DYNAMICAL X-RAY DIFFRACTION IN ICOSAHEDRAL Al-Pd-Mn QUASICRYSTAL. Y. Zhang, R. Colella, Purdue Univ., Physics Dept., W. Lafayette, IN 47907, U.S.A., S. Kycia, A. Goldman, Iowa State Univ., Physics Dept., Ames, IA 50011, U.S.A.

Large size crystals of Al₇₀Pd_21.5 Mn_8.5 were grown by the Bridgman method. Berg-Barrett topographs of a thick specimen showed three large grains approximately equal in size. Regions extending several mm² in size could be identified, over which the diffracted intensity was uniform, an indication of crystal perfection. The integrated intensity of several reflections was measured on an absolute basis with [[lambda]] = 1.54 Å and the structure factor was extracted from the experimental value of the integrated intensity by making use of dynamical theory. Such procedure is justified by the fact that the rocking curves were extremely sharp (. 30 arcseconds) and that anomalous transmission has been observed in similar samples (S.W. Kycia et al., Phys. Rev. B48, 3544 (1993)). While good agreement is found between theory and experiment for the strong reflections, large discrepancies are observed for the weak reflections, which are more sensitive to the details of the atomic surfaces in 6D space.

Berg-Barrett topographic photo. Symmetric Bragg geometry; [[lambda]] = 1.54 Å. Reflection excited: (0 2 4 0 2 4) on a two-fold axis, normal to the specimen surface. The dark circle is due to surface stain. The white lines are regions of strain.