Silver on

Phase-coexistence of a $\chem{Ag/Pt}$ surface-alloy and a pure $\chem{Pt}$-hex reconstructed phase has been observed for sub-0.3$\un{ML}$ $\chem{Ag}$ coverage on a $\chem{Pt(100)}$ surface by scanning tunneling microscopy and temperature-programmed desorption of $\chem{Ag}$. We argue that, despite a stronger homoatom interaction, mixing at the surface is observed because the intrinsic tensile stress of the $\chem{Pt(100)}$ surface facilitates the incorporation of the slightly larger $\chem{Ag}$ atoms into the surface layer in order to minimize strain energy of the surface. Thus, surface-alloying presents an alternative to surface reconstruction as a means of strain minimization. Therefore, a competition between surface reconstruction and alloying to reduce the stress at the surface exists and, consequently, a two-phase coexistence of an alloy-phase and a pure $\chem{Pt}$ hex-reconstructed phase are observed