Shear softening in tantalum at megabar pressures

We have experimentally and theoretically investigated the aggregate elasticity of tantalum to pressures exceeding 1 Mbar. Our inelastic x-ray scattering measurements show a softening in the aggregate shear velocity in the 90–100 GPa range with typical pressure dependence above 120 GPa. Our calculations suggest that, in analogy with theoretical predictions on vanadium and niobium, this anomalous behavior is likely due to the intraband nesting of the Fermi surface that leads to an electronic topological transition and a concomitant transverse-acoustic-phonon mode softening.

Figure 1

IXS spectrum collected at ambient temperature and 120 GPa, for $q=6{\text{nm}}^{-1}$. The experimental data and their statistical error bars are shown together with the best fits.

Figure 2

Aggregate longitudinal sound velocity as a function of density. Solid circles, IXS measurements; open triangles, ISLS (Ref. 19); open squares, RXRD (Ref. 20); and open diamonds, shock-wave compression (Ref. 34). The solid line is the extrapolation of the ultrasonic measurements (Ref. 18) [relative uncertainty $\sim 20\mathrm{\%}$ at 1 Mbar (Ref. 7)]. Inset: computational results (Voigt-Reuss-Hill average).

Figure 3

Aggregate shear sound velocity as a function of density. Solid circles, IXS measurements; open triangles, ISLS (Ref. 19); open squares, RXRD (Ref. 20). The solid line is the extrapolation of the ultrasonic measurements (Ref. 18) [relative uncertainty $\sim 20\mathrm{\%}$ at 1 Mbar (Ref. 7)]. Inset: computational results (Voigt-Reuss-Hill average).