Predicting distinct regimes of hydrogen behavior at nano-cavities in metals

•Nano-cavities containing hydrogen can be characterized as undersaturated or oversaturated.•In some oversaturated nano-cavities, H molecule formation is possible.•Geometry and energetics inform a predictive model for H saturation.•Results are verified using atomistic simulation with DFT and Monte Carlo.

Hydrogen interacts strongly with structural defects and is often implicated in degradation of materials. Precise conditions and local structures favoring the formation of H2 molecules are unclear so far, and not directly accessible experimentally. We present a computational and theoretical study of properties and behavior of hydrogen at nano-cavities based on the saturation of the vacancy cluster surfaces. A predictive model is developed which characterizes two regimes of behavior: in the undersaturated regime, non-interacting atomic hydrogen decorates the surface of a nano-cavity. In the oversaturated regime, the surface is maximally covered, and stable molecular hydrogen can be formed within the bubble. We verify this model with ab initio Monte Carlo simulations to extensively explore low energy states of small vacancy clusters containing multiple hydrogen atoms. The present model is expected to be easily transferable to the study of H in other metallic systems.