Visible light consists of electromagnetic waves with wavelengths between 400 and 800 nanometers. This is more than 1,000 times longer than the diameter of a typical atom. Therefore, visible light cannot be used to study atomic structures. In x-ray diffraction we employ x-rays - electromagnetic waves with much shorter wavelengths. In principle, the sample is illuminated by an x-ray beam. The scattered radiation is observed.
Very similar to an optical grating for visible light the atom lattice generates sharp reflections for distinct angles of incidence and exit. The planes of the lattice act almost like mirrors for the x-ray beam.
In addition to the sharp reflections caused by the periodic lattice we observe a much weaker diffuse scattering for all other angles. This scattering is generated by all non-periodic perturbations of the lattice, for example local distortions, irregular configuration of the atom species in an alloy, vacancies, other defects and thermal vibrations.
The picture on the left shows the diffuse scattering pattern of a titanium-vanadium alloy at room temperature.
An important question in my work was, how the differently sized atoms in an alloy distort the crystal lattice. This can get quite complicated, since the atoms displaced by a particularily big neighbor push their own neighbors in turn. Die distortion field of a single defect atom can extend over ten or twenty lattice sites.
Understanding these phenomena is very important. They can influence the mechanlical properties of a material, for example how hard or soft, how tough or brittle it is. Besides that, the distortion fields constitute an interaction mechanism, For example, they can determine how close two defect atoms can get to each other. The most simple consequence can be, that a certain combination of elements cannot be mixed at all.
High Energy X-Rays at the Synchrotron Source
The experiments for this project have been conducted at the European Synchrotron Radiation Source (ESRF) in Grenoble (France). This is a storage ring that keeps electrons very close to the speed of light on a circular path. Each time when they are forced to change their flight direction they emit a shower of x-rays that can be used for experiments. We use unusual short wavelengths, corresponding to x-rays with very high energies.
H. Reichert, V. Bugaev, O. Shchyglo, A. Schöps, I. Ramsteiner, Y. Sikula, H. Dosch, V. Honkimäki:
Strain-induced nonanalytic short-range order in the spin glass Cu83Mn17 – Reply,
Phys.Rev.Lett. 88 (20) 209604 (2002)
High-Energy X-Ray Study of Short Range Order and Phase Transformations in Titanium-Vanadium
Dissertation, Max-Planck-Institut für Metallforschung, Universität Stuttgart (2005)
H. Reichert, A. Schöps, I.B. Ramsteiner, V.N. Bugaev, O.Shchyglo, A. Udyansky, H. Dosch, M. Asta, R. Drautz, V. Honkimäki:
Competition between order and phase separation in Au-Ni,
Phys.Rev.Lett. 95 (23) 235703 (2005)
further publications in preparation
|The work described here was done
at the Max-Planck-Institut für
Metallforschung in Stuttgart in group of Prof.
Helmut Dosch. The experiments have been performed at beamline ID15
at ESRF in Grenoble.