Among the over eighteen different forms of water ice, only the common hexagonal phase and a cubic phase are present in nature on Earth. The existence of these two polytypes, almost degenerate in energy, represents one of the most important and unresolved topics in the physics of ice. It is now widely recognised that all the samples of cubic ice obtained so far are instead a stacking-disordered form of ice I (i.e. ice Isd), in which both hexagonal and cubic stacking sequences of hydrogen-bonded water molecules are present. Here we describe a new method to obtain cubic ice Ic in large quantities, and demonstrate its unprecedented structural purity from two independent neutron diffraction experiments performed on two of the leading neutron diffraction instruments in Europe.
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