On the formation of two-dimensional alloys of Sn and Pb co-adsorbed on Ru(0001)

Following recent experimental work of Yuhara et al. [Surf. Sci. 616 (2013) 131], indicating that the atoms of immiscible Sn and Pb metals deposited on Ru(0001) form locally a close-packed two-dimensional alloy structure, we employ DFT simulations to establish the conditions for the occurrence of Sn-Pb ordered alloys on this substrate. To this end, we calculate the surface phase diagram of Sn and Pb co-adsorbed on Ru(0001) and determine ranges of doses of deposited Sn and Pb when the formation of a binary alloy is energetically more favorable than the growth of segregated pure phases of Sn and Pb. We find a general tendency towards alloying of Sn and Pb on Ru(0001), however, the observed dominant alloy phase depends on the assumed deposition procedure of compositionally minor and major elements. We identify the preferable Sn-Pb alloy structures in terms of their stoichiometry, lateral symmetry, as well as the optimal adatom configuration within the surface unit cell, and contrast the DFT-predicted structural models with available STM images.