Diffraction, amorphous substances

CPAs should, at least partially, solidify in the amorphous state. However amorphous state alone does not assure protection. Izutsu et al. [3.4] showed, for beta-galactosidase by X-ray diffraction, that only such additives avoid denaturation, which do not crystallize. A dilution of protein in the solidified protective agent reduces the chance of reactions between the protein molecules. Amorphous substances dry more slowly, which makes it easier to avoid overdrying. 

The 500 nm size is a limit value crystallites below this size tend to broaden the diffraction peaks in a spectrum, while size distributions above this value produce particularly sharp signals whose half width is a function only of the wavelength of the X-ray beam and the equipment. Signal broadening is at its maximum in materials known as X-ray amorphous substances, featuring particle size distributions below 8 nm. These afford flattened, washed-out spectra of little analytical value. 

The basic modem data describing the atomic stmcture of matter have been obtained by the using of diffraction methods - X-ray, neutron and electron diffraction. All three radiations are used not only for the stmcture analysis of various natural and synthetic crystals - inorganic, metallic, organic, biological crystals but also for the analysis of other condensed states of matter - quasicrystals, incommensurate phases, and partly disordered system, namely, for high-molecular polymers, liquid crystals, amorphous substances and liquids, and isolated molecules in vapours or gases. This tremendous... 

Especially methods of electron microscopy are important at study of X-ray amorphous substances and polyphase nanomixtures which are distributed very widely in the nature such as agate, bauxite, bitumen, coal, natural glasses etc., as X-ray diffraction is almost useless at analyzing such mostly disordered materials. 

Interpretation of diffraction effects of non-crystalline substances. It has been pointed out in Chapter V that there is no sharp dividing line between crystalline and amorphous substances with decrease of crystal size, X-ray diffraction patterns become more and more diffuse until, finally, any attempt to calculate crystal size by the method given earlier in this chapter gives a figure of only a few Angstrom units— that is, about one unit cell in these circumstances the word crystal , with its implication of pattern repetition, is inappropriate. The alternative word amorphous is not entirely satisfactory either on account of the sizes of atoms and their preference for particular environments, the distribution of atomic centres.cannot be entirely random. The word non-crystalline is really preferable. 

Silicon Dioxide occurs as an amorphous substance that shows a noncrystalline pattern when examined by X-ray diffraction. It is produced synthetically, either by a vapor-phase hydrolysis process, yielding fumed silica, or by a wet process, yielding precipitated silica, silica gel, colloidal silica, or hydrous silica. Fumed silica is produced in an essentially anhydrous state, whereas the wet-process products are obtained as hydrates or contain surface-adsorbed water. 

Structural chemistry of solutions started in Japan rather late compared with other studies of solutions, although crystallographic investigations were very highly developed and actively investigated in Japan. This fact may be due to lack of the concept of structure of liquids in most Japanese physical chemists. The concept of "structure was soundly applied to solids and molecules in the gas phase, but not to liquids and solutions. X-Ray diffraction studies on liquids and amorphous substances were already examined in 1916 by Debye and Sherrer , immediately after the first work of Debye for the X-ray diffraction. Studies on liquid metals and molten salts by using X-... 

G.IO Andre Guinier. X-Ray Crystallographic Technology (Lx>ndon Hiiger and Watts, 1952). Excellent treatment of the theory and practice of x-ray diffraction. The title is not fair to the book, which includes a considerable body of theory and detailed experimental technique. The theory and applications of the reciprocal lattice are very well described. Includes treatments of focusing monochromators, small-angle scattering, and diffraction by amorphous substances. 

The data for the NaP03-Si02 system are given in Table IX. Crystalline phases of all the samples are composed of a-cristobalite and trisodium trimetaphosphate. Some of them contain small amounts of or-tridymite. Ortho-, di-, and triphosphate were found by paper chromatography in the aqueous solutions of the samples. These ortho- and chain phosphates may be derived from amorphous substances, because the x-ray diffraction patterns of these samples do not exhibit any peaks attributable to phosphates other than trisodium trimetaphosphate. However, the structures of these amorphous substances cannot be determined from the data presented here. It has been shown that the crystals of neither of the systems contain P-O-Si linkages. 

