  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 ($\beta$-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 $\beta$-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.

Materials Science