Sound velocity softening in body-centered cubic niobium under shock compression

Our understanding of pressure-induced anomalies in the physical and mechanical properties of Nb with a simple body-centered cubic structure remains limited both experimentally and theoretically. In this work we experimentally investigate the sound velocity of the compressional, bulk, and shear waves of Nb up to about 69 GPa and 1100 K under shock compression. These measurements reveal a significant softening in both the compressional and shear wave sound velocities in Nb compressed at 50 to 60 GPa. First-principles computations indicate that this anomalous behavior is most likely due to a pressure-induced Lifshitz-type electronic topological transition at 50 to 60 GPa. This study thus indicates that high-order phase transformations associated with the changes of electronic structures play an important role in the physical modification of transition metals.