Sunday, June 16, 2013

Principles of X-ray Diffraction

Diffraction effects are observed when electromagnetic radiation impinges on peri-odic structures with geometrical variations on the length scale of the wavelength of the radiation. The interatomic distances in crystals and molecules amount to 0.15-0.4 nm which correspond in the electromagnetic spectrum with the wave- length of x-rays having photon energies between 3 and 8 keV. Accordingly, phe-nomena like constructive and destructive interference should become observable when crystalline and molecular structures are exposed to x-rays. 
In the following sections, firstly, the geometrical constraints that have to be obeyed for x-ray(x ray protection) interference to be observed are introduced. Secondly, the results are exemplified by introducing the 0/20 scan, which is a major x-ray scattering technique in thin-film analysis. Thirdly, the 0/20 diffraction pattern is used to out- line the factors that determine the intensity of x-ray reflections. We will thereby re- ly on numerous analogies to classical optics and frequently use will be made of the fact that the scattering of radiation has to proceed coherently, i.e. the phase infor-mation has to be sustained for an interference to be observed. 
In addition, the three coordinate systems as related to the crystal {c1}, to the sam-ple or specimen {s,} and to the laboratory {11} that have to be considered in diffrac-tion are introduced. Two instrumental sections (Instrumental Boxes 1 and 2) relat-ed to the B/20 diffractometer and the generation of x-rays(medical equipments) by x-ray tubes supple-ment the chapter.One-elemental metals and thin films composed of them will serve as the material systems for which the derived principles are demonstrated. Abrief presentation of one-elemental structures is given in Structure Box 1.