Electronic driving mechanisms for displacive reconstruction and its lifting by hydrogen adsorption on a metallic surface alloy

Abstract

We investigated a bilayer ${\mathrm{Pd}}_{3}\mathrm{Ti}$ surface alloy formed on Pd(100) by low-energy electron diffraction and angle-resolved photoelectron spectroscopy (ARPES). The surface alloy has $p(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}p4g$ symmetry and hydrogen adsorption induces transformation to c(2\ifmmode\times\else\texttimes\fi{}2) symmetry. On the $p(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}p4g$ phase, the first layer is composed of a laterally distorted Pd(100) plane and the second layer consists of a $c(2\ifmmode\times\else\texttimes\fi{}2)$ Pd-Ti alloy. The adsorption of hydrogen removes the first layer distortion, yielding the surface with $c(2\ifmmode\times\else\texttimes\fi{}2)$ symmetry which is analogous to the surface of ${\mathrm{Pd}}_{3}\mathrm{Ti}$ bulk alloy. The ARPES band mapping gives a rational description of the electronic driving mechanisms. The $p(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}p4g$ reconstruction is related to the strong polar interaction between the first-layer Pd and second-layer Ti atoms. Hydrogen-induced lifting of the reconstruction is ascribed to the repulsion among the first-layer Pd atoms due to the occupation of an in-plane antibonding state.