Anomalous X-ray diffraction study of the AlPdMn icosahedral phase

Abstract

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 FPd of the Pd atoms. The Pd is found to be very ordered in the six-dimensional description of the quasicrystal, as was previously inferred from X-ray and neutron single-crystal diffraction data. However, a phason Debye-Waller-like 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 intensity scales as (1-exp(-BperpQ2perp)) as expected from random fluctuations of the atomic surfaces. The average fluctuation is found to be of the order of 1 AA. 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.