Tellurite And Fluorotellurite Glasses For Active And Passive

5. Crystallisation studies; MDO 136
the range of 1 to 10 Å (10 -10 to 10 -9 m) which corresponds to the wavelength of the X-ray
part of the electromagnetic spectrum. When this X-ray radiation impinges on a material,
two effects are observed: absorption and scattering [1]. Absorption results in excitation of
inner orbital electrons of the atoms of the material which Auger electron spectroscopy
(AES) and X-ray fluorescence (XRF) utilise for chemical analysis [1].
When X-rays are scattered by a material their wavelength is either changed
(incoherent scattering) or their wavelength remains unchanged (coherent scattering) [1].
The coherent X-rays scattered from different electrons of the same atom lead to
interference effects. These effects are more likely for X-rays coherently scattered by
atoms of the same or adjacent planes [1]. This phenomena, known as X-ray diffraction, is
analogous to the diffraction of light from an optical grating. The closely spaced lines of
the grating are the two-dimensional equivalent of the atomic planes in a crystal. Once the
diffraction pattern is obtained the atomic structure can be calculated [1].
The scattering of X-rays was first treated by von Laue [2]. He considered a row of
atoms periodically spaced, scattering a parallel set of rays, which are only observable if
they are travelling in phase. This means the path difference between each of the scattered
rays should be an integral multiple of the wavelength, λ (i.e. nλ, where n = an integer 1,
2, 3 etc.).
W.L. Bragg found that beams of diffracted X-rays can be treated in terms of
‘reflections’ from the lattice planes of crystals [3]. Fig. (5.2) illustrates the diffraction of
incident X-rays from parallel rows of atoms in a crystalline lattice, described below.