ANOMALOUS X-RAY-DIFFRACTION STUDY OF THE ALPDMN ICOSAHEDRAL PHASE

The atomic structure of the perfect AlPdMn icosahedral phase has been studied on a single grain by anomalous x-ray diffraction close to the Pd edge. A method is proposed for relative normalization of the data, in order to extract the partial structure factor F(Pd) of the Pd atoms . The Pd is found to be very ordered in the six-dimensional descriptio n of the quasicrystal, as was previously inferred from x-ray and neutr on single-crystal diffraction data. However, a phason Debye-Waller-lik e term has to be introduced to account for the data. Comparison of the present data set with low-resolution data indicates the existence of diffuse scattering located close to the Bragg reflections whose intens ity scales as [1-exp(- B(perp) Q2(perp)] as expected from random fluct uations of the atomic surfaces. The average fluctuation is found to be of the order of 1 angstrom. Moreover the comparison of the data with a spherical model shows that the atomic surface describing Pd atoms is likely to be made of several disjointed pieces.