Phonon dispersion curve of icosahedral Mg–Zn–Y quasicrystals

We present new results on the phonon dispersion curves of the icosahedral quasicrystal (QC) i-Mg–Zn–Y measured by coherent inelastic scattering of X-rays. Measurements have been made both along both the 5-fold (longitudinal only) and the 2-fold axis (both longitudinal and transverse). We find acoustic branches in agreement with the speed of sound up to about q=4 nm−1 in all cases, but with a linewidth which progressively increases with scattering vector. We find as well ‘optical’ branches with less dispersion but very large line widths for both longitudinal and transversal geometry, differing from the well-known i-Al–Pd–Mn. This difference may be related to the different structural units in the two QCs: the i-Al–Pd–Mn being built of (modified) MacKay and Bergmann clusters, while the i-Mg–Zn–Y are of the Frank–Kasper type.