Ab initio molecular dynamics of pipe diffusion in fcc Ni beyond transition state theory

Abstract

Advances in first principles methods provide a means to explore the rate constants of mechanisms that constitute diffusion. We apply these to atom-vacancy exchanges near a partial core of a 12〈11¯0〉 screw dislocation in fcc Ni at finite temperatures. Intrinsic properties of this dynamic approach contrast limitations of transition state theory (TST) and widely used harmonic TST (HTST) approximations, revealing direction-dependent free energy profiles within the zero-temperature period of the dislocation and temperature-dependent correlation effects. The calculated free energy profiles indicate increased dislocation periods at elevated temperatures, similar to temperature-dependent period doubling in semiconductors. In contrast to static HTST, the dynamic approach indicates an intra-cell asymmetry in energy profile that is related to the dynamic geometry rearrangements. This arises from different philosophies underlying the two methodologies. The absolute values of the rate constants, however, are within the same order of magnitude, with the HTST method generally underestimating rates by a factor of 2–5. Combined with a twofold increase in rates due to thermal expansion at high temperatures, this partially explains why previous theoretical works report lower diffusivities for pipe diffusion than experiments.