THE ICE VII-ICE X PHASE TRANSITION IN ONE-COMPONENT AND TWO-COMPONENT SYSTEMS

A multitude of prior research in the one-component system of high pressure polymorphs of H₂O have detailed the lattice structure and density of these phases, particularly ice VI, ice VII, and ice X; phases potentially present in the interiors of large icy planetary bodies. Conversely, data concerning high pressure polymorphs of a two-component system, high pressure ices with an impurity present, are few, especially the transition of ice VII to ice X. This study examined the ice VII-ice X transition, which occurs at approximately 62 GPa, with both a one-component and select two-component systems. Solutions of pure H₂O, 1.6 mol% NaCl in H₂O, and 1.60 mol% CH₃OH in H₂O were loaded into a symmetric diamond anvil cell (DAC). Experiments were performed at the GSECARS 13-BM-D beam line at the Advanced Photon Source at Argonne National Laboratory. Monochromatic X-ray radiation of a wavelength of 0.2755 angstroms and a MAR 345 online imaging system were used to collect powder diffraction data of the ice samples. Isothermal compression of the samples attained pressures of approximately 72, 74, and 68 GPa respectively, with data collected at pressure increments of 3-5 GPa. A peak split on the ice VII (110) diffraction line was observed in both the ice formed from the one-component and the two-component systems, appearing at approximately 20 GPa and became difficult to distinguish around 65 GPa. Analysis of the data provided pressure-volume relations, at 298 K, which were used to detail the ice VII-ice X transition. Equation of state variables were determined from the pressure-volume relations for all three solutions. The zero pressure volume, the bulk modulus, and its pressure derivative for H₂O were 39.3 ± 0.3, 2.7 ± 1.2, and 4.54 ± 0.08 respectively; for NaCl-H₂O were 39.7 ± 0.1, 25.8 ± 0.3, and 4.15 (fixed value); and for CH₃OH-H₂O were 38.7 ± 0.5, 31.7 ± 2.6, and 3.8 ± 0.1. All these values are in good agreement with previous studies of both one-component and two-component systems. The incorporation of an impurity into the lattice structure of a high pressure polymorph of H₂O does not appear to substantially alter the equation of state of the phase.