Amorphous materials/solids

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. 

Angell CA (1986) Recent developments in fast ion transport in glassy and amorphous materials. Solid State Ionics 18 19 72-88... 

The things that we have been talking about so far - metal crystals, amorphous metals, solid solutions, and solid compounds - are all phases. A phase is a region of material that has uniform physical and chemical properties. Water is a phase - any one drop of water is the same as the next. Ice is another phase - one splinter of ice is the same as any other. But the mixture of ice and water in your glass at dinner is not a single phase because its properties vary as you move from water to ice. Ice + water is a two-phase mixture. 

Beside the crystalline material, a certain portion of amorphous lead dioxide is always observed. In the working electrode such amorphous material is apparently hydrated and forms a gel structure at the phase boundary between the solid material and the electrolyte (cf. Ref. [6]). 

Fabrication techniques, especially the preparation of thin films of functional materials, have made major progress in recent years. Thin-film solid electrolytes in the range of several nanometers up to several micrometers have been prepared successfully. The most important reason for the development of thin-film electrolytes is the reduction in the ionic resistance, but there is also the advantage of the formation of amorphous materials with stoichiometries which cannot be achieved by conventional techniques of forming crystalline compounds. It has often been observed that thin-film electrolytes produced by vacuum evaporation or sputtering provide a struc-... 

Solid state materials have been studied by nuclear magnetic resonance methods over 30 years. In 1953 Wilson and Pake ) carried out a line shape analysis of a partially crystalline polymer. They noted a spectrum consisting of superimposed broad and narrow lines which they ascribed to rigid crystalline and amorphous material respectively. More recently several books and large articles have reviewed the tremendous developments in this field, particularly including those of McBrierty and Douglas 2) and the Faraday Symposium (1978)3) —on which this introduction is largely based. 

One of the most important areas of application of the solid-state NMR technique is the investigation of the structures of cross-linked amorphous materials in cases where X-ray diffraction technqiues are not applicable. Polymeric resins are one such important class of materials. A lot of work has been done in this area by several investigators 36,37 38 since the beginning of the 80. Some solid-state NMR data of phenolic resins are presented in Fig. 10. Comparison with liquid state data for... 

Thermal decomposition of [Cu0Si(0 Bu)3]4 in the solid phase begins at ca. 100 °C under argon (by TGA) and results in formation of an amorphous material until roughly 600 °C, at which temperature Cu metal was detected (by PXRD) [105]. Conversely, decomposition under oxygen led initially to a material with Cu crystalhtes and small amounts of CU2O and CuO, and subsequent heating beyond 800 °C resulted in oxidation of all the copper to CuO. 

The model proposed by Anderson and Phillips gives a phenomenological explanation of the properties of the amorphous materials without supplying a detailed microscopic description [42]. Low-temperature measurements of the specific heat of amorphous solids have however shown that instead of a linear contribution as expected from the TLS theory, the best representation of data is obtained with an overlinear term of the type [43,44] ... 

Equation (12.6) is in the shape predicted by the tunnelling theory for the amorphous materials [38,39] and 8 of eq. (12.7) is within the range of values obtained for other disordered solids [40]. 

The sorption of water vapor onto nonhydrating crystalline solids below RHq will depend on the polarity of the surface(s) and will be proportional to surface area. For example, water exhibits little tendency to sorb to nonpolar solids like carbon or polytetrafluorethylene (Teflon) [21], but it sorbs to a greater extent to more polar materials such as alkali halides [34-37] and organic salts like sodium salicylate [37]. Since water is only sorbed to the external surface of these substances, relatively small amounts (i.e., typically less than 1 mg/g) of water are sorbed compared with hydrates and amorphous materials that absorb water into their internal structures. 

As shown in Fig. 2 [37], and also in the work of Barraclough and Hall [34], moisture uptake onto sodium chloride as a function of relative humidity is reversible as long as RH0 is not attained. This is evidence that actual dissolution of water-soluble crystalline substances does not occur below RH0. This is consistent with thermodynamic rationale that dissolution below RHo would require a supersaturated solution (i.e., an increased number of species in solution would be necessary to induce dissolution at a relative humidity below that of the saturated solution, RH0). In this regard, one should only need to consider the solid state properties of a purely crystalline material below RH0. As will be described, other considerations are warranted for a substance that contains amorphous material. 

To illustrate this more quantitatively, consider the hypothetical sucrose example discussed by Ahlneck and Zografi [80]. Assuming that all the sorbed water is taken up by the amorphous portion of material, 0.1% total moisture would correspond to approximately 20%, 10%, 4%, and 2% moisture content in the amorphous material, respectively, for 0.5%, 1%, 2.5%, and 5% of amorphous solid. The glass transition temperatures for the amorphous portions... 

Pressure-induced amorphization of solids has received considerable attention recently in physical and material sciences, although the first reports of the phenomenon appeared in 1963 in the geophysical literature (actually amorphization on reducing the pressure [18]). During isothermal or near isothermal compression, some solids, instead of undergoing an equilibrium transition to a more stable high-pressure polymorph, become amorphous. This is known as pressure-induced amorphization. In some systems the transition is sharp and mimics a first-order phase transition, and a discontinuous drop in the volume of the substance is observed. Occasionally it is strictly not an amorphous phase that is formed, but rather a highly disordered denser nano-crystalline solid. Here we are concerned with the situation where a true amorphous solid is formed. 

In contrast to crystalline solids characterized by translational symmetry, the vibrational properties of liquid or amorphous materials are not easily described. There is no firm theoretical interpretation of the heat capacity of liquids and glasses since these non-crystalline states lack a periodic lattice. While this lack of long-range order distinguishes liquids from solids, short-range order, on the other hand, distinguishes a liquid from a gas. Overall, the vibrational density of state of a liquid or a glass is more diffuse, but is still expected to show the main characteristics of the vibrational density of states of a crystalline compound. 

For example, whereas the solid oxidation catalyst MCM-41-entrapped perruthenate can be used for the conversion of benzyl alcohols only, a similarly perruthenated-doped amorphous ORMOSIL is equally well suited for a variety of different alcohol substrates.35 On the other hand, a uniform pore structure ensures access to the active centres, while in an amorphous material made of non-regular porosity hindered or even blocked sites can well exist (Figure 1.16), rendering the choice of the polycondensation conditions of paramount importance. 

X-ray crystallographic techniques when extended to polymeric solids some interesting features of the internal structure of these substances. It was found that good majority of polymers diffract X-rays like any crystalline substance but many behave like amorphous materials giving very broad and diffuse X-ray diffraction patterns. This is seen in following figure. 

The solubility of the drug is affected by several physiological and physicochemical factors [26], Drug properties are defined not only by the chemical structure but also by the solid material, and a drug can potentially exist in many different solid state forms which may differ in solubility. Amorphous materials tend to show much higher aqueous solubility than crystalline forms of the same compound and different crystal modifications of the same compound may also have different solubility (e.g., [25]). 

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