Paper detail

New diversity form of ice polymorphism: Discovery of second hydrogen ordered phase of ice VI

More than 20 crystalline and amorphous phases have been reported for ice so far. This extraordinary polymorphism of ice arises from the geometric flexibility of hydrogen bonds and hydrogen ordering, and makes ice a unique presence with its universality in the wide fields of material and earth and planetary science. A prominent unsolved question concerning the diversity is whether a hydrogen-disordered phase of ice transforms into only one hydrogen-ordered phase, as inferred from the current phase diagram of ice, although its possible hydrogen configurations have close energies. Recent experiments on a high-pressure hydrogen-disordered phase, ice VI, revealed an unknown hydrogen-ordered form ($β$-XV) besides the known ordered phase, ice XV, which would be a counterexample of the question. However, due to lack of experimental evidence, it has not been clarified whether $β$-XV is a distinct crystalline phase. Herein we report a second hydrogen-ordered phase for ice VI, ice XIX, unambiguously demonstrated by neutron diffraction measurements. The phase boundary between ice VI and ice XIX shows that ice VI contracts upon the hydrogen ordering, which thermodynamically stabilizes ice XIX in higher-pressure region than ice XV because of its smaller volume than ice XV. The pressure-driven phase competition between hydrogen-ordered phases, also theoretically suggested in other ice polymorphs, can induce hydrogen ordering of ice in different manners. Thus, this study demonstrates a hitherto undiscovered polymorphism of ice.