Pressure effect on Kohn anomaly and electronic topological transition in single-crystal tantalum

The Kohn anomaly and topological change of the Fermi surface in d-block metals can occur under high pressure with affiliated significant changes in elastic, mechanical, and transport properties. However, our understanding of their origin and associated physical phenomena remains limited both experimentally and theoretically. Here we study the pressure effect on the Kohn anomaly, electronic topological transition (ETT), and the associated anomalies in physical properties of body-centered cubic (bcc) single-crystal tantalum (Ta). The phonon dispersions of Ta crystal were directly measured up to ∼47 GPa using high-energy resolution inelastic x-ray scattering in a diamond anvil cell with hydrostatic helium medium. A Kohn anomaly in Ta was observed and became significantly stronger at 47.0 GPa at the reduced wave vector of ∼0.7 in the longitudinal acoustic mode along the [ζ,0,0] direction. Our theoretical and experimental results indicate that the electron-phonon coupling and Fermi surface nesting mainly contribute to the Kohn anomaly, and the latter plays a dominant role at high pressures of 17-47 GPa. First-principles calculations further reveal an ETT with a topology change of the Fermi surface to occur at ∼100 GPa in Ta, which causes a softening in the elastic constants (C11 and C44) and mechanical properties (shear, Young's, and bulk moduli). Our study shows that the d-orbital electrons in Ta play a key role in the stability of its electronic topological structure, where electron doping in Ta could significantly depress its ETT and elastic anomaly at high pressures. It is conceivable that our observed Kohn anomaly and ETT in a representative bcc Ta are much more prevalent in d-block transition metals under compression than previously thought.