Molecular Mechanism of Gas Diffusion in Ice-Ih

Abstract

Atmospheric gases trapped in polar ice have been used to reconstruct polar and global climate changes, providing better time resolution when less diffused. Experiments have shown that gas diffusion in ice is negligible on a laboratory time scale, but its cumulative impact on old glacial ice (>1M yr remains unclear. Here, we employ density functional theory calculations to investigate the diffusion mechanism of gases trapped in ice-Ih from the atomistic level. The results suggest that the diffusion energy barrier between interstitial sites is primarily dependent on the atomic size and charge distribution of hopping gases. The diffusion of noble gases (He, Ne, Ar, Kr, and Xe) primarily occurs via the interstitial mechanism, consistent with previous results of classical molecular dynamics simulations. In contrast, the precisely determined diffusion paths and energy barriers for CO2, O-2, and N-2 suggest that these molecular gases prefer to hop along the hexagonal channel also via the interstitial mechanism, and the bond-breaking mechanism proposed previously to explain the diffusion of those molecular gases as fast as Ne may be unnecessary.