Diffraction studies EXAFS

I. V. Tomov, D. A. Oulianov, P. Chen, and P. M. Rentzepis, Ultrafast time-resolved transient structures of solids and liquids studied by means of X-ray diffraction and EXAFS. J. Phys. Chem. B 103(34), 7081-7091 (1999). 

Nowadays there is a general consensus that the Ti(IV) atoms are incorporated as isolated centers into the framework and are substituting Si atoms in the tetrahedral positions forming [Ti04] units. The model of isomorphous substitution has been put forward on the basis of several independent characterization techniques, namely X-ray [21-23] or neutron [24-26] diffraction studies, IR (Raman) [52-57], UV-Vis [38,54,58], EXAFS, and XANES [52, 58-62] spectroscopies. 

In order to gain information on the environments of certain atoms in dissolved species, in melts or in solids (crystalline or noncrystalline), which are not accessible to diffraction studies for one reason or another, X-ray absorption spectrometry (XAS) can be applied, with the analysis of the X-ray absorption near-edge structure (XANES) and/or the extended X-ray absorption fine structure (EXAFS). Surveys of these methods are available 39,40 a representative study of the solvation of some mercury species, ElgX2, in water and dimethylsulfoxide (DMSO) by EXAFS and XANES, combined with quantum-chemical calculations, has been published.41... 

EXAFS (Extended X-ray Absorption Fine Structure) measurements using synchrotron radiation have been successfully applied to the determination of structural details of SCO systems and have been particularly useful when it has not been possible to obtain suitable crystals for X-ray diffraction studies. Perhaps the most significant application has been in elucidating important aspects of the structure of the iron(II) SCO linear polymers derived from 1,2,4-triazoles [56]. EXAFS has also been applied to probe the dimensions of LIESST-generated metastable high spin states [57]. It has even been used to generate a spin transition curve from multi-temperature measurements [58]. 

The solid curves below 7 A are calculated accurately for a model (Section 4) that fits the osmotic coefficient data. The curves above 7 A are merely schematic, showing in exaggerated form the oscillations that appear in gab at large r when the concentration is large, even for the models in Section 4. The dashed curve indicates the location and intensity of the peak in g+. (r) identified in aqueous NiCle in neutron diffraction and EXAFS studies, as reviewed in Section 5. 

At this time diffraction data for ion-ion distributions in aqueous solutions of moderate concentration are beginning to become available. In aqueous NiCl2 solutions very refined neutron diffraction studies indicate that the Ni2+-Cl pair correlation function has a peak near 3.l8 under conditions in which the Cl does not penetrate the Ni(H20)g2+ unit. (J+2 ) It is reported that EXAFS studies give the same result. (1 3) While the information is most welcome it is puzzling because a geometrical calculation indicates that the closest center to center distance for the Ni2+ and a Cl that does not penetrate the hydration shell is closer to 3.98. (7)... 

Metal-oxygen and metal-metal distances have been established for many oxo- and hydroxo-bridged dimers and polynuclear cations, for example (57)220 and several of type (58).217 Early work was on solids, but recently X-ray diffraction studies have established the geometries of a few such species in solution. EXAFS has proved valuable in establishing the geometry of the various di- and tri-nuclear forms of the various oxidation states of molybdenum in aqueous solution.23... 

Double salt problems are of practical importance in the chemical industry. However, only limited discussion has appeared concerning an elucidation of the mechanism of formation, because reliable information on the structure of ions in solution is not available. The development of solution X-ray diffraction and EXAFS techniques can throw new light on the problem to elucidate the mechanism of formation of double salts. As a case study we take a series of double salts M C1-MgCl2,nH20, where M1 denotes an alkali metal or an ammonium ion. 

The direct X-ray diffraction study is undoubtedly the most optimal and informative structural method for all compounds. Unfortunately, no one has obtained crystals of the binuclear iron(II) azineoximates, which are suitable for X-ray crystallography [193], The structure of these complexes was deduced from EXAFS data, which allow one to obtain the function of the radial atom distribution (RAD) and hence to determine coordination numbers and the distances from the encapsulated iron ions to the clathrochelate framework atoms, and from Fe Mdssbauer spectra, which make it possible to predict the distortion angle (p (see above). Macrobicyclic iron(II) tris-dioximates... 

Pioneering development of a capillary based micro reaction cell was published by Clausen et al., for studies of catalysts under real operating conditions. The reaction cell was suited for both in situ EXAFS and transmission geometry powder diffraction studies (and was also used for combined... 

MODERN TECHNIQUES OF STUDYING SOLVAHON AXD DIFFRACTION AND EXAFS 541 4nr g(i)... 

Modern techniques of studying solvation AXD diffraction and EXAFS... 

For practical applications in X-ray and neutron-diffraction studies under pressure (and in volumetric or, more, recently in extended X-ray-absorption fine-structure spectroscopy (EXAFS) studies), a single (p-v )-isotherm at ambient temperature is insufficient, so low- and high-temperature isotherms are also required. Thus, the complete (thermal) equation of state (EOS), P - or F = V p,T), must be known for at least one solid to establish... 

Details of the ferrous coordination environments are available only for deoxyHr. A difference electron density map from X-ray diffraction studies of deoxyHr and metHr suggests only small structural differences between the two forms (19). Although the crystal structure of deoxyHr is of low resolution (3.9 A), there appears to be a decrease in the correlated motions of the two iron atoms suggesting a weakening of the Fe-p.-0 bonds and an increase in the Fe-Fe distance. These changes in core dimensions are confirmed by an EXAFS analysis of deoxyHr (rpe-o 1.98 A, rpe-Fe = 3.57 A) (39) they suggest that the oxo bridge has become protonated in deoxyHr, in line with the diminished Lewis acidity of Fe(II). 

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