Metastable surface structures of the bimetallic Sn/Pt(1 0 0) system

An ordered Sn overlayer on Pt(100) and several metastable Sn/Pt(100) surface alloys were studied using scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). These surfaces were studied previously using alkali-ion scattering spectroscopy (ALISS), LEED, and CO chemisorption, in order to determine conditions for alloy formation [Surf. Sci. 330 (1995) 193]. In those earlier studies, structural transitions with increasing annealing temperatures from a c(2×2)-Sn overlayer to a c(2×2)-Sn/Pt(100) surface alloy and finally to a (3√2×√2)R45° alloy structure were observed. Here STM is used to determine surface morphologies and atomic scale structures of these alloys. Sn (0.67 ML) deposited on the reconstructed Pt(100)-hex surface and annealed to 550 K formed a c(2×2)-Sn overlayer with compact, monolayer-high islands with step edges oriented along the [100] and [010] azimuths. Annealing the sample to 750 K initiated alloying and a c(2×2) surface alloy formed locally. However, the c(2×2) alloy structure does not appear to be stable even at this early stage of alloying and narrow channels along the 〈100〉 crystallographic directions form. For higher annealing temperatures, these channels evolve to form a two-domain structure with almost equidistant channels along the [100] and [010] directions, respectively. A dominant channel separation of 3√2 times the surface-lattice constant of Pt(100) is observed. This channel separation gives rise to a streaky (3√2×√2)R45° LEED pattern. The channels are characterized by three missing atomic rows. Between these channels are narrow terraces that are predominantly three-atomic-rows wide. The step edges of these terraces are Pt-terminated, while Sn is alloyed in the center of these terraces. A local p(2×2) ordering of alloyed Sn is often observed for wider terraces. Annealing to 1000 K results in "fragmentation" of the narrow terraces to form small, often square, islands with side-lengths as small as √2 times the surface-lattice constant of Pt(100). The structural characterization of these Sn/Pt(100) surfaces is used to interpret previous chemical adsorption studies.