  The high-pressure melting curve of tantalum (Ta) has been the center of a
long-standing controversy. Sound velocities along the Hugoniot curve are
expected to help in understanding this issue. To that end, we employed a
direct-reverse impact technique and velocity interferometry to determine sound
velocities of Ta under shock compression in the 10-110 GPa pressure range. The
measured longitudinal sound velocities show an obvious kink at ~60 GPa as a
function of shock pressure, while the bulk sound velocities show no
discontinuity. Such observation could result from a structural transformation
associated with a negligible volume change or an electronic topological
transition.
