Crystallization of Ice Ic from Ice XVII: Outward, then Inward

The rare Scheiner’s halo at 28° around the sun, first reported nearly 400 years ago in Rome, suggests the presence of cubic ice (ice Ic) in the sky. However, humans only held its pure form in hand just 5 years ago, as the synthesis had been hindered by the competitive formation of hexagonal ice (ice Ih) and the mixture of hexagonal and cubic stacking sequence, resulting in stacking disordered ice (ice Isd). Since then, methods for creating pristine ice Ic from ice XVII and C2 hydrogen hydrate have been established. However, the underlying molecular mechanisms remain largely unexplored. Here, we unveil the microscopic details that enable the preferential formation of ice Ic from ice XVII through spontaneous crystallization in molecular dynamics (MD) simulations. The phase transition proceeds in two steps: first, the outward surface freezing of ice Ic within the premelting layer on the ice XVII prismatic surface, followed by the inward, layer-by-layer propagation of the stable Ic-XVII coexistence. The matching of lattice lengths and the defect-free layerwise conversion are essential. This mechanism could provide valuable insights for the precise synthetic control of target crystals over other polymorphs with similar stability, with broad applications in various fields.