Absence of Amorphous Forms When Ice XIc Is Compressed at Low Temperature

It is generally believed that ice crystal-to-crystal transitions do not occur below the glass-transition temperature. For instance, under compression, ice I becomes a metastable state but does not transform into other high-pressure ice crystals, and applying excessive pressure ends up causing its collapse into high-density amorphous ice (HDA). Here, we perform molecular dynamics (MD) simulations to demonstrate that a hydrogen-ordered form of cubic ice (ice Ic) transforms to a hydrogen-ordered form of ice IV without yielding HDA. Our comprehensive search for different configurations combined with density functional theory (DFT) calculations indicates that the hydrogen-ordered ice IV formed by compression would be the thermodynamically most stable hydrogen-ordered ice IV configuration. We also predict the phase boundary of hydrogen order-disorder transition of ice IV, which remains unknown in experiments. Our findings suggest that hydrogen ordering enables an immediate transition between ice polymorphs that is impossible when the hydrogens are disordered.