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Surface geometry of BaO on W(100): A surface-extended x-ray-absorption fine-structure study

1991; American Physical Society; Volume: 44; Issue: 11 Linguagem: Inglês

10.1103/physrevb.44.5818

1095-3795

A. Shih, C. Hor, W. T. Elam, J. P. Kirkland, D. Mueller,

Advanced Condensed Matter Physics

A surface-extended x-ray-absorption fine-structure study of ordered monolayers of coadsorbed barium and oxygen on a single-crystal W(100) surface is described. A (\ensuremath{\surd}2 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}2 )R45\ifmmode^\circ\else\textdegree\fi{} structure with a stoichiometric barium-to-oxygen ratio, and a (2 \ensuremath{\surd}2 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}2 )R45\ifmmode^\circ\else\textdegree\fi{} structure with a nearly 1:2 barium-to-oxygen atomic ratio both form on W(100). The surface-extended x-ray-absorption fine-structure results indicate that all the barium and oxygen atoms are nearly coplanar in the (\ensuremath{\surd}2 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}2 )R45\ifmmode^\circ\else\textdegree\fi{} overlayer. The Ba-to-O distance in this overlayer is 3.20 \ifmmode\pm\else\textpm\fi{}0.05 \AA{} and \ensuremath{\beta}=82\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}5\ifmmode^\circ\else\textdegree\fi{}, where \ensuremath{\beta} is the angle between the Ba-O internuclear axis and the surface normal. For the (2 \ensuremath{\surd}2 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}2 )R45\ifmmode^\circ\else\textdegree\fi{} overlayer, there are two types of oxygen sites. Oxygen atoms nearly coplanar with the barium atoms are also present in these films with a Ba-to-O distance of 3.27\ifmmode\pm\else\textpm\fi{}0.05 \AA{} and \ensuremath{\beta}=75\ifmmode\pm\else\textpm\fi{}4\ifmmode^\circ\else\textdegree\fi{}. Additional oxygen atoms lie outside the barium plane at a distance 2.03\ifmmode\pm\else\textpm\fi{}0.05 \AA{} and \ensuremath{\beta}=23\ifmmode\pm\else\textpm\fi{}10\ifmmode^\circ\else\textdegree\fi{} from the nearest barium atoms.