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 ∼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 ∼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.

Figure 1

Principal Hugoniot of platinum in the $U_s\text{−}{U}_p$ (a) and the P-η (b) space. The open spheres are the present three-stage and two-stage gas gun data. The open squares and open up-pointing triangles are the two-stage gas gun data of McQueen et al. [43] and Holmes et al. [11], respectively. The Z-machine data are displayed as the open diamonds [13]. The dash, dash-dot, dash-dot-dot, and dash-plus curves are the calculations of Elkin et al. [19], SESAME3730 [17], QEOS [18], and SESAME3732 [17], respectively. The AIMD simulations with LDA and PBEsol [13] functionals are represented by the solid and short-dash lines, respectively. The linear $U_s-U_p$ relation obtained by fitting the present gas gun data is shown as the black solid line.

Figure 2

(a) The radial distribution function of Pt under shock conditions; (b) the melting temperature of Pt under static and dynamic high pressures. The solid and dash-dot lines represent the shock Hugoniot calculated by the AIMD simulations with PBEsol functional [13] and SESAME3730 EOS [17], respectively. The short dash and dash-dot-dot lines are the melting curve by fitting the DAC experimental data of Anzellini et al. [9] and Geballe et al. [44], respectively. The long dash lines are the melting curve from the semiempirical EOS of Elkin et al. [19]. The open square is the melting temperature of Pt under shock compressions based on our RDF calculations.

Figure 3

Ab initio calculations for the free electron number density $n_e$ (a) and dc conductivity ${\sigma }_0$ (b) at shock conditions of Pt. The superlinear behavior for both $n_e$ and ${\sigma }_0$ appears when the shock pressure exceeds ∼1.06 TPa.

Figure 4

Ab initio calculations of the DOSs of the $6s$ band (left panel) and $5d$ band (right panel) for the shocked Pt. The black dash lines represent the Fermi level.