Dynamics of the ZnMgY icosahedral phase

The dynamics of the icosahedral ZnMgY phase has been investigated on a single-grain quasicrystalline sample using inelastic neutron scattering Similarly to other quasicrystals we observe well defined acoustic modes for values of the wavevector smaller than 0.3 Angstrom(-1). We find a longitudinal sound velocity equal to 4800 m s(-1) and an isotropic transverse sound velocity equal to 3100 m s(-1). The acoustic character and the broadening of transverse excitations has been studied extensively. We find that transverse excitations remain acoustic up to q = 0.5 Angstrom(-1). For q lying between 0.3 and 0.5 Angstrom(-1) we observe a broadening, which is independent of the propagation or polarization direction, while the dispersion relation departs from linearity. For q = 0.6 Angstrom(-1) there is a resonance with an optic-like excitation located at about 10 meV and the signal can no longer be described by a single acoustic mode. For larger wavevectors we observe a broad distribution of modes, which can be split into three dispersionless optical bands located at 8, 12 and 17 meV. The 8 and 12 meV bands are associated with the crossing of pseudo-zone boundaries by the acoustic branch. A detailed comparison with results obtained in the i-AlPdMn phase shows significant differences: in particular the broadening of transverse acoustic modes is much less pronounced in the ZnTMgY phase than in the i-AlPdMn one.