Two-dimensional alloy of immiscible Pb and Sn atoms on Ru(0001)

We investigated the atomic arrangements and phase diagrams of single and binary two-dimensional (2D) films of Pb and Sn on Ru(0001) using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The Pb films formed the (√7 × √7) and c(2 × 4) structures in the saturated and unsaturated coverages, respectively, and their arrangements of Pb atoms were identified as hexagonal and rectangular structures. The Sn films formed the c(2 × 8) and incommensurate (“√6 × √3”), namely IC-1, at low coverages. Upon increasing the coverage, the IC-1 phase transformed into the commensurate (√7 × √3) structure and gradually extended to an incommensurate (“√10 × √3”) structure, namely IC-2, at saturated coverages. The atomic arrangements of the Sn atoms were primarily rectangular-like structures. The 2D binary film exhibited an incommensurate structure close to (√7 × √7) at the Pb and Sn coverages of 0.15 ± 0.05 ML and 0.40 ± 0.05 ML, respectively. Atomically resolved STM images of the 2D binary films revealed a highly ordered hexagonal arrangement of Pb and Sn atoms with an average composition of PbSn3.