Critical test for the icosahedral structure of the quasicrystal Al70Pd20Mn10

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Abstract
The technique of secondary-electron imaging reveals the presence of twofold-, threefold- and fivefold-symmetry axes in icosahedral quasicrystalline Al70Pd20Mn10. These axes represent the internuclear directions in a near-surface region of the quasicrystalline structure. Single-scattering cluster spherical wave calculations applied on a Mackay-type cluster can satisfactorily reproduce the experimental patterns. In experiments that are sensitive to only a few surface atomic layers, pentagonal features in the pattern disappear, while faint decagonal spots remain. These observations can be reproduced by computations that consider four atomic layers at the surface of the cluster. Hence, all the observations can be accounted for by the complete or partial Mackay-type cluster. Computations with different atomic distributions in the cluster further suggest that the results do not significantly depend on chemical order.

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