Viscosity and structure correlations in NaCl and KCl melts at high pressures

Knowledge of the structure and physical property change of melts and fluids at high pressures is important for understanding the nature of the Earth's interior. Recently, we have developed synchrotron techniques that are capable of conducting structure measurement of liquid at high-pressure and high-temperature conditions in a Paris-Edinburgh cell by using multi-angle energy-dispersive x-ray diffraction at the 16-BM-B, HPCAT at the Advanced Photon Source (APS), in collaboration with GSECARS. In addition to the structure measurement, we newly developed falling sphere viscosity measurement using x-ray radiography with high-speed camera (> 1000 frame/second), which enables us to investigate viscosity of not only high viscos melts such as silicate or oxide melts but also low viscos liquids and fluids such as H2O and CO2 (around 1 mPa s or less at ambient pressure). Here we report a study of viscosity and structure change in NaCl and KCl melts at high pressures to 7.3 GPa. Viscosity of the NaCl melt continuously increased with increasing pressure to 7.3 GPa. In contrast, viscosity of the KCl melt first increased up to 2.2 GPa, and then remained at a certain level at higher pressures in 2.2-5.9 GPa. Structure measurement of NaCl and KCl melts revealed that the nearest (r1) and the second nearest (r2) neighbor distance gradually shortened with increasing pressure, while the ratio (r2/r1) changes as a function of pressure resembled the behavior of viscosity of both melts. These observations suggest that viscosities of NaCl and KCl melts at high pressures strongly correlate with the changes of the r2/r1 ratio rather than the distance variation only.