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Ultrasonic measurements were performed on a fci Zn-Mg-Y quasicrystal and on the related parent compound Zn2Mg. Absolute values of both the longitudinal and the transverse sound velocities as well as their temperature dependence were determined. The temperature dependence of the quasicrystal sound velocity in the millikelvin temperature range is very similar to the logarithmic behaviour observed for amorphous materials.Introduction. -Since the discovery of the first quasicrystalline structure by Shechtman et al. [1] in 1984 in the system Al-Mn, there is a great interest in these long-range ordered but not periodic structures. However, only a few systems exist which are thermodynamically stable, most of them in the Al-TM system (TM = transition metal) like Al-Li-Cu [2], Al-Cu-Fe [3] and Al-Pd-Mn [4]. In 1993 a stable icosahedral phase was discovered in the system Zn-Mg-RE by Luo et al. [5]. This icosahedral phase turned out to be identical with the Z-phase found in 1982 by Padezhnova et al. [6] with a composition of Zn 6 Mg 3 Y 1 , whose structure could not be determined at that time. A detailed investigation showed the stability of the i-phase for the RE elements Y, Gd, Tb, Dy, Ho and Er [7]. In contrast to most of the other known i-phases, this system is based on Zn -not Al-as the main constituent. Another interesting feature of this quasicrystalline alloy is the possibility of incorporating RE elements with 4f -electron carrying a localized magnetic moment (Tb, Ho, Gd) in the quasicrystalline lattice.Since the determination of the primary solidification area in 1997 by Langsdorf et al. [8], large face-centred icosahedral (fci) Zn-Mg-RE single grains can be grown from the melt by different techniques [9][10][11]. Since then, measurements on single grains could be performed. Recent results of neutron scattering experiments show that the fci Zn-Mg-RE quasicrystals do not develop long-range magnetic order [12], however, diffuse-scattering data can be interpreted as short-range spin correlations in a six-dimensional hypercubic crystal [13]. So far, no ultrasonic investigations have been done in this system.

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We report about our work on the preparation and characterization of icosahedral Zn-Mg-(Y, RE) (RE = rare earth element) quasicrystals. We obtained single grains of the icosahedral (i-) phase via slow cooling experiments and with the Bridgman technique. We revealed a composition range of the i-phase, within which the structure changes. Additionally a systematic investigation of the Zn-Mg-Y phase diagmm, including the determination of the primary solidification area of the icosahedral phase is presented. It was found that for a growth from the melt a starting composition with a low Y-content is needed.