Solid argon precipitation in a metallic glass: Does free-volume help?

We find that small particles (1 nm) of solid argon are densely precipitated in a Pd₄₀Ni₂₀P²⁰ metallic glass matrix after argon ion-beam irradiation with an accelerating voltage of 4 keV. This is in sharp contrast to the case of noble-gas implantations into crystalline metals, for which high-energy beam (50-300 keV) has been essential to promote creations of precursor defects, such as voids or vacancy aggregates for initial confinements of noble-gas atoms. Therefore, the present low-energy implantation implies that the argon confinements have occurred at pre-existing low-density regions in the glass matrix, providing an important clue to phenomenological free-volume entities.