Ordered alloying of Pd with the Mo(100) Surface

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The interaction of Pd with the Mo(100) surface has been studied with low-energy electron microscopy and diffraction, and first-principles total-energy calculations. Deposition of Pd at elevated temperature leads initially to the development of an intense $c(2\ifmmode\times\else\texttimes\fi{}2)$ diffraction pattern. Theoretical calculations demonstrate that a $c(2\ifmmode\times\else\texttimes\fi{}2)$ substitutional alloy is energetically favored compared to an overlayer structure at half monolayer coverage. The creation of a large number of islands upon Pd deposition is consistent with the formation of a substitutional alloy. Accommodation of Pd in excess of the ideal $c(2\ifmmode\times\else\texttimes\fi{}2)$ coverage leads to the formation of a $c(2\ifmmode\times\else\texttimes\fi{}n)$ structure $(n=8,10),$ although with little apparent further change of surface morphology. Proliferation of antiphase domain walls in the $c(2\ifmmode\times\else\texttimes\fi{}2)$ alloy is proposed to explain the $c(2\ifmmode\times\else\texttimes\fi{}n)$ periodic structure. Theoretical calculations indicate that a pseudomorphic Pd overlayer is the most stable configuration at 1 monolayer coverage, and that the Pd-covered Mo(100) surface may become unstable with respect to faceting to the {112} orientations if pseudomorphic growth can be realized up to 2 monolayer coverage. However, the $c(2\ifmmode\times\else\texttimes\fi{}n)$ structure is found experimentally to be stable at the interface between Mo and thicker Pd overlayers. These results suggest that there is a kinetic limitation to the formation of the pseudomorphic structure and the faceting instability is preempted by the formation of more complex surface alloy structures.