What Determines the fcc-bcc Structural Transformation in Shock Compressed Noble Metals?

High pressure structural transformations are typically characterized by the thermodynamic state (pressure-volume-temperature) of the material. We present in situ x-ray diffraction measurements on laser-shock compressed silver and platinum to determine the role of deformation-induced lattice defects on high pressure phase transformations in noble metals. Results for shocked Ag show a copious increase in stacking faults (SFs) before transformation to the body-centered-cubic (bcc) structure at 144–158 GPa. In contrast, shock compressed Pt remains largely free of SFs and retains the fcc structure to over 380 GPa. These findings, along with recent results for shock compressed gold, show that SF formation promotes high pressure structural transformations in shocked noble metals that are not observed under static compression. Potential SF-related mechanisms for fcc-bcc transformations are discussed.

Figure 1

(a) Schematic of the experimental configuration. (b)–(f) Representative XRD results for shocked Pt and Ag. Pt shocked to 146 and 383 GPa remains in the fcc structure. Ag shocked to 118 GPa remains fcc. Ag shocked to 170 GPa has the bcc structure and Ag shocked to 221 GPa shows a broad liquid scattering ring. The localized spots are from the single crystal LiF window.

Figure 2

(a) Experimental and simulated line profiles for silver at several representative stresses. Only a single broad peak is observed at 203.6 and 220.7 GPa [no observable ${(200)}_{\mathrm{bcc}}$ or ${(211)}_{\mathrm{bcc}}$ peaks] indicating the molten phase, but the noticeably more pointed hump at 203.6 GPa possibly indicates that shock melting is not completed by 203.6 GPa. (b) Experimental and simulated line profiles for platinum at several representative stresses.

Figure 3

Stacking fault probability versus volume compression in silver. Corresponding longitudinal stresses and calculated temperatures are shown at the top. The longitudinal stresses were determined using the reported Ag Hugoniot [21, 22, 33]. The calculated temperatures were determined using the silver equation of state (#2720) from the SESAME library developed at Los Alamos National Laboratory [42].

Figure 4

(a) Comparison of $P_x-V$ states for shock compressed silver determined using XRD with the reported Ag Hugoniot [21, 22, 33]. (b) Comparison of $P_x-V$ states for shock compressed platinum determined using XRD with the reported Hugoniot [18, 33].