Shock temperature and reflectivity of precompressed H₂O up to 350 GPa:Approaching the interior of planets

Using a combination of static precompression and laser-driven shock compression, shock temperature and reflectivity of H₂O have been measured up to 350 GPa and 2.1×10⁴ K. Here, two calibration standards were applied to enhance temperature measurement reliability. Additionally, in temperature calculations, the discrepancy in reflectivity between active probe beam wavelength and self-emission wavelength has been taken into account to improve the data's precision. Precompressed water's temperature-pressure data are in very good agreement with our quantum molecular dynamics model, suggesting a superionic conductor of H₂O in the icy planets' deep interior. A sluggish slope gradually approaching Dulong-Petit limit at high temperature was found at a specific heat capacity. Also, high reflectivity and conductivity were observed at the same state. By analyzing the temperature-pressure diagram, reflectivity, conductivity and specific heat comprehensively at conditions simulating the interior of planets in this work, we found that as the pressure rises, a change in ionization appears; it is supposedly attributed to energetics of bond-breaking in the H₂O as it transforms from a bonded molecular fluid to an ionic state. Such molecular dissociation in H₂O is associated with the conducting transition because the dissociated hydrogen atoms contribute to electrical properties.