Nature of low-temperature dense ice up to 80 GPa observed by x-ray diffraction

TL;DR

This study investigates the structural properties of ice VIII under high pressure up to 80 GPa using x-ray diffraction, revealing its volume behavior, phase transition characteristics, and symmetry at low temperatures.

Contribution

It provides new measurements of ice VIII's lattice volume and phase behavior under high pressure and low temperature, with detailed structural parameters and symmetry analysis.

Findings

Ice VIII's volume increases under isothermal compression at low temperatures.

Ice VIII transitions to a high-pressure phase with tetragonal symmetry.

The high-pressure phase is identified as paraelectric.

Abstract

We have measured the lattice volume of ice VIII in different pressure-temperature pathways and found isothermal compression at low-temperature conditions makes the volume larger. Ice VIII has become its high-pressure phase with the molar volume of 6.45 cm^3 at 10 K where the pressure can be estimated as 60.4 GPa based on the third-order Birch-Murnaghan equation with parameters determined in this study (K_0= 32.4 GPa, K'_0= 3.7, and V_0= 11.9 cm^3). The present results indicate that this high-pressure state is paraelectric with tetragonal symmetry.