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Observation of Hexagonal Close-Packed Water Ice at Extreme Pressures and Temperatures

Alexis Forestier, Gunnar Weck, Sandra Ninet, Gaston Garbarino, Mohamed Mezouar, Frédéric Datchi, Paul Loubeyre

Abstract

The determination of the phase diagram of water ice under extreme conditions remains a fundamental challenge in high-pressure physics. While theoretical predictions have long suggested the existence of compact phases, such as face-centered cubic (fcc) and hexagonal close-packed (hcp) structures, yet only the fcc phase has been experimentally confirmed. Here, we report the first direct observation of a hcp ice phase using synchrotron x-ray diffraction in laser-heated diamond anvil cells. Between 80 and 200 GPa, we observe the coexistence of fcc and hcp ice, arising from stacking disorder in the fcc oxygen layers, similar to phenomena seen in compressed noble gases. Above 200 GPa, the hcp phase becomes dominant and is recovered at 300 K, indicating its increased thermodynamic stability at ultrahigh pressures. These findings not only expand our understanding of water's complex behavior under extreme conditions but also may play a crucial role in the interiors of giant icy planets.

Observation of Hexagonal Close-Packed Water Ice at Extreme Pressures and Temperatures

Abstract

The determination of the phase diagram of water ice under extreme conditions remains a fundamental challenge in high-pressure physics. While theoretical predictions have long suggested the existence of compact phases, such as face-centered cubic (fcc) and hexagonal close-packed (hcp) structures, yet only the fcc phase has been experimentally confirmed. Here, we report the first direct observation of a hcp ice phase using synchrotron x-ray diffraction in laser-heated diamond anvil cells. Between 80 and 200 GPa, we observe the coexistence of fcc and hcp ice, arising from stacking disorder in the fcc oxygen layers, similar to phenomena seen in compressed noble gases. Above 200 GPa, the hcp phase becomes dominant and is recovered at 300 K, indicating its increased thermodynamic stability at ultrahigh pressures. These findings not only expand our understanding of water's complex behavior under extreme conditions but also may play a crucial role in the interiors of giant icy planets.

Paper Structure

This paper contains 1 section, 1 figure.

Table of Contents

  1. Materials and Methods

Figures (1)

  • Figure 1: (a) Sketch of the sample assembly loaded in the DAC for XRD measurements on H$_2$O at extreme pressures and temperatures, scaled at around $200$ GPa. (b) Series of diffraction images from run 1 on hot ice at 82 GPa, recorded while decreasing the temperature from 1986 K to 1086 K. The sequence illustrates the evolution of the 111 and 200 fcc reflections (denoted F111 and F200), the onset of X-ray diffuse scattering associated with the development of stacking faults, and the subsequent appearance of hcp Bragg reflections (H100, H002, and H101) below 1310 K. The final image, collected at 1086 K, corresponds to the end of the temperature ramp, during which the cell was rotated by 20$^\circ$ during XRD acquisition. (c) Integrated diffraction patterns measured at 82 GPa, 1086 K (bottom), 197 GPa, 2250 K (middle) and 219 GPa, 2630 K (top). XRD peaks from ice crystals are labeled in pink (hcp) and blue (fcc). The measured diffraction also includes contributions from bcc ice, the Re gasket, Al$_2$O$_3$ insulating layers (corundum phase), and BDD heaters, indicated by ticks.