Structure determination for the W(100)-(2 x 2)-O surface phase

For oxygen adsorbed on the W(100) surface and annealed at a temperature of 1000 C a (2 x 2) reconstruction is observed for which an O coverage larger than 1.0 ML has been reported. Although the existence of this surface phase has been known for long, its structure is still unknown. We investigated this W(100)-(2 x 2)-O phase by means of quantitative LEED. The analysis reveals that the surface comprises two coexisting domains with p(2 x 2) and p(2 x 1) order, respectively. The p(2 x 1) structure is the already known missing row reconstruction with 1 ML oxygen atoms residing in the three-fold sites along the tungsten chains[2]. The p(2 x 2) domain consists of a single W atom per (2 x 2) unit cell decorated by four O atoms. Another oxygen atom is located on top of the 3rd layer tungsten atoms, resulting in a total oxygen coverage of 1.25 ML for this domain. The surface is covered with the two domains at a fraction of 60% for the (2 x 2) order and 40% for the (2 x 1) order. The W(100)-(2 x 2)-O surface phase can be considered as a precursor for the (2 x 1) missing row structure which is fully developed for higher annealing temperatures