Liquid-phase epitaxy to ice VI and ice XII in accordance with Ostwald’s step rule

Hexagonal ice (ice Ih) collapses into high-density amorphous ice (HDA) under low-temperature compression, and HDA subsequently transforms into ice IV, VI, or XII upon heating at different pressures. However, the microscopic details that prioritize one among multiple crystallization pathways remain poorly understood. Recently, our molecular dynamics (MD) simulations revealed that ice Ih can avoid pressure-induced amorphization if it adopts a specific hydrogen-ordered configuration, instead transforming into an experimentally unreported phase (called ice M), which then transforms to ice XII upon heating. However, the molecular mechanisms responsible for triggering the ice M-to-XII transition have not yet been clarified. In this study, we perform MD simulations of solid-liquid coexistence between ice M and its hydrogen-disordered counterpart (ice Md). We identify a specific crystallographic plane that promotes the nucleation of ice XII. Moreover, we find that another plane of ice M/Md facilitates the formation of ice VI through a previously unreported intermediate phase, denoted ice Md2. Calculated chemical potentials suggest that these sequential liquid-phase epitaxies follow Ostwald's step rule.