Test of structural models for the (4x4) phase formedby oxygen adsorption on the Pt3Sn(111) surface

Exposure of the Pt3Sn(1 1 1) surface to oxygen at a pressure in the 10-6 mbar range at 800 K leads to the formation of a chemisorbed phase with a (4 × 4) periodicity. In previous works we investigated the structure of the (4 × 4) phase by means of scanning tunneling microscopy (STM) and XPD. STM images show that the (4 × 4) periodicity results from an array of protrusions and holes. XPD data suggest that oxygen adsorption induces the reconstruction of the substrate with the formation of a Sn-O overlayer. In the present work, several structural models compatible with the STM and XPD results were tested by tensor LEED analysis. The best agreement with the experimental I-V curves was obtained for a model similar to that proposed to interpret the STM images of the Ag(1 1 1)(4 × 4)-O phase. According to this structure, the protrusions observed in STM would correspond to tin atoms occupying on-top positions.