Diffraction methods EXAFS

Mercury(II) halides can be inserted into porosils (crystalline microporous silicon dioxide modifications) via the vapor phase [60-63]. The guest molecules HgX2 (X Cl, Br, I) are disordered within the voids of the host and the positions of their atoms cannot be determined by diffraction methods. EXAFS as a local spectroscopic tool is well suited to gather information about the local structure of the guest species. 

XAFS (EXAFS and XANES) methods X-ray diffraction method Biological investigations Flydrolysis of [OrganotinllV)]" Cations Interactions of [OrganotinllV)]" with Biological Molecules... 

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. 

Diffraction methods are not the only way of determining structure. Other techniques that sometimes make use of bond valences include NMR discussed in Section 13.5.1 and EXAFS discussed in Section 13.6.1. 

During the last two decades, studies on ion solvation and electrolyte solutions have made remarkable progress by the interplay of experiments and theories. Experimentally, X-ray and neutron diffraction methods and sophisticated EXAFS, IR, Raman, NMR and dielectric relaxation spectroscopies have been used successfully to obtain structural and/or dynamic information about ion-solvent and ion-ion interactions. Theoretically, microscopic or molecular approaches to the study of ion solvation and electrolyte solutions were made by Monte Carlo and molecular dynamics calculations/simulations, as well as by improved statistical mechanics treatments. Some topics that are essential to this book, are included in this chapter. For more details of recent progress, see Ref. [1]. 

X-ray and neutron diffraction methods and EXAFS spectroscopy are very useful in getting structural information of solvated ions. These methods, combined with molecular dynamics and Monte Carlo simulations, have been used extensively to study the structures of hydrated ions in water. Detailed results can be found in the review by Ohtaki and Radnai [17]. The structural study of solvated ions in lion-aqueous solvents has not been as extensive, partly because the low solubility of electrolytes in 11011-aqueous solvents limits the use of X-ray and neutron diffraction methods that need electrolyte of -1 M. However, this situation has been improved by EXAFS (applicable at -0.1 M), at least for ions of the elements with large atomic numbers, and the amount of data on ion-coordinating atom distances and solvation numbers for ions in non-aqueous solvents are growing [15 a, 18]. For example, according to the X-ray diffraction method, the lithium ion in for-mamide (FA) has, on average, 5.4 FA molecules as nearest neighbors with an... 

New techniques such as laser Raman spectrophotometry, NMR spectroscopy and X-ray and neutron diffraction methods, as well as EXAFS and XANES spectroscopy, provide us tools to observe solution phenomena from the microscopic point of view on the bases of structural chemistry and reaction dynamics. Thus, structural and dynamic studies of solutions have been developed as new streams of solution chemistry. 

In the following section, diffraction results are presented and discussed for various groups of ions. The basis for discussing these ions is a selection of results from the neutron and X-ray diffraction difference methods (ND and XD), total X-ray and neutron diffraction methods (TX and TD), and EXAFS (EX). A selection of results are contained in Tables II-VII and, where appropriate, will be contrasted with results derived from other methods. A more complete tabulation of TX results is available in table 2 of ref. (53) or in ref. (47). 

In crystals, impurities can take simple configurations. But depending on their concentration, diffusion coefficient, or chemical properties and also on the presence of different kind of impurities or of lattice defects, more complex situations can be found. Apart from indirect information like electrical measurements or X-ray diffraction, methods such as optical spectroscopy under uniaxial stress, electron spin resonance, channelling, positron annihilation or Extended X-ray Absorption Fine Structure (EXAFS) can provide more detailed results on the location and atomic structure of impurities and defects in crystals. Here, we describe the simplest atomic structures more complicated structures are discussed in other chapters. To explain the locations of the impurities and defects whose optical properties are discussed in this book, an account of the most common crystal structures mentioned is given in Appendix B. 

This review deals with several types of polymer hosts that have been investigated. These include polyethylene oxide and its several modified forms, comb like polymers such as polyacrylates and inorganic polymers such as polyphosphazenes and polysiloxanes. Various instrumental techniques have been employed in the structural characterization of polymer electrolytes. The structural information obtained from methods such as Extended X-ray Absorption Fine Structure (EXAFS), X-ray diffraction methods, vibrational spectroscopy and nuclear magnetic resonance (NMR) have also been discussed. 

