Ice Ic without stacking disorder by evacuating hydrogen from hydrogen hydrate

Abstract

Water freezes below 0 °C at ambient pressure ordinarily to ice I_h, with hexagonal stacking sequence. Under certain conditions, ice with a cubic stacking sequence can also be formed, but ideal ice I_c without stacking-disorder has never been formed until recently. Here we demonstrate a route to obtain ice I_c without stacking-disorder by degassing hydrogen from the high-pressure form of hydrogen hydrate, C₂, which has a host framework isostructural with ice I_c. The stacking-disorder free ice I_c is formed from C₂ via an intermediate amorphous or nano-crystalline form under decompression, unlike the direct transformations occurring in ice XVI from neon hydrate, or ice XVII from hydrogen hydrate. The obtained ice I_c shows remarkable thermal stability, until the phase transition to ice I_h at 250 K, originating from the lack of dislocations. This discovery of ideal ice I_c will promote understanding of the role of stacking-disorder on the physical properties of ice as a counter end-member of ice I_h.

Fig. 1. Phase diagram of hydrogen hydrate and ice with experimental paths in this study.

Phase boundaries for hydrogen hydrates and ices are drawn using thick blue lines and thin black lines, respectively. Experimental p-T paths are shown as black arrows in alphabetical sequence from a to g. The structural models for a high-pressure form of hydrogen hydrate, C₂, and ice I_c are schematically drawn with a newly found amorphous-like state as an intermediate transitional state from C₂ to ice I_c. Red, white, and light blue balls in the structure model depict oxygen, hydrogen in water molecules, and hydrogen in guest molecules, respectively. Note that hydrogens in water molecules are disordered, so that two of four possible sites surrounding one oxygen are actually occupied.

Fig. 2. Results of Rietveld analyses for neutron diffraction patterns.

The patterns of a hydrogen hydrate, C₂, and b ice I_c were obtained at 3.3 GPa and 300 K (at d in Fig. 1), and at 0 GPa and 130 K (in the path f → g). The inset diffraction pattern in b shows the expanded area for 111 reflections, shown as a box in the main figure, with logarithmic scale. The calculated peak positions of ice I_h are also shown as light blue lines with their indices in the inset. Structure models for C₂ and ice I_c are also shown as insets in a and b, respectively.

Fig. 3. Neutron diffraction patterns showing the transformation from C₂ to ice I_c.

The patterns were obtained with decreasing pressure at 100 K (path e → f) and with increasing temperature at 0 GPa (path f → g). Corresponding temperatures and pressures are shown at the right side of the respective patterns, and the arrows mean that temperature or pressure kept constant. Most observed peaks are identified as C₂, ice I_c, Mg(OD)₂, or ice I_h. The peak marked by an asterisk is a parasitic peak from the high-pressure cell.