Shock Compression of Silver to 300 GPa: Solid State Transition, Melting, and Liquid State Response

To gain insight into the shock compressed Ag states corresponding to the fcc-to-bcc transformation and melting observed using in-situ x-ray diffraction (XRD) measurements [PRL 124, 235701 (2020)], we measured wave profiles and longitudinal sound speeds in Ag at peak stresses ranging from 30 to 300 GPa. The measured U_s-u_p results were fitted very well by a linear relationship over the entire stress range, providing an accurate determination of the Hugoniot curve. The measured sound speeds increased linearly with density compression to 171 GPa, showing that the sound speeds (and longitudinal moduli) in the fcc and bcc phases are very similar. Between 171 and 218 GPa, the sound speed dropped significantly, consistent with the melting reported using XRD measurements. From 218 to 300 GPa, the increasing sound speeds and Hugoniot states provide a determination of the liquid phase Ag response. Determination of the Grüneisen parameter (Γ) showed that Γ/V for liquid Ag is constant, but differs significantly from that for solid Ag at ambient conditions. Thus, the Mie- Grüneisen equation of state can be used to describe the Hugoniot and off-Hugoniot response of liquid Ag.