Quantitative structure factor and density measurements of high-pressure fluids in diamond anvil cells by x-ray diffraction: Argon and water

We report quantitatively accurate high-pressure, structure-factor measurements of fluids in diamond anvil cells (DAC's) using x-ray diffraction. In the analysis of our diffraction data, we found it possible (and necessary) to determine the density directly. Thus, we also present a diffraction-based determination of the equation of state for fluid water. The analysis of these measurements is difficult since the diamond anvils are many times as thick as the sample and excessive care must be taken in the background subtraction. Due to the novel nature of the experiment and the complexity of the analysis, this paper is concerned primarily with a careful exposition of our analytical methods. Our analysis is applicable to both atomic and molecular fluids and glasses, and we present results for the structure factor and density of two relatively low-$Z$ liquids: argon and water. In order to validate our methods we present an extensive comparison of our measurements on water at $P\ensuremath{\simeq}0$ in a DAC to recent state-of-the-art x-ray and neutron diffraction experiments and to first-principles simulations at ambient conditions.