Transient Phase of Ice Observed by Sum Frequency Generation at the Water/Mineral Interface During Freezing.

We observed a transient noncentrosymmetric phase of ice at water/mineral interfaces during freezing, which enhanced the intensity of the IR-visible sum frequency generation intensity by up to 20-fold. The lifetime of the transient phase was several minutes. Since the most stable form of ice, hexagonal and cubic ice, are centrosymmetric, our study suggests the transient existence of stacking-disordered ice during the freezing process at water/mineral interfaces. Stacking-disordered ice, which has only been observed in bulk ice at temperatures lower than -20 °C, is a random mixture of layers of hexagonal ice and cubic ice. However, the transient phase at the ice/mineral interface was observed at temperatures as high as -1 °C. It suggests that the mineral surface may play a role in promoting and stabilizing the formation of stacking-disordered ice at the interface.