Elastic constants and charge ordering in alpha '-NaV2O5

We present the temperature dependence of shear and longitudinal elastic con stants in alpha'-NaV2O5. For the longitudinal c(22) and c(33) modes we find anomalies at T-c in contrast to the spin-Peierls substance CuGeO3 where on ly the longitudinal mode along the chain shows a pronounced anomaly at T-SP . The c(66) shear mode (propagation along the chain in b direction, polariz ation in a direction) shows strong softening of 12%. Such a large effect is absent for all sheer modes in CuGeO3. We can interpret this softening with a coupling of the epsilon(xy) symmetry strain to the charge fluctuation of Big symmetry. We give the possible low-temperature charge distribution. [S 0163-1829(99)07537-2].