Ice forms in two main crystal structures. Cubic ice (Ic) packs water molecules in a diamond-type arrangement. Hexagonal ice (Ih) — the familiar structure of snowflakes — stacks the same tetrahedral units in a different sequence. Both are ordered. Both are stable under different conditions. The question is how one becomes the other.
Huang, Wang, and colleagues used cryogenic transmission electron microscopy and molecular dynamics simulations to watch the transition during vapor deposition onto a substrate. Cubic ice nucleates first, forming embryonic cores on the surface. As the crystal grows, it must transition to hexagonal ice — the thermodynamically preferred bulk structure. But the transition does not happen directly. Between the cubic core and the hexagonal dendrites, intermediate stacking-disordered layers appear. These layers mix cubic and hexagonal stacking sequences without committing to either. They are a fluctuating bridge: ordered enough to maintain the crystal lattice, disordered enough in their stacking sequence to connect two incompatible symmetries.
The mechanism is surface-constrained symmetry breaking. The substrate favors cubic nucleation. The bulk favors hexagonal growth. The stacking-disordered intermediate reconciles these two preferences by providing a gradient from one symmetry to the other. The crystal doesn't snap from cubic to hexagonal. It passes through a state of ambiguous stacking that accommodates both.
This is the structural inverse of a result in metallic glasses. Across twelve different alloys, the transition between two disordered glassy states was found to require an ordered intermediate — crystallization followed by re-melting into a structurally distinct liquid. Disorder to disorder through order. Here, the path between two ordered states requires a disordered intermediate. Order to order through disorder.
The pair reveals a general principle about phase transitions between similar states. Two kinds of order cannot transform directly into each other because the path must break the first symmetry before establishing the second. Two kinds of disorder cannot transform directly either — the path must pass through order to reorganize the local structure. The transition between like states requires passing through their opposite. The bridge is always built from the other material.