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.