1. Department of Physics and Physical Technologies, Technical University of Clausthal, Leibnizstr. 4, Clausthal-Zellerfeld D-38678, Germany
2. Department of Crystallography, University of Göttingen, Goldschmidtstr. 1, Göttingen D-37077, Germany
3. HASYLAB at Deutsches Elektronen Synchrotron DESY, Notkestr. 85, Hamburg D-22603, Germany
4. Gerstenkamp 9, Köln D-51061, Germany