Kohn anomaly and elastic softening in body-centered cubic molybdenum at high pressure

Transition metals in body-centered cubic (bcc) structures under compression can display several novel physical properties because of their complex electronic structures and electron-phonon interactions. Here, we used inelastic x-ray scattering experiments in a diamond-anvil cell up to \ensuremath{\sim}45 GPa and density-functional theory calculations up to 210 GPa to investigate the phonon dispersions, and electronic and elastic properties of single-crystal molybdenum (Mo). Our results show a pressure-induced Kohn anomaly at $q\ensuremath{\sim}0.5$ along the [\ensuremath{\xi}00] direction in the longitudinal acoustic mode at \ensuremath{\sim}45 GPa; this anomaly is triggered by the pressure-enhanced Fermi-surface nesting effect. Theoretical calculations show that electron redistributions in the $s$-to-$d$ orbitals of bcc-Mo contribute to the shear modulus anomaly at \ensuremath{\sim}50 GPa. In contrast, the Young's modulus anomaly in bcc-Mo at \ensuremath{\sim}210 GPa results from a Lifshitz-type electronic topological transition. Our results shed light on the complex electronic behaviors that are associated with macroscopic elastic properties in typical bcc $d$-block transition metals under compression.