Migration of helium-pair in metals

TL;DR

This study uses first-principles calculations to analyze how helium pairs migrate in various metals, revealing migration barriers and configurations that depend on metal type and interstitial stability, which is crucial for fusion reactor materials.

Contribution

It provides detailed first-principles insights into the migration mechanisms and barriers of helium pairs in bcc and fcc metals, highlighting differences from previous atomistic studies.

Findings

Migration barriers are slightly higher for helium pairs than single atoms in bcc metals.

He-pair migration in fcc metals involves either sequential or simultaneous movement depending on the metal.

Strong He-He interactions facilitate the associative motion of helium pairs, modeled by Morse potentials.

Abstract

Understanding helium accumulation in plasma-facing or structural materials in a fusion reactor starts from uncovering the details of the migration of single and paired He interstitials. We have carried out a first-principles density functional theory investigation into the migration of both a single interstitial He atom and an interstitial He-pair in bcc (Fe, Mo and W) and fcc (Cu, Pd and Pt) metals. By identifying the most stable configurations of an interstitial He-pair in each metal and decomposing its motion into rotational, translational, and rotational-translational routines, we are able to determine its migration barrier and trajectory. Our first-principles calculations reveal that the migration trajectories and barriers are determined predominantly by the relatively stable He-pair configurations which depend mainly on the stability of a single He in different interstices. Contrary to atomistic studies reported in literature, the migration barrier in bcc Fe, Mo, and W is 0.07, 0.07, and 0.08 eV respectively, always slightly higher than for a single interstitial He (0.06 eV for all three). Configurations of a He-pair in fcc metals are much more complicated, due to the stability closeness of different interstitial sites for a single He atom. In both Cu and Pd, the migration of a He-pair proceeds by moving one He at a time from one tetrahedral site to neighboring octahedral site; whereas in Pt the two He move simultaneously because the bridge interstitial site presents an extremely low barrier. The migration barrier for a He-pair is 0.05, 0.15, and 0.04 eV for Cu, Pd, and Pt, slightly lower than (in Cu), or similar to (in Pd and Pt) a single He, which is 0.08, 0.15, and 0.03 eV, respectively. The associative motions of a He-pair are ensured by the strong He-He interactions in metals which are chemical bonding-like and can be described very well with Morse potentials.