Glassy Dynamics at Pre-melted Grain Boundaries in Ice I_h

Using first-principles and classical molecular dynamics simulations, we study pre-melting phenomena in pristine coincident-site-lattice grain boundaries in proton-disordered hexagonal ice I_h at temperatures just below the melting point T_m. The results are consistent with experimental estimates for the pre-melt layer thickness of low-disorder impurity-free GBs and provide key insight into the mobility of water molecules in the pre-melted layers. In particular, the translational motion of the water molecules is found to be subdiffusive for time scales longer than 10 ns. Furthermore, it is well-described by a continuous-time random walk model characterized by a waiting-time distribution with a power-law decay, suggesting that the dynamics in the pre-melt layers at GBs in ice I_h is glassy in nature, even at temperatures close to T_m.