Simultaneous high-pressure and high-temperature volume measurements of ice VII and its thermal equation of state

Abstract

We conducted simultaneous high-pressure ͑P͒ and high-temperature ͑T͒ unit-cell volume ͑V͒ measurements of H 2 O ice VII in an externally heated diamond anvil cell using in situ synchrotron x-ray diffraction technique with an angle-dispersive system. Isothermal unit-cell volume of ice VII was collected at P = 33-50 GPa with T = 873 K. Nonisothermal experiments at T = 430-740 K and P = 19-37 GPa were also conducted to further constrain P-V-T properties of ice VII. Using the existing 300 K compression data for an equation of state ͑EoS͒ at the reference temperature, a thermal EoS of ice VII was constructed from the collected high-P-T data. The melting temperature of ice VII was computed up to P = 40 GPa by thermodynamic calculations with the newly constructed EoS of ice VII. The resulting melting temperatures are higher than that of previous calculations since the new EoS gives a smaller molar volume of ice VII. In contrast, the calculated melting curves are consistent with recent experimental estimates.

Experimental evidence of superionic conduction in H2O ice

“…The formation of PtH X was not detected by the XRD measurements throughout the experiments. 26 The previous melting experiment using an EHDAC demonstrated that H 2 O reacted with Re to form rhenium hydride, confirmed by the presence of O 2 possibly arising from the loss of hydrogen to Re. 10 However, they observed the reaction only above melting temperature, which indirectly denies the possible reaction between solid H 2 O (ice) and Re.…”

“…22 Our data at P ≤ 50 GPa is in good agreement with the calculated 873-K isotherm of ice VII. 26 However, at P > 50 GPa the volume data deviates from the 873-K isotherm of ice VII, abruptly decreasing within a narrow pressure range of P = 50-53 GPa, which seems anomalously compressible compared to P ≤ 50 GPa. With further compression at P > 53 GPa, the volume decreases rather gently, showing smaller compressibility compared to the highly compressible regime.…”

“…The volume measurements were conducted by using in situ angle-dispersive XRD method at the SPring-8, the experimental procedures of which are described in Ref. 26. The P-T conditions for the ionic conductivity and the volume measurements are shown in Fig.…”

“…Some of the data at P ≤ 50 GPa were reported in our previous paper. 26 The previous isothermal compression data at T = 300 K were also plotted in Fig. 5 as well as the 300-K EoS of ice VII and ice X .…”

“…25 However, at a high temperature of 873 K those transformations were not observed up to P = 50 GPa. 26 The relationships between those transitions and the proposed triple point should be clarified in order to understand the origin of the steep rise of melting temperature. Although there have been several reports of the subsolidus phase transitions in ice, they were based on indirect information such as optical observations including Raman spectroscopy.…”