Modeling of the thermal migration mechanisms of atomic oxygen in Ar, Kr, and Xe crystals

Abstract

Accommodation and migration of the ground-state (2s²2p^{4 3}P) oxygen atom in the ideal Ar, Kr, and Xe rare gas crystals are investigated using the classical model. The model accounts for anisotropy of interaction between guest and host atoms, spin–orbit coupling, and lattice relaxation. Interstitial and substitutional accommodations are found to be the only thermodynamically stable sites for trapping atomic oxygen. Mixing of electronic states coupled to lattice distortions justifies that its long-range thermal migration follows the adiabatic ground-state potential energy surface. Search for the migration paths reveals a common direct mechanism for interstitial diffusion. Substitutional atoms are activated by the point lattice defects, whereas the direct guest–host exchange meets a higher activation barrier. These three low-energy migration mechanisms provide plausible interpretation for multiple migration activation thresholds observed in Kr and Xe free-standing crystals, confirmed by reasonable agreement between calculated and measured activation energies. An important effect of interaction anisotropy and a minor role of spin–orbit coupling are emphasized.