Temperature variation of ultrasonic attenuation and phase velocity in VO2 and V2o3 crystals

We report measurements of the change in the ultrasonic attenuation and phase velocity in the vicinity of the phase transitions of VO2 and V2O3. We show that there exists a variation of the attenuation and of the velocity in the high temperature phase of VO 2 which is compatible with the existence of a soft-mode at the R-point of the Brillouin zone. The forces involved should be long-ranged.