Surface X-Ray Diffraction Study on Polar Oxide Surface and Interface

Abstract

An atomic scale study of surface/interface structure is required to properly understand physical and chemical phenomena such as crystal growth, lubrication and electrochemistry. The stability of polar oxide surface has long been an interesting question. A bulk-terminated polar oxide surface comprises alternating layers of opposite charges, thus resulting in diverging surface energies. In order to reduce the surface energy, various reconstruction-stabilized MgO (111) surfaces have been reported experimentally. However, the atomic structure of the MgO (111)rt3×rt3R30o reconstructed surface remains unclear. Using a third-generation X-ray source is one of the feasible methodologies to probe such a system due to its increase of sensitivity on the interface layer. Surface X-ray diraction (SXRD) experiments were performed for the MgO(111) rt3×rt3R30o reconstructed surface at Advanced Photon Source, Argonne National Laboratory. The sample surface was prepared at home laboratory by annealing in a tube furnace for 36hrs at 1050oC, with N2 blowing at rate 1 to 2 scft. Crystal truncation rod (CTR) and super structure rod (SSR) measurements were acquired in both the absence and presence of a thin layer of water, obtained by compressing the bulk water layer with a thin Kapton sheet. A differential evolution algorithm, GenX, was used to search for the appropriate atomic model of reconstructed structure. Some reasonable models are presented and discussed with quantitative calculation of optimizing parameters (R factor and chi square). Preliminary SXRD results of the dry surface and solid-liquid interface are compared. This determination will shed light on whether physical (as opposed to chemical) factors are operant in the formation of ice-like layers.