USING SYNCHROTRON RADIATION TO DETERMINE THE ORIENTATION DISTRIBUTION OF MOSAICX BLOCKS
Johannes Zellner, Kerstin Hölzer, Ralf Müller, Klaus Schroer, Edgar Weckert
Institut für Kristallographie, Universität Karlsruhe (TH), 76128 Karlsruhe, Germany
Keywords: mosaicity, tomography
Crystal mosaicity is a spoiling factor in many diffraction experiments. Examples are the influence of mosaicity on multiple-beam diffraction experiments for experimental phase determination. Furthermore an exact knowledge of the mosaic distribution should enable a more quantitative extinction correction. Therefore, knowing the orientation distribution of crystal domains is often desirable if not indispensable for estimating experimental errors or even the feasibility of an experiment. The angular distribution of mosaic domains can be investigated, if the resolution of the experimental setup is better than the features of the orientation distribution which are to be resolved. For a typical mosaic spread of some hundredth to thenth of a degree and a sampling accuracy of some hundred points, well collimated synchrotron radiation of narrow wavelength bandpass is needed and the angular resolution of the diffractometer should be less than 1/10 000o.
In the absence of strain it is sufficient to measure the diffracted intensity distribution in the vicinity of reciprocal lattice points perpendicular to the corresponding lattice vector.
Two methods were applied:
An analyzer crystal perpendicular to the reflection plane of the crystal was used to measure the angular distribution directly (reciprocal space mapping).
Another technique is to record rocking curves at different angles Y, where Y is a rotation about the diffraction vector. For a single rocking curve, the diffraction vector is moved perpendicular through the Ewald sphere. If the mosaicity is small compared to the radius of the Ewald sphere, each scan point represents the integral of the diffraction intensity perpendicular to the scanning direction. Thus if a full range of 360o in Y is covered with the set of measured profiles, the projection of the diffraction intensities in each of these directions is obtained. There's a striking resemblance between this technique and ordinary (real space) tomography. Like in the real space counterpart, well known computer tomography algorithms can be used to reconstruct the orientation distribution of the mosaic domains.
Both methods were carried out on a Y-circle diffractometer at the Swiss-Norwegian beamline and ID22 (both ESRF). For the tomographic reconstruction routines of the RECLBL-Library were used.
First results from measurements on decagonal quasicrystals and distorted semiconductor materials will be presented.
This work was supported by the German Ministry of Education and Research (BMBF), Förderkennzeichen 05 647KA 5.