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

We conducted simultaneous high-pressure $(P)$ and high-temperature $(T)$ unit-cell volume $(V)$ measurements of H₂O 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 (EoS) 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.