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.)

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.

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