Dynamical diffraction and x-ray standing waves from atomic planes normal to a twofold symmetry axis of the quasicrystal AlPdMn

We have observed dynamical diffraction in the [0240 (2) over bar 4] and [0460 (4) over bar 6] reflections of the icosahedral quasicrystal AlPdMn in the back-reflection geometry (theta(B) = 90 degrees). The x-ray fluorescence From the A1 and Pd atoms exhibits strong standing wave behavior, similar to that observed in crystalline materials. The data indicate a long-range order of each species of atoms, with the coherent positions attributable to distributions of the Al and Pd, which we compare to a centrosymmetric model. We observe deviations from the model which imply small departures from inversion symmetry along the twofold symmetry axis and from the expected coherent fractions for Al. [S0031-9007(99)08910-3].