Crystalline solids specific structures

EXAFS is a nondestructive, element-specific spectroscopic technique with application to all elements from lithium to uranium. It is employed as a direct probe of the atomic environment of an X-ray absorbing element and provides chemical bonding information. Although EXAFS is primarily used to determine the local structure of bulk solids (e.g., crystalline and amorphous materials), solid surfaces, and interfaces, its use is not limited to the solid state. As a structural tool, EXAFS complements the familiar X-ray diffraction technique, which is applicable only to crystalline solids. EXAFS provides an atomic-scale perspective about the X-ray absorbing element in terms of the numbers, types, and interatomic distances of neighboring atoms. 

Table 8.53 shows the main features of XAS. The advantages of EXAFS over diffraction methods are that the technique does not depend on long-range order, hence it can always be used to study local environments in amorphous (and crystalline) solids and liquids it is atom specific and can be sensitive to low concentrations of the target atom (about 100 ppm). XAS provides information on interatomic distances, coordination numbers, atom types and structural disorder and oxidation state by inference. Accuracy is 1-2% for interatomic distances, and 10-25 % for coordination numbers. 

Rather specific structural features appear to be necessary for high ionic conductivity in crystalline solids, and as a result it may seem surprising that glasses can support high ionic conductivity. For example, a Li" conductivity of 0.16mScm at 25 °C for a glass with the composition... 

Most solid materials are crystalline. The atoms that are contained in the solid are arranged so that a specific structural motif is repeated in a periodic manner in all directions. This property is remarkable in that it permits the description of the location of every atom in a macroscopic crystalline substance by... 

Beside the chemical composition, the crystalline structure of the mineral has an important effect on the adsorption ability of its surface. This is due to the fact that lattice bindings are usually not equivalent and space disproportions occur, so that fission surface areas have specific properties. Typical examples are layer lattices of graphite or talc where the main valences proceed in the layer plains whereas these are interconnected with feeble valences. Fission areas of such minerals are hydrophobic. The effect of the structure on adsorption properties of a mineral surface increases with increasing adsorption density and with decreasing force of the adsorption binding of the solid phase5. A crystalline lattice contains structural defects (which include physical and chemical surface imperfections and deficiencies in the volume phase) which can influence the chemical reactivity of a crystal surface. 

The surfaces of minerals (or other crystalline solids) differ from the bulk material in terms of both crystal structure and electronic structure. A great variety of spectroscopic, diffraction, scanning, and other techniques are now available to study the nature of solid surfaces, and models are being developed to interpret and explain the experimental data. These approaches are discussed with reference to a few examples of oxide and sulfide minerals. Although relatively few studies have been undertaken specifically of the surfaces of minerals, many of the reaction phenomena... 

The chemical structure of a polymer determines whether it will be crystalline or amorphous in the solid state. Both tacticity (i.e., syndio-tactic or isotactic) and geometric isomerism (i.e., trans configuration) favor crystallinity. In general, tactic polymers with their more stereoregular chain structure are more likely to be crystalline than their atactic counterparts. For example, isotactic polypropylene is crystalline, whereas commercial-grade atactic polypropylene is amorphous. Also, cis-pol3nsoprene is amorphous, whereas the more easily packed rans-poly-isoprene is crystalline. In addition to symmetrical chain structures that allow close packing of polymer molecules into crystalline lamellae, specific interactions between chains that favor molecular orientation, favor crystallinity. For example, crystallinity in nylon is enhanced because of... 

One may define a solvatomorph as a crystalline solid in which solvent molecules have become included in the structure through the existence of positional substitution at positions that are site specific and that are related to other solvent molecules through translational symmetry. Other types of structural solvation exist but will not be discussed here. Since water is such a ubiquitous substance, it is not surprising that the most important type of solvatomorphism involves the incorporation of water into a crystal lattice. 

Crystalline solids composed of higftly asymmetric molecules may undergo stepwise increases in disorder as the temperature is increased. Such transitions are reversible and occur at a fixed characteristic temperature [23]. Movement in specific crystallographic directions may become possible, resulting in a liquid crystal in which structure and properties are anisotropic. 

Thus the simulation of quenching may not necessarily be an adequate model of the real process. This is an important fact if we take into consideration the fact that an amorphous state is generally metastable so that its structural characteristics depend upon the history of its formation. This distinguishs the amorphous state from the stable crystalline state whose structure is in principle determined by the thermodynamic parameters, e.g., pressure and temperature, and not upon the history of its formation. To be more specific, let us consider the simple case when a crystalline structure of a solid may be determined... 

The X-ray diffraction technique is the most commonly used experimental method for investigating the crystal structures of crystalline solids. As the underlying theory and methods are detailed in several specific textbooks (e.g.. Ref. [1]), only a brief description of the essential features will be provided at this point. 

Strontium carbonate is a colorless or white crystalline solid having a rhombic structure below 926°C and a hexagonal structure above this temperature. It has a specific gravity of 3.70, a melting point of 1497°C at 6 MPa (60 atm), and it decomposes to the oxide on heating at 1340°C. It is insoluble in water but reacts with acids, and is soluble in solutions of ammonium salts. 

In Section 14 we considered crystalline solids and approximated their structures as if the materials were infinite and periodic in all three dimensions. This may be a good approximation when studying the bulk properties of the material, but for some properties the fact that the material is finite is of ultimate importance. Among these are those properties that are related to the existence of surfaces. The fact that the atoms at the surfaces have a lower coordination than those in the interior and accordingly have dangling bonds leads to the existence of surface-specific properties. These include surface reconstructions for which the surface atoms adopt a structure different from that of the interior, as well as a higher reactivity of the surface atoms. The latter is the topic of the present section. 

In the case of a crystalline solid it is possible to determine, by diffraction methods, the equilibrium positions and vibrational amplitudes of all the atoms involved and this information specifies the structure of the crystal. A liquid, by its very nature, cannot have a structure in this sense. The environment of each atom or molecule is continually changing and we must usually be satisfied with some sort of time-averaged specification of the environment or, which is essentially the same thing in this case, a space average over the environments of many different molecules. 

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