Ramp Compression of Gold to 690 GPa

Gold is a face-centered-cubic (fcc) transition metal with wide applications as a pressure standard and experimental component in high-pressure science. At multimegabar pressures, theoretical studies have predicted transformations to hexagonal-close-packed (hcp), double hexagonal-close-packed, body-centered-cubic (bcc), or stacking disordered phases. Static experimental studies above 200 GPa have produced conflicting results on high-pressure polymorphism in gold. In this study, we used the Omega Laser Facility (U. of Rochester) to ramp-compress gold to 690 GPa. Our target packages consisted of a diamond ablator, gold foil and either a diamond or a LiF window. Samples were compressed over 5-10 ns timescales via laser-ablation. Pressure was determined from measured VISAR wave profiles. The \textit{in-situ }lattice-level structure was probed using X-ray diffraction with a laser-plasma X-ray source. We observe the fcc phase at pressures up to 240 GPa, a mixed or intermediate phase from 240-390 GPa, and the bcc phase from 390-690 GPa. Our results will be compared with existing theoretical calculations and experimental data.