The cylindrical sample is kept as small as possible to minimize the absorption of diffracted radiation. The optimum thickness is jpmP, where p is the sample density. Cylinders are generally kept at 0.5rmm or less diameter. When dilution is necessary, an amorphous substance such as flour is used as the diluent. It is best that the cylindrical sample not be in a container, but in many cases this is not possible. Satisfactory container materials include lithium borate ( Lindemann glass) or various plastics because of their low mass-absorption coefficients the container is a tube with a wall-thickness of about 0.01-mm. 

An ingenious approach to the autoionization process was suggested by Tomelini and Fanfoni [32]. They examined diffraction processes in the framework of autoionization, i.e., a second-order process. More exactly, they studied only the second stage of autoionization, namely, the secondary electron emission from the intermediate state where the core hole is coherently distributed over the crystal. With such an approach it was shown that in crystals this diffraction contribution enhances fine structure as compared to the amorphous substance. However, the applicability of such a model is determined by the probability of the occurrence of the core hole distributed coherently over the crystal. More conventional... 

Since reducing the grain sizes leads to broadening of XRD lines, one can compute when these lines merge into a halo characteristic of an amorphous substance. For MgO the critical size is around 1 nm, for Si02 twice that. Obviously, the higher the symmetry of the crystal, i.e. the fewer lines are present in the XRD pattern, the broader the diffraction lines must be (i.e. the smaller the grain size) to form a... 

Amorphous substances (e.g., Ga, Se, Ge, Si02, C) are mostly derived from face-centered cubic crystals or hexagonal ones. The law of Donnay and Marker [4] allows one to connect the diffraction pattern to the external shape of a crystal. The more developed face of a crystal is the one where the atomic density is maximum (i.e., when the atoms are in contact). Correspondingly, electrostatic... 

Amorphous substances give very much more featureless spectra than do well-crystallized compounds. Nevertheless, infrared spectroscopy is a unique tool in deriving information on the structure of such substances, and this aspect will be discussed more fully later. Amorphous substances absorb as strongly as crystalline substances, and so their presence in admixture with crystalline components is less likely to be overlooked in infrared than in X-ray examination. On the other hand, small amounts of crystalline components in the presence of larger amounts of amorphous material of similar composition will often be more readily detectable by their X-ray diffraction pattern. 

Many polymers show partial crystallinity. This is apparent from the study of X-ray diffraction patterns, which for polymers generally show both the sharp features associated with crystalline regions as well as less well-defined features which are characteristic of disordered substances with liquid-like arrangements of molecules. The co-existence of crystalline and amorphous regions is typical of the behaviour of crystalline polymers. 

In the crystalline state of a substance, the molecules are arranged in a defined unit cell that is repeated in a three-dimensional lattice [1], Since the crystal lattice can act as a diffraction grating for X-rays, the X-ray diffraction pattern of a crystal consists of a number of sharp lines or peaks, often with baseline separation. Figure 1 shows the X-ray powder diffraction pattern of the crystalline and amorphous forms of nedocromil sodium trihydrate. 

Investigations for the occurrence of polymorphism have been undertaken by ir spectroscopy, differential scanning calorimetry and x-ray powder diffraction (Guinier-de Wolff). No polymorphism has been observed so far. An amorphous form may be prepared artificially by rapid evaporation of a methanolic solution of the drug substance. 

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. 

Another characteristic point is the special attention that in intermetallic science, as in several fields of chemistry, needs to be dedicated to the structural aspects and to the description of the phases. The structure of intermetallic alloys in their different states, liquid, amorphous (glassy), quasi-crystalline and fully, three-dimensionally (3D) periodic crystalline are closely related to the different properties shown by these substances. Two chapters are therefore dedicated to selected aspects of intermetallic structural chemistry. Particular attention is dedicated to the solid state, in which a very large variety of properties and structures can be found. Solid intermetallic phases, generally non-molecular by nature, are characterized by their 3D crystal (or quasicrystal) structure. A great many crystal structures (often complex or very complex) have been elucidated, and intermetallic crystallochemistry is a fundamental topic of reference. A great number of papers have been published containing results obtained by powder and single crystal X-ray diffractometry and by neutron and electron diffraction methods. A characteristic nomenclature and several symbols and representations have been developed for the description, classification and identification of these phases. 

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