We report on the microscopic structure of water at sub- and supercritical conditions studied using X-ray Raman spectroscopy, ab initio molecular dynamics simulations, and density functional theory. Systematic changes in the X-ray Raman spectra with increasing pressure and temperature are observed. Throughout the studied thermodynamic range, the experimental spectra can be interpreted with a structural model obtained from the molecular dynamics simulations. A spatial statistical analysis using Ripley’s K-function shows that this model is homogeneous on the nanometer length scale. According to the simulations, distortions of the hydrogen-bond network increase dramatically when temperature and pressure increase to the supercritical regime. In particular, the average number of hydrogen bonds per molecule decreases to ≈0.6 at 600 °C andp= 134 MPa.

Significance: The combination of multichannel collimator and diamond anvil cells enabled the investigation of the real-space structure of an amorphous material >100 GPa. We have measured the structure of SiO ₂ glass by angle-dispersive X-ray diffraction up to 172 GPa. Our results are in agreement with existing data up to 50 GPa showing a sharp change from fourfold to sixfold Si–O coordination number (CN). However, at higher pressures, CN continuously increases to values beyond 6 without sharp structural changes. The behavior of SiO ₂ glass at high pressure serves as a model for more complex silicate glasses and melts. Thus, our results provide experimental insight into the structural evolution of silicate glasses and melts at ultrahigh pressures.

Oxide potentials which transfer well between different materials have to account explicitly for many-body contributions to the interaction potentials between the ions. These include dipole and quadrupole polarization effects and the compression and deformation of an oxide ion by its immediate coordination environment. Such complex potentials necessarily involve many parameters. We examine how the results of ab initio electronic structure calculations, based upon planewave DFT methods, on general configurations of ions derived from simulations at finite temperature, may be used to parameterize an “aspherical ion method” (AIM) potential (A. J. Rowley, P. emmer, M. Wilson and P. A. Madden, J. Chem. Phys., 1998, 108, 10 209). Dipoles and quadrupoles on the individual ions are obtained via a transformation of the Kohn–Sham orbitals to localized orbitals on each ion, which enables a distorted charge density for each ion to be obtained. The dipoles and quadrupoles appearing in polarization parts of the AIM potential are fit to those obtained from the ab initio ionic charge densities obtained in this way. The remaining parts of the potential, describing short-range repulsive interactions between ions with compressed and deformed charge densities, are fit to the ab initio forces and the stress tensor. By using a sufficiently large and varied set of configurations on which to carry out this optimization, an excellent transferable potential is obtained.