Crystalline solids classes

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. 

Nearly all of the structures that are mentioned in this chapter are crystalline solids, but due to the structural variations, there are scarce reports on studies on particular classes of compounds and as such the primary references should be consulted for this. 

We use the same approach to classify the different nanostructures for Titania. The term one-dimensional (ID) nanostructures indicate nanocrystals in which elongation only in one direction is above this threshold (about 10 nm). This class of ID nanostructures comprises different types of nano-ordered materials, such as nanorods, -wires, -coils, -fibers, -pillars (or -columns) and -tubes. We prefer to use the term quasi one-dimensional nanostructures, because often the dimensions are larger than the indicated threshold, although elongation along one main axis still exists. When the diameter of the nanorod, nanowire or nanotube becomes smaller, there is often a significant change in the properties with respect to crystalline solids or even two-dimensional systems. A bismuth nanowire is an excellent example, which transforms into a semiconductor, as the wire diameter becomes smaller.145... 

These particular molecules were chosen since they exemplify the interesting and unique properties - of an entire class of 17-donor species. These properties include the formation of stable, colored radical cations and the known ability to interact strongly in the crystalline solid state. - ... 

The class of nitrolic acids (nitro-oximes) was introduced by Victor Mayer as early as 1873 with the simplest representative methylnitroUc acid (HC(N02)=N0H) °. Tscher-niak, a student of Mayer, was the first to isolate the very labile methylnitroUc acid at low temperatures, while Wieland improved the synthesis by detailing the exact reaction conditions to increase the low yields. MethylnitroUc acid can be prepared as crystalline solid from nitromethane and KNO2/H2SO4 in situ generation of nitrous acid) in water at low temperatures, followed by extraction from this weak acidic solution into ether (Scheme 17). Removal of ether yields the pme, highly unstable free acid. Salts of methylnitroUc acid, NNtM, can be generated by extracting the acid into sodium carbonate solution. ... 

The forward search starts from the name of a chemical compound, proceeds to finding its molecular structure, and then its physical and chemical properties, such as the boiling point, melting point, density, etcetera, in a handbook. Many databases for single compounds are also organized by classes and families of similar structures. Fluid solutions represent the next level of complexity. For the most important fluids, such as water, air, and some refrigerants, we can find extensive tables for the thermal properties of mixtures. For complex fluids, such as paint and emulsion, which are difficult to characterize and to reproduce, specialized books and journals should be consulted. The properties of some crystalline solids can be found, but usually not for multicrystal composite and amorphous solids. 

In this chapter we have described two classes of solvent-less chemical reactions those occurring between molecular solids (mter-solid reactions) to yield a new crystalline solid, and those occurring between a molecular solid and a gas (solid-gas reactions) to yield a solid product. We have also described solid-state reactions taking place between molecules within a crystal (intra-solid reactions). The distinction between intra- and inter-solid reactions carries no implications for the reaction mechanisms, which we have decided not to discuss. Indeed, the mechanical stress produced by grinding or milling together two solids, as in the case of inter-solid reactions, may well generate molecular... 

Liquid Crystalline Polymers. One class of polymers that requires some special attention from a structural standpoint is liquid crystalline polymers, or LCPs. Liquid crystalline polymers are nonisotropic materials that are composed of long molecules parallel to each other in large clusters and that have properties intermediate between those of crystalline solids and liquids. Because they are neither completely liquids nor solids, LCPs are called mesophase (intermediate phase) materials. These mesophase materials have liquid-like properties, so that they can flow but under certain conditions, they also have long-range order and crystal structures. Because they are liquid-like, LCPs have a translational degree of freedom that most solid crystals we have described so far do not have. That is, crystals have three-dimensional order, whereas LCPs have only one- or two-dimensional order. Nevertheless, they are called crystals, and we shall treat them as such in this section. 

The above simple picture of solids is not universally true because we have a class of crystalline solids, known as Mott insulators, whose electronic properties radically contradict the elementary band theory. Typical examples of Mott insulators are MnO, CoO and NiO, possessing the rocksalt structure. Here the only states in the vicinity of the Fermi level would be the 3d states. The cation d orbitals in the rocksalt structure would be split into t g and eg sets by the octahedral crystal field of the anions. In the transition-metal monoxides, TiO-NiO (3d -3d% the d levels would be partly filled and hence the simple band theory predicts them to be metallic. The prediction is true in TiO... 

The structures of several members of this class of compounds are shown below. They are crystalline solids that are produced by hypochlorite oxidation of Q -aininonitriles. 

List the four main classes of crystalline solids, and give a specific example of each. 

In 1991, scientists at AT T Bell Laboratories discovered a new class of high-temperature superconductors based on fullerene, the allotrope of carbon that contains Cgo molecules (Sections 10.10 and 19.6). Called "buckyballs," after the architect R. Buckminster Fuller, these soccer ball-shaped Cgo molecules react with potassium to give K3C6o- This stable crystalline solid contains a face-centered cubic array of buckyballs, with K+ ions in the cavities between the Cgo molecules (Figure 21.16). At room temperature, K3Q,o is a metallic conductor, but it becomes a superconductor at 18 K. The rubidium fulleride, Rb C o, and a rubidium— thallium-Cfio compound of unknown stoichiometry have higher Tc values of 30 K and 45M8 K, respectively. 

A new class of pentacoordinate zwitterionic silicates (52-70) has been developed and reported by Tacke and his coworkers28-31 46-52 (cf Section II.A.3). These are generally high melting crystalline solids, which are almost insoluble in nonpolar organic solvents, and only slightly soluble in polar solvents. 

Moreover, low temperature NMR spectra indicate a pyramidal structure of the carbanion in triphenylphosphonium cyclopropylid 6, and an X-ray diffraction analysis of the crystalline solid confirms this result (14). Contrary to earlier predictions, cyclo-propylids are thus found to be the first class of ylids to contain non-planar carbanions with an unusual ylidic bonding (Fig.2). They form stable metal complexes, as illustrated by a gold complex (7) +... 

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