Mechanism of molecular diffusion in ice crystals

We used molecular dynamics (MD) simulations to investigate the diffusion of O₂, N₂, CH₄, and CO₂ in a crystal of ice Ih. The results show that the diffusion mechanism for the molecules differs significantly from the interstitial mechanism that applies to small atoms such as helium. The air molecules hopped between stable sites by a new mechanism called the breaking-bond mechanism in which hydrogen bonds in the lattice are broken. The diffusion coefficients determined from the MD calculations are several orders of magnitude larger than the estimates under the assumption of the interstitial mechanism. We conclude that the repulsive interactive between the air and water molecules in ice is the dominant factor governing the diffusion mechanism.