Experimental evidence for the existence of a second partially-ordered phase of ice VI

Ice exhibits extraordinary structural variety in its polymorphic structures. The existence of a new form of diversity in ice polymorphism has recently been debated in both experimental and theoretical studies, questioning whether hydrogen-disordered ice can transform into multiple hydrogen-ordered phases, contrary to the known one-to-one correspondence between disordered ice and its ordered phase. Here, we report a high-pressure phase, ice XIX, which is a second hydrogen-partially-ordered phase of ice VI. We demonstrate that disordered ice undergoes different manners of hydrogen ordering, which are thermodynamically controlled by pressure in the case of ice VI. Such multiplicity can appear in all disordered ice, and it widely provides a research approach to deepen our knowledge, for example of the crucial issues of ice: the centrosymmetry of hydrogen-ordered configurations and potentially induced (anti-)ferroelectricity. Ultimately, this research opens up the possibility of completing the phase diagram of ice. Water ice exhibits several hydrogen-ordered and disordered phases and it's unclear if a disordered phase can transform into only one ordered phase. Here, the authors identify a partially hydrogen-ordered phase at high pressure, ice XIX, as the second hydrogen-ordered phase of ice VI beside ice XV.

Automated summary

The study reveals that under high pressure, ice can exhibit different forms of hydrogen-ordered phases, deepening our understanding of the polymorphism of ice and opening up the possibility of completing the phase diagram of ice.