Evidence and Stability Field of fcc Superionic Water Ice Using Static Compression

Structural transformation of hot dense water ice is investigated by combining synchrotron x-ray diffraction and a laser-heating diamond anvil cell above 25 GPa. A transition from the body-centered-cubic (bcc) to face-centered-cubic (fcc) oxygen atoms sublattices is observed from 57 GPa and 1500 K to 166 GPa and 2500 K. That is the structural signature of the transition to fcc superionic (fcc SI) ice. The sign of the density discontinuity at the transition is obtained and a phase diagram is disclosed, showing an extended fcc SI stability field. Present data also constrain the stability field of the bcc superionic (bcc SI) ice up to 100 GPa at least. The current understanding of warm dense water ice based on ab initio simulations is discussed in the light of present data.

Figure 1

Typical XRD patterns associated to the ${\mathrm{CO}}_2$ laser heating configuration, as obtained for the heating run at 57 GPa. (a) Section of the x-ray diffraction 2D images collected at 57 GPa–1560 K (left), $-1320\mathrm{K}$ (middle), and $-300\mathrm{K}$ (right), respectively, above and below the bcc-fcc transition and after the heating cycle. The Bragg peaks of bcc and fcc phases of ice are tagged. (b) XRD patterns. The intensity of the diffraction peaks is displayed in a log scale. The spectra have been shifted vertically, and the peaks coming from the diamond anvils have been masked for clarity. The double peak structure of bcc 110 at 1320 K is indicative of the transition to bcc SI. The inset shows a sketch of a cross-sectional view of the ${\mathrm{H}}_2\mathrm{O}$ sample cavity under direct ${\mathrm{CO}}_2$ laser heating, with the hot ice volume in red.

Figure 2

Typical XRD patterns associated to the Yb laser heating configuration, as obtained for the heating run at 166 GPa. (a) Section of the XRD 2D images collected at 166 GPa–1750 K (left) and 166 GPa–2500 K (right) collected just before and after the bcc-fcc transition. Tick marks indicate the Bragg peak positions of the compounds present in the sample chamber. The diamond anvil diffraction peaks are tagged in blue and masked. (b) Integrated XRD patterns of water measured at several temperatures around 166 GPa. The spectra have been shifted vertically for clarity. The inset shows a sketch of a cross-sectional view of the ${\mathrm{H}}_2\mathrm{O}$ sample cavity with the C:B absorber disk positioned in a pit made on the anvil tip.

Figure 3

Comparison between calculated and measured densities versus pressure for bcc, bcc SI, and fcc SI ice. The color scale corresponds to the temperature of the hot spot measured by spectroradiometry. The symbols indicate experimental data of present work and previous works [5, 21, 40, 41, 42]. Pressure values for Ref. [5] were corrected to account for the updated ruby scale [43]. Full, dotted, and dashes lines indicate DFT-PBE calculations for bcc, bcc SI, and fcc SI [13, 44], respectively.

Figure 4

Phase diagram of dense ${\mathrm{H}}_2\mathrm{O}$. The various colored domains correspond to the stability fields of: in gray, the molecular fluid; in blue, the bcc ices (ice VII, ${\mathrm{VII}}^{\prime }$, X, and bcc SI); in yellow, the fcc SI. The white zones represent the uncertainties on the transition lines. Blue circles represent the points where only the bcc phase was observed. The blue diamonds are the data where both bcc SI and the fluid were observed. The red circles are the data points where the diffraction signal of fcc SI was observed. The blue and red triangles correspond to bcc and fcc SI data obtained under dynamic compression in Ref. [20]. The gray lines represent the experimental [21] (plain line) and theoretical [7] (dashed line) melting curves. The Uranus and Neptune isentropes [7] are represented as the yellow and green lines, respectively. The boundary lines between bcc and fcc SI ${\mathrm{H}}_2\mathrm{O}$ as calculated in various works [11, 12, 13, 14] are drawn with different line styles.