Sound speed measurements in silver shock compressed to 300 GPa: Solid-state transition, melting, and liquid-state response

Abstract

To gain insight into the thermodynamic response of shock-compressed Ag states corresponding to the face-centered-cubic (fcc) to body-centered-cubic (bcc) transformation and melting observed using in situ x-ray diffraction (XRD) measurements [Phys. Rev. Lett. 124, 235701 (2020)], we present longitudinal sound speed results and their analysis in Ag at peak stresses ranging from 60 to 300 GPa. 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 in shock-compressed Ag. From 218 to 300 GPa, the increasing sound speeds and Hugoniot states provide a determination of the liquid phase Ag response. In particular, determination of the Gr\"uneisen parameter (\ensuremath{\Gamma}) showed that the density--Gr\"uneisen parameter product (\ensuremath{\rho}\ensuremath{\Gamma}) for liquid Ag is constant, but differs significantly from that for solid Ag at ambient conditions. Thus, the Mie-Gr\"uneisen equation of state can be used to describe the Hugoniot and off-Hugoniot response of liquid Ag.