Charge Balance Controls the (100) Surface Structure of the Ba8Au5.25Ge40.75 Clathrate

Abstract

Controlling material surface properties is crucial for many applications. While coatings and functional layers are widely used to tailor surface properties, another route to control topography and electronic surface charge relies on the design of new material surfaces. Here, we present a detailed study of the (100) surface structure of a BaAuGe intermetallic clathrate, a class of covalently bonded cage compounds with tunable electronic properties. Using a combination of experimental and computational methods, we demonstrate that the surface structure preserves the cages of the bulk structure up to the surface and contains an ordered arrangement of Ba surface atoms. Ab initio calculations show that the surface structure is stabilized through electron charge transfer from protruding Ba to surface Ge and Au atoms, saturating the dangling bonds. As a consequence, surface atoms recover an electronic environment similar to that of the bulk phase. Such a charge balanced stabilization mechanism may be general at the surface of intermetallic clathrates.