Solid X-ray diffraction

Because a chemical bond is only about 10 10 m long, special techniques have to be used to measure its length. There are two principal techniques one for solids and the other for gases. The technique used for solids, x-ray diffraction, is described in Major Technique 3, billowing Chapter 5. Microwave spectroscopy, discussed here, is used to determine bond lengths in gas-phase molecules. This branch of spectroscopy makes use of the ability of rotating molecules to absorb microwave radiation, which has a wavelength close to 1 cm. 

In studying both polynuclear aromatic hydrocarbons and carbonaceous solids, X-ray diffraction has proved to be a valuable tool this book contains elegant structural studies on both aromatic anions (65) and aromatic cations (66). NMR, including both 13C NMR in the solid state (67-72) and various two-dimensional spectroscopies (73-75), will have a dramatic impact in the future on both polynuclear aromatic hydrocarbons and carbonaceous solids. 

Since its discovery in 1912 by von l.aue. X-ray diffraction has provided a wealth of important information to science and industry. For example, much of what is known about the arrangement and the spacing of atoms in crystalline materials has been determined directly from diffritciion studies. In addition, such studies have led to a much clearer tinderstftnding of the physical properties of metals, polymeric materials, and other solids. X-ray diffraction is one of the ntost important methods for determining the structures of such complex natural products as steroids, vittimins, and an tibtoiics. The details of these applications are beyxmd the scope of this book. 

Because x-rays are particularly penetrating, they are very usefiil in probing solids, but are not as well suited for the analysis of surfaces. X-ray diffraction (XRD) methods are nevertheless used routinely in the characterization of powders and of supported catalysts to extract infomration about the degree of crystallinity and the nature and crystallographic phases of oxides, nitrides and carbides [, ]. Particle size and dispersion data are often acquired with XRD as well. 

Tredgold R H, Allen R A and Hodge P 1987 X-ray-diffraction and optical studies of Langmuir-Blodgett films formed from azobenzene derivatives Thin Solid Films 155 343-52... 

Cane sugar is generally available ia one of two forms crystalline solid or aqueous solution, and occasionally ia an amorphous or microcrystalline glassy form. Microcrystalline is here defined as crystals too small to show stmcture on x-ray diffraction. The melting poiat of sucrose (anhydrous) is usually stated as 186°C, although, because this property depends on the purity of the sucrose crystal, values up to 192°C have been reported. Sucrose crystallines as an anhydrous, monoclinic crystal, belonging to space group P2 (2). 

From shock compression of LiF to 13 GPa [68] these results demonstrate that X-ray diffraction can be applied to the study of shock-compressed solids, since diffraction effects can be observed. The fact that diffraction takes place at all implies that crystalline order can exist behind the shock front and the required readjustment to the shocked lattice configuration takes place on a time scale less than 20 ns. Another important experimental result is that the location of (200) reflection implies that the compression is isotropic i.e., shock compression moves atoms closer together in all directions, not just in the direction of shock propoagation. Similar conclusions are reached for shock-compressed single crystals of LiF, aluminum, and graphite [70]. Application of these experimental techniques to pyrolytic BN [71] result in a diffraction pattern (during compression) like that of wurtzite. 

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. 

Solid state NMR is a relatively recent spectroscopic technique that can be used to uniquely identify and quantitate crystalline phases in bulk materials and at surfaces and interfaces. While NMR resembles X-ray diffraction in this capacity, it has the additional advantage of being element-selective and inherently quantitative. Since the signal observed is a direct reflection of the local environment of the element under smdy, NMR can also provide structural insights on a molecularlevel. Thus, information about coordination numbers, local symmetry, and internuclear bond distances is readily available. This feature is particularly usefrd in the structural analysis of highly disordered, amorphous, and compositionally complex systems, where diffraction techniques and other spectroscopies (IR, Raman, EXAFS) often fail. 

Another major difference between the use of X rays and neutrons used as solid state probes is the difference in their penetration depths. This is illustrated by the thickness of materials required to reduce the intensity of a beam by 50%. For an aluminum absorber and wavelengths of about 1.5 A (a common laboratory X-ray wavelength), the figures are 0.02 mm for X rays and 55 mm for neutrons. An obvious consequence of the difference in absorbance is the depth of analysis of bulk materials. X-ray diffraction analysis of materials thicker than 20—50 pm will yield results that are severely surface weighted unless special conditions are employed, whereas internal characteristics of physically large pieces are routinely probed with neutrons. The greater penetration of neutrons also allows one to use thick ancillary devices, such as furnaces or pressure cells, without seriously affecting the quality of diffraction data. Thick-walled devices will absorb most of the X-ray flux, while neutron fluxes hardly will be affected. For this reason, neutron diffraction is better suited than X-ray diffraction for in-situ studies. 

Some of the techniques included apply more broadly than just to surfaces, interfaces, or thin films for example X-Ray Diffraction and Infrared Spectroscopy, which have been used for half a century in bulk solid and liquid analysis, respectively. They are included here because they have by now been developed to also apply to surfaces. A few techniques that are applied almost entirely to bulk materials (e.g.. Neutron Diffraction) are included because they give complementary information to other methods or because they are referred to significantly in the 10 materials volumes in the Series. Some techniques were left out because they were considered to be too restricted to specific applications or materials. 

When Max Planck wrote his remarkable paper of 1901, and introduced what Stehle (1994) calls his time bomb of an equation, e = / v , it took a number of years before anyone seriously paid attention to the revolutionary concept of the quantisation of energy the response was as sluggish as that, a few years later, whieh greeted X-ray diffraction from crystals. It was not until Einstein, in 1905, used Planck s concepts to interpret the photoelectric effect (the work for which Einstein was actually awarded his Nobel Prize) that physicists began to sit up and take notice. Niels Bohr s thesis of 1911 which introduced the concept of the quantisation of electronic energy levels in the free atom, though in a purely empirical manner, did not consider the behaviour of atoms assembled in solids. 

Metallurgists originally, and now materials scientists (as well as solid-state chemists) have used erystallographic methods, certainly, for the determination of the structures of intermetallic compounds, but also for such subsidiary parepistemes as the study of the orientation relationships involved in phase transformations, and the study of preferred orientations, alias texture (statistically preferential alignment of the crystal axes of the individual grains in a polycrystalline assembly) however, those who pursue such concerns are not members of the aristocracy The study of texture both by X-ray diffraction and by computer simulation has become a huge sub-subsidiary field, very recently marked by the publication of a major book (Kocks el al. 1998). 

In this chapter shock modification of powders (their specific area, x-ray diffraction lines, and point defects) measurements via analytical electron microscopy, magnetization and Mossbauer spectroscopy shock activation of catalysis, solution, solid-state chemical reactions, sintering, and structural transformations enhanced solid-state reactivity. 

X-ray diffraction studies show that solid N2O5 consists of an ionic array of linear NO2+ (N-O 115.4 pm) and planar NO3 (N-O 124 pm). In the gase phase and in solution (CCI4, CHCI3, OPCI3) the compound is molecular the structure is not well established but may be O2N-O-NO2 with a central N-O-N angle close to 180°. The molecular form can also be obtained in the solid phase by rapidly quenching the gas to — 180°, but it rapidly reverts to the more stable ionic form... 

The product is a black-brown solid that is very sensitive to oxygen. The same cation can be obtained by oxidation of S4N4 with AsFs and is unusual in being the only sulfur-nitrogen (paramagnetic) radical that has been obtained as a stable crystalline salt. X-ray diffraction analysis shows the structure to be a planar 5-membered ring with approximate... 

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