(425f) First-Principles Analysis of Surface Terminations in Stoichiometric Metal Hexaborides

Metal hexaborides (MB6) are materials with 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. Using density functional theory (DFT) calculations, we study the energetics and structural features of the surface terminations of Ca, Ba, Sr, and La hexaborides. There is significant uncertainty in the literature about the nature of the surface compositions in these materials regarding metal versus boron terminations. From electronic structure calculations we show that segregated regions of metal and boron-terminations produce the lowest energies for the metal hexaborides with di-cations, namely CaB6, SrB6, and BaB6. For the LaB6 case, the surface energy is minimized by arranging the metal ions in parallel rows on the surface. XPS measurements also show that CaB6 and SrB6 have surfaces that are close to the stoichiometric ratio for the compound, while BaB6 show surfaces that are Ba-rich. Energetic barriers are also calculated for transitions between each of the surface geometries studied. We find a substantial increase in the activation energy for the lanthanum migrations compared to the case for di-valent 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.