High-resolution X-Ray diffraction

In recent years, high-resolution x-ray diffraction has become a powerful method for studying layered strnctnres, films, interfaces, and surfaces. X-ray reflectivity involves the measurement of the angnlar dependence of the intensity of the x-ray beam reflected by planar interfaces. If there are multiple interfaces, interference between the reflected x-rays at the interfaces prodnces a series of minima and maxima, which allow determination of the thickness of the film. More detailed information about the film can be obtained by fitting the reflectivity curve to a model of the electron density profile. Usually, x-ray reflectivity scans are performed with a synchrotron light source. As with ellipsometry, x-ray reflectivity provides good vertical resolution [14,20] but poor lateral resolution, which is limited by the size of the probing beam, usually several tens of micrometers. 

The main sources of error in charge density studies based on high-resolution X-ray diffraction data are of an experimental nature when special care is taken to minimise them, charge density studies can achieve an accuracy better than 1% in the values of the structure factor amplitudes of the simplest structures [1, 2]. The errors for small molecular crystals, although more difficult to assess, are reckoned to be of the same order of magnitude. 

Lecomte, C. (1995) Experimental electron densities of molecular crystals and calculation of electrostatic properties from high resolution X-ray diffraction, Adv. in Molec. Struct. Res., 1, 261-302. 

For the crystalline materials, high resolution X-ray diffraction experiment is a powerful tool to derive accurate electron density even for large systems like zeolites. In this study, we are interested in the experimental electron density distribution in the scolecite CaAl2Si3O10 3H20 in order to make comparison with its sodium analogue natrolite Na2Al2Si3Oi0 2H20 for which the electron density has been reported recently [1,2],... 

Scolecite gave the opportunity to relate the electron density features of Si-O-Si and Si-O-AI bonds to the atomic environment and to the bonding geometry. After the multipolar density refinement against Ag Ka high resolution X-ray diffraction data, a kappa refinement was carried out to derive the atomic net charges in this compound. Several least-squares fit have been tested. The hat matrix method which is presented in this paper, has been particularly efficient in the estimation of reliable atomic net charges in scolecite. 

Ghermani, N.E., Lecomte, C. and Dusausoy, Y. (1996) Electrostatic properties in zeolite-type materials from high-resolution X-ray diffraction the case of natrolite, Phys. Rev. B, 53, 5231-5239. 

Figure 7.11 Pictures obtained with high-resolution X-ray diffraction data of G-wire of d(TG4T) (left) and the B-DNA duplex of d(CGCGAATTCGCG). (Reprinted with permission of Wiley— VCH from Chemistry—A European Journal, Vol. 6, p. 3249 ad ff., copyright 2000.)...

There are three methods of collecting high-resolution X-ray diffraction data diffractometry, photographically, and by electronic area detector. Each method has advantages and disadvantages for a particular crystalline protein, but for very accurate data acquisition beyond 2 A... 

In this chapter we introduce high resolution diffraction studies of materials, beginning from the response of a perfect crystal to a plane wave, namely the Bragg law and rocking curves. We compare X-rays with electrons and neutrons for materials characterisation, and we compare X-rays with other surface analytic techniques. We discuss the definition and purpose of high resolution X-ray diffraction and topographic methods. We also give the basic theory required for initial use of the techniques. 

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