Melm)(02) EtOH, where a short Fe—Nim and a long Fe—O bond are observed both from the structure revealed by single-crystal x-ray diffraction methods and by EXAFS data. On the other hand, for solvate-free Fe(PF)(2-MeIm)(02) and for Fe(PF)(l,2-Me2lm)(02), the EXAFS patterns are interpreted in terms of a short Fe—O and long Fe—Im bond. ... 

The RMC method is more general than this simple algorithm in that any set(s) of data that can be directly related to the structure can be modelled. It can be applied to isotopic substitution in neutron diffraction, or equivalently to anomalous scattering in X-ray diffraction, to EXAFS (multiple edges) and possibly to NMR data. All data sets can be modelled simultaneously simply by adding the respective y1 values. 

Zeolites aie crystalline. Diffraction methods ate therefore the prime technique for structural characterization. Local stmctural features (such as Brpnsted add sites, non-frramewcHlr cation configurations etc.) that do not obey the full crystallographic symmetry are often present These contribute to the diffraction pattern only wealdy if they are dilute, or they t pear as disorder in the diffraction-averaged stracture. Such features must be probed by techniques such as solid state nmr or EXAFS that are sensitive to local enviromnent, but proper interpretations of such data must still be made in the context of the crystal structure provided by the di raction results. 

Table 2 summarizes selected results among various experimental results reported. Generally, results from NMR agree with those obtained from diffraction methods. However, if we define the hydration number to be the number of water molecules in the nearest neighbor of an ion, the results obtained by the diffraction methods are more reliable than those by NMR, because NMR cannot measure the number of water molecules moving fast within less than ca. 10 s well. Alternatively, the hydration numbers found by XRD, ND, and EXAFS methods for alkali and alkali earth metal ions do not always agree with the results derived by Frank-Wen s and Samilov s considerations, because the hydrated water molecules determined by diffraction methods do not always imply water molecules strongly combined with the central ions. 

In the realm of impurities such a task is certainly not simple as standard diffraction methods cannot be used for that purpose. Moreover the EXAFS technique requires a minimum of impurity concentration around 1% and the uncertainty in the obtained Re values is higher than 1pm [2,3]. 

Studying structures of liquids is a slow and thankless task, so you need to start when you are young if you want to give yourself time to get somewhere For a start, there are very few techniques available. NMR studies can give valuable information about structures of simple compounds in solutions in liquid crystals (Section 4.15), but otherwise the diffraction methods and EXAFS (Section 10.15) are the only generally available techniques. It is not difficult to collect diffraction data it can be done relatively easily using electrons. X-rays or neutrons. The hard part is to know what to do with the data once they have been obtained and how to interpret the results. It is the inherent structural complexity of liquids and glasses that makes their stmcture so extremely difficult to determine [16]. 

So we are left with the situation that, by and large, we are ignorant of the structures of liquids and solutions. As the majority of chemical reactions occur in solution, and chemical reactivity is highly dependent on structure, this is unfortunate. But at present it is simply not possible to find out as much about hquids by diffraction methods as we routinely do for the sohd and gaseous states. However, the EXAFS method is often apphed to (frozen) solutions and may give some hmited information on the local surrounding of certain atoms. 

Kamiya K., Tatsumi M., Matsuoka J., Nasu H. Structure study of sol-gel-derived sodium germanate glasses by X-ray diffraction and EXAFS methods. Phys. Chem. Glasses 1998b 39 9-16 Kamiya K., Oka A., Nasu H., Hashimoto T. Comparative study of structure of silica gels from different sources. J. Sol-Gel Sci. Technol. 2000 19 495-499 Kamiya K. The comparative structure study of sol-gel and biology-derived silicas (in Japanese). Seramikkusu (Ceramics) 2002 37 169-172... 

Computer simulations have been applied to studies of the structure of molten salts along two lines one is the fi ee standing application of the computer simulation to obtain the partial pair correlation functions, the other is the refining of x-ray and neutron diffraction and EXAFS measurements by means of a suitable model. In both cases a suitable potential function for the interactions of the ions must be employed, as discussed in Sect. 3.2.4. Such potential functirms are employed in both the Monte Carlo (MC) and the molecular dynamics (MD) simulation methods. A further aspect that has been considered in the case of molten salts is the long range coulombic interaction that exceeds the limits of the periodic simulation boxes usually involved (for 1000 ions altogether), requiring the Ewald summation that is expensive in computation time and is prone to truncation errors if not applied carefully. 

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