Helium ions implanted into metals can evolve into ordered bubbles isomorphic to the host lattice. Long-range elastic interaction is generally believed to drive the formation of bubble superlattice, but little is known about the thermodynamics at the very initial stage. Our first-principles calculations demonstrate that in molybdenum, Friedel oscillations induced by individual helium generate both potential barriers and wells for helium clustering at short He-He distances. Such repulsion and attraction at high concentration provide a thermodynamic diving force to assist lining up randomly distributed He atoms into ordered bubbles. Friedel oscillations might have general impact on solute-solute interactions in alloys.

11 pages, 4 figures