Absence of Amorphous Forms When Ice XIc Is Compressed\nat Low Temperature

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