Surface termination analysis of stoichiometric metal hexaborides: Insights from first-principles and XPS measurements

Abstract We present a modeling framework and discuss the energetics and structural features of the surface terminations of Ca, Ba, Sr and La hexaborides using density functional theory analysis in combination with X-ray photoelectron spectroscopy. There is significant uncertainty in the literature about the nature of the surface terminations in metal hexaborides in terms of metal versus boron terminations. We show from electronic structure calculations that segregated regions of metal and boron-terminations produce the lowest energies for di-cations of CaB6, SrB6 and BaB6, while trivalent LaB6 minimizes the surface energy by arranging the metal ions in parallel rows on the surface. XPS measurements show that CaB6 and SrB6 have surfaces that are close to stoichiometric for the compound, while BaB6 has surfaces that are Ba-rich. Energetic barriers are calculated for transitions between each of the surface geometries considered. There is a substantial increase in the activation energy for the lanthanum migrations compared to the divalent cations. We also find that the boron octahedra units in these materials tend to contract or expand from their bulk values depending on the proximity to regions of high metal concentrations. These materials have many attractive features, such as low work functions, high hardness, low thermal expansion coefficients, and high melting points, among many other properties of interest for industrial applications. Promising uses of these materials also include catalytic applications for chemical dissociation reactions of various molecules such as hydrogen, water and carbon monoxide, for example, thus, the interest in determining relevant surface properties.