  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.
