terms of coordination and bond lengths for a particular material based solely on its chemical composition. Direct space modeling approaches will be discussed, to some extent, in Chapter 6. 

Finally a new class of crystal-glass transformations are discussed by the homogeneous transition due to an instability of the crystal [2.11]. Again, the... 

There is a class of crystallizers that are not agitated tanks, rather a bed of growing crystals is fluidized by a recirculating stream of... 

An interesting class of crystals are the hybrid crystals, in which an organic partner is built into an inorganic crystal lattice in a stoichiometric ratio. A nice example of this are the lead halides. 

A new situation, however, appears in crystals. Experiments show that, in effect, there are two large classes of crystals ... 

It s the electrons in the atoms, that cause the x-rays to diffract. Because it s the electrons, the diffraction can also tell you something about the coordination of the atoms in the solid, and this in turn can show what class of crystal it is. The CN can help distinguish between ionic and covalent solids. For example, ionic solids often have low CN. 

Phillips, F.C. (1972) An Introduction to Crystallography, 4th edition, Wiley, New York. Includes a clear description of the Herman-Mauguin notation and the 32 classes of crystal symmetry. First published in 1946. 

Once a particular class of crystallizer is decided upon, the choice of a specific unit depends upon such considerations as the initial and operating costs, the space availability, the type and size of crystals required, the physical characteristics of the mother liquor and crystal slurry, the need for corrosion resistance, sterile operation, environmental protection, and so on. 

Some substimted diacetylenes (R-C=C-CMI -R, hereinafter referred to as DA) constitute a class of crystals polymerizable in the solid state [1]. One may thus obtain large single crystals, built of poly diacetylene (PDA) chains parallel to one another. A pronounced anisotropy of the systems, due to the presence of long TC-conjugated chains, resulted in an interest concerning electrical and opticd properties of DA [2,3]. Much less attention has been paid to studies of their dielectric properties [4-6]. 

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