We present the temperature dependence of elastic modes of Fe3−xZnxO4 with x = 0, x = 0.02 and x = 0.032. The c44 shear modes show a pronounced softening which can be normalized to a common behavior for all these substances. We can explain the softening with a bilinear coupling of the elastic strain to an order parameter linked to charge ordering processes. The other elastic modes (c11, c12 and the bulk modulus) do not show any softening. We present a symmetry analysis for the charge ordering model. PACS. 71.30.+h Metal-insulator transitions and other electronic transitions -62.65.+k Acoustical properties of solids -75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)

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.

We present data of the elastic properties of the Laves phase compound CeRu2 in the normal and superconchtcting state obtained from ultrasound measurements. In the temperature range between the superconducting transition temperature of 6.1 K and 300 K we used a deJormation potential coupling approach to describe the temperature dependence oJ" the elastic constants. The elastic anomalies in the superconducting state are characterised with the model of thermally assisted Jlux flow ( TAFF). This enables us to construct a detailed superconducting B-T phase diagram inchtding the vortex dynamics and interpret the peak effect region. Especially the ultrasonic measurements exhibit clearly the connection between the normal state and the superconducting properties. They show that structural fluctuations have strong effects on the superconductivity and on the vortex dynamics.

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The results of the heat capacity and elastic constant studies of Fe3-x Ζn x O 4 (x < 0.04) series are reported. Adiabatic heat capacity measurements, performed on single crystals, show the clear change of the transition character from first order for low Zn content (x < 0.012) to the higher order in 0.012 < x < 0.035 regime. The change of transition order is accompanied by the change in the heat capacity base lines below the Verwey transition temperature Tv. From the elastic constants measurements we found that cli, (Cl1 -c12) and c44 modes exhibit anomalies at Tv but only shear wave c44 show critical softening while approaching transition from high temperatures. This behavior can be linked to the results obtained by the heat capacity studies. These point to the important role of the lattice contribution to the mechanism of the Verwey transition.