Ultrahigh pressure shock compression of platinum up to 1.1 TPa: Melting and ionization

Shock compression experiments on platinum (Pt) were carried out by combining a two-stage gas gun with the unique high-Z three-stage gas gun launcher technique. The shock Hugoniot of Pt was determined in a wide pressure range of 0.2--1.1 TPa. These data fill the gap between the two-stage gas gun and magnetically driven shock experiments, and thereby we observe a clear softening behavior of the Pt Hugoniot at the beginning of \ensuremath{\sim}1.1 TPa. Ab initio molecular dynamics simulations were also performed to reveal the response of Pt under ultrahigh pressure compressions. The simulation results show that the melting of Pt under shock compression occurs at \ensuremath{\sim}570 GPa, and a sudden increase in conductivity and free electron number density emerges at the beginning of the Hugoniot softening. The further electronic structure calculations indicate that the Hugoniot softening may arise from the thermal excitation (ionization) of the $5d$ electrons and the localized $6s$ electrons in Pt under extreme shock compression. This work has gained further insight into the ionization behaviors of high-Z metal elements with complex electronic structures under ultrahigh pressure compressions, which is of significance for understanding the partial ionization effect in the transition region from condensed matter to warm dense matter.