Atomic structure of two-dimensional binary surface alloys: Si(111)-√21 × √21 superstructure

► Two-dimensional Ag-Au alloy phases on Si(111) surface have been investigated. ► Chemical environments of Ag and Au atoms were revealed by core-level spectra. ► Atomic coordinates were determined by reflection high-energy positron diffraction. ► Atomic structures for the alloys with different stoichiometries are almost identical.