Ag Cu Au in Si

The extremely fast diffusivity of Cu and its pronounced tendency to precipitate on cooling indicate that Cu is dissovled interstitially in the Si lattice.
By contranst, extended investigations of the Au diffusion in Si strongly support the view that the majority of Au atoms are incorporated on substitutional sites.
Ag appears to be predominantly interstitial with a much lower solubility than Cu and Au.

Ref: J.Phys. D: Appl Phys 20 (1987) p1148-1155