Hydrogen Ordering in Ice Ih Facilitates the Transition from HDA to Ice XII

Under low-temperature compression, ice I becomes metastable but does not transform into other high-pressure ice crystals. Excess pressure results in its collapse to high-density amorphous ice (HDA), which predominantly transforms into ice IV or XII upon heating. Understanding how to avoid pressure-induced amorphization and molecular origin prioritizing nucleation of a metastable phase provides insights into the nature of HDA. Here, we perform molecular dynamics (MD) simulations to demonstrate that when hexagonal ice (ice Ih) exhibits a particular hydrogen ordering, the hydrogen-ordered ice Ih can transform into ice XII via a partially ordered HDA. The crystalline region is identified as an unreported ice form (named ice M) with pentagonal-helical hydrogen-bond networks identical to those in ice XII. The remanent ice M configuration facilitates the transition of HDA to ice XII. Combined with our previous study on a hydrogen-ordered form of ice Ic, hydrogen ordering in ice I appears to enable crystal-to-crystal transitions, whether direct or indirect, even at low temperatures.