Structural Transformation and Melting in Gold Shock Compressed to 355 GPa.

Gold is believed to retain its ambient crystal structure at very high pressures under static and shock compression, enabling its wide use as a pressure marker. Our in situ x-ray diffraction measurements on shock-compressed gold show that it transforms to the body-centered-cubic (bcc) phase, with an onset pressure between 150 and 176 GPa. A liquid-bcc coexistence was observed between 220 and 302 GPa and complete melting occurs by 355 GPa. Our observation of the lower coordination bcc structure in shocked gold is in marked contrast to theoretical predictions and the reported observation of the hexagonal-close-packed structure under static compression.

The laser shock station in the dynamic compression sector. I.

The Laser Shock Station in the Dynamic Compression Sector (DCS) [Advanced Photon Source (APS), Argonne National Laboratory] links a laser-driven shock compression platform with high energy x-ray pulses from the APS to achieve in situ, time-resolved x-ray measurements (diffraction and imaging) in materials subjected to well-characterized, high stress, short duration shock waves. This station and the other DCS experimental stations provide a unique and versatile facility to study condensed state phenomena subjected to shocks with a wide range of amplitudes (to above ∼350 GPa) and time-durations (∼10 ns-1 µs). The Laser Shock Station uses a 100 J, 5-17 ns, 351 nm frequency tripled Nd:glass laser with programmable pulse shaping and focal profile smoothing for maximum precision. The laser can operate once every 30 min. The interaction chamber has multiple diagnostic ports, a sample holder to expose 14 samples without breaking vacuum, can vary the angle between the x-ray and laser beams by 135°, and can translate to select one of the two types of x-ray beams. The x-ray measurement temporal resolution is ∼90 ps. The system is capable of reproducible, well-characterized experiments. In a series of 10 shots, the absolute variation in shock breakout times was less than 500 ps. The variation in peak particle velocity at the sample/window interface was 4.3%. This paper describes the entire DCS Laser Shock Station, including sample fabrication and diagnostics, as well as experimental results from shock compressed tantalum that demonstrate the facility's capability for acquiring high quality x-ray diffraction data.