Elastic constants and charge ordering in α′-NaV2O5

We present the temperature dependence of shear and longitudinal elastic constants in a'-NaV2O5. For the longitudinal c22 and c33 modes we find anomalies at Tc in contrast to the Spin Peierls substance CuGeO3 where only the longitudinal mode along the chain shows a pronounced effect at TSP. The c66 shear mode (propagation along the chain in b-direction polarization in a-direction) shows strong softening of 12%. Such a large effect is absent for all shear modes in CuGeO3. We can interpret this softening with a coupling of the exy symmetry strain to the charge fluctuation of B1g symmetry. We give the possible low temperature charge distribution.