The water inside a carbon nanotube is restricted to forming certain geometries of hydrogen bonds in the plane perpendicular to the axis of the tube. When the temperature of the water drops below freezing point, the ice that forms is n-gonal (pentagonal, hexagonal) with the diameter of the surrounding carbon tube determining the geometry of the icy core.

However, the smallest possible size (which forms cubic ice) maintains some structure into the liquid phase. This might account for some unusual phase transition between solid and liquid. The liquid water is already 'half-frozen' by inclusion in the cavity of the carbon tube.

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