Surface termination of MgB2 unveiled by a combination of adsorption experiments and theoretical calculations

Abstract

Superconductivity in polycrystalline and thin-film MgB₂ is strongly affected by the termination of its surface, but a reliable determination of the surface termination is still a challenging task of surface chemistry. Here, the surface properties of superconducting MgB₂ were investigated using a combination of inverse gas chromatography and van der Waals corrected density functional theory calculations. The dispersive surface energy was measured as a function of the surface coverage and its value (58 mJ m⁻² to 48 mJ m⁻²) was verified by high-level non-local EXX + RPA calculations, which predicted that the dispersive contribution to the cleavage energy was 56 mJ m⁻². The isosteric adsorption enthalpies of cyclohexane, dioxane, acetone and acetonitrile molecules were measured on an MgB₂ sample and compared to the DFT calculated enthalpies for the Mg-terminated MgB₂, B-terminated MgB₂ and MgO(001) surfaces. The close agreement between theory and experiment for the Mg-terminated surface suggested that the magnesium termination is the dominant surface phase of MgB₂. Thus, combining inverse gas chromatography experiments with theoretical calculations may provide information about the surface termination.