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Migration of helium-pair in metals

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.