Refined measurement of the compressibility of solid platinum under ramped compression to 500 GPa at the Z machine

Quasi-isentropic, shockless ramp-wave experiments at Sandia’s Z machine provide high-precision, high-accuracy absolute measurements of the compressibility of solids at relatively low temperatures and up to multi-megabar pressures, where isothermal diamond-anvil techniques have limited pressure accuracy due to reliance on theoretical equations of state for calibration standards. One such commonly used standard is platinum, due to its phase stability, chemical inertness, and high X-ray scattering power. A recently published calibration of platinum [D. E. Fratanduono et al, Science 372, 1063–1068 (2021)] relied on data from experiments at Z under 400 GPa to constrain the result from National Ignition Facility experiments at higher pressure and attain an unprecedented one-sigma pressure uncertainty of less than 4% at 800 GPa. Additional, higher-precision Z experiments not included in that work offer an opportunity to further reduce uncertainty. We present results from eleven independent measurements of platinum ramp-compressed to as high as 500 GPa using both inverse Lagrangian analysis and Bayesian calibration methods.