Hydrogen-Induced and Defect-Mediated Structural Transition (5×1)-hex → (5×1)-H on Ir(100)

Abstract

Abstract Ir(100) is well known to exhibit a (5×1) periodic surface reconstruction denoted as (5×1)-hex. The atoms of the outermost layer are quasi-hexagonally arranged rather than being of quadratic order as the layers below and in the bulk. Accordingly, an excess of 20% of atoms is accommodated in the top layer and the surface is buckled with atomic rows protruding from the surface. It is shown by the combined application of atomically resolved scanning tunnelling microscopy (STM) and quantitative low-energy electron diffraction (LEED) that hydrogen adsorption at sufficiently high temperature induces the surface to restructure. The new phase, (5×1)-H, develops by the extraction of surface protruding and close packed atomic Ir wires from the hexagonal layer. The remaining atoms of the latter reassume quadratic order and the wires expelled reside on this layer. The extraction of the Ir wires proceeds in a zip-like process and starts preferentially at surface defects, in particular at step edges or domain boundaries. The wires’ lateral periodicity normal to their spatial orientation is again 5-fold, but only on average. The precise lateral arrangement depends sensitively on the details of preparation, in particular the amount of hydrogen exposure and the sample’s temperature during, or temperature treatment after, exposure.