The many facets of “cubic ice.” In water’s notoriously complex phase diagram, a solid phase commonly known as “cubic ice” or “ice Ic” is frequently encountered in various transitions between liquid, gaseous, and other solid phases. It has been seen, for example, in crystals nucleated out of supercooled water, in cirrus clouds and contrails in the atmosphere, and in the dissociation of gas hydrates. But the phase is metastable, decaying with time and temperature to the more familiar hexagonal ice, ice Ih. Moreover, the phase is not truly cubic. Instead of having an ordered arrangement of two-dimensional layers, the phase is replete with stacking faults, and crystals formed via different routes have different degrees of disorder. Werner Kuhs and colleagues at the University of Göttingen and the Institut Laue-Langevin now show that the stacking order is more complex than previously reported: It is governed by topological...

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