Extraordinary case of acoustic wave acceleration due to electrical shorting of piezoelectrics

The regions of anomalous (decelerating) influence of piezoelectricity on the phase velocities of surface and bulk acoustic waves are numerically found near the orientations where the phase velocities of two acoustic modes coincide. Calculations of surface acoustic waves (SAWs) are performed for a potassium niobate crystal. An unusual, inverse in sign, velocity shift of Rayleigh-type SAWs caused by electrical shorting at the surface of the crystal is accompanied by a rise of the tranverse (shear-horizontal) displacement component. Nevertheless, the velocity shift is inverse in sign even in the case when this transverse component tends to zero. Bulk acoustic waves are studied in a semiconductor selenium crystal. The cone of angles confining the region of anomalous “piezoelectric softening” with increasing the conductivity of the crystal is determined.