Dynamics of theZnMgY icosahedral phase

Abstract

The dynamics of the icosahedral ZnMgY phase has beeninvestigated on a single-grain quasicrystalline sample usinginelastic neutron scattering. Similarly to other quasicrystalswe observe well defined acoustic modes for values of thewavevector smaller than 0.3 Å-1. We find a longitudinal sound velocity equal to 4800 m s-1 and anisotropic transverse sound velocity equal to 3100 m s-1.The acoustic character and the broadening of transverseexcitations has been studied extensively. We find thattransverse excitations remain acoustic up toq = 0.5 Å-1. For q lying between 0.3 and0.5 Å-1 we observe a broadening, which is independentof the propagation or polarization direction, while thedispersion relation departs from linearity. Forq = 0.6 Å-1 there is a resonance with an optic-like excitation located at about 10 meV and the signal can no longerbe described by a single acoustic mode. For larger wavevectorswe observe a broad distribution of modes, which can be splitinto three dispersionless optical bands located at 8, 12 and17 meV. The 8 and 12 meV bands are associated with the crossingof pseudo-zone boundaries by the acoustic branch. A detailedcomparison with results obtained in the i-AlPdMn phase showssignificant differences: in particular the broadening oftransverse acoustic modes is much less pronounced in the ZnMgYphase than in the i-AlPdMn one.