XRD of gold and platinum to above 600 GPa.

The aim of this proposal is to extend XRD measurements of Au and Pt from 150 GPa to above 600 GPa under hydrostatic conditions using a Ne pressure-transmitting medium in a toroidal DAC. Simultaneous determination of the volumes of these three elements at various pressures will be useful to test and potentially refine the recently proposed calibration of gold and platinum, based on shockless compression experiments, as standards up to 1 TPa. Platinum is expected to remain in the fcc structure up to 1 TPa. While a sequence of fcc-hcp-bcc phase transitions has been predicted for gold, the transition pressures remain debated. Here, we expect to unambiguously observe the fcc-hcp transition by reaching slightly above 600 GPa. Finally, this data set will allow for a critical assessment of XRD measurements of these two elements under nanosecond compression, the current approach used to investigate the atomic structure of matter at TPa pressures.