High-frequency dielectric and magnetic anomaly at the phase transition in NaV2O5

We found anomalies in the temperature dependence of the dielectric and magn etic susceptibility of NaV2O5 in the microwave and far-infrared frequency r anges. The anomalies occur at the phase transition temperature T-c, at whic h the spin gap opens. The real parts of the dielectric constants epsilon(a) and epsilon(c) decrease below T-c, while epsilon(b) remains constant. The change in epsilon(a) is proportional to the square of the ordering paramete r below T-c. The dielectric anomalies found indicate a redistribution of ch arges at the phase transition. The character of the dielectric constant ano malies is of the antiferroelectric type, which is in agreement with the mod els assuming the zigzag charge ordering in thr ab plane below T-c. The anom aly of the microwave magnetic losses is probably related to the coupling be tween the spin and charge degrees of freedom in vanadium ladders.