Modeling and Characterization of Metal Hexaboride Materials


Metal hexaborides are non-oxide ceramic materials with unique electronic, magnetic and optical properties that have a broad variety of applications with great potential for new uses in fields such as lightweight armor development, gas storage, and n-type thermoelectrics. The cubic crystal structure consists of covalently bonded boron octahedra surrounding a loosely-bonded metal ion, which donates electrons to the boron framework and directly influences the compound’s conductivity and other thermophysical properties. In this work, we present extensive modeling results using density functional theory (DFT) and abinitio molecular dynamics (AMD) to describe hydrogen interaction with these materials including detail modeling and characterization of equilibrium surface structures, work functions, and atom migration energetics. We observe both chemisorption and physisorption phenomena when hydrogen interacts with surfaces of calcium, strontium, and barium hexaborides. The adsorption/absorption phenomena seem to occur preferentially on the first two layers of the materials. The nature of the surface, metal terminated versus boron terminated, also seems to play a major role on the energetic interactions of hydrogen with these surfaces, with hydrogen adsorption preferentially occurring on the metal terminated surfaces, which is the type of termination we have experimentally determined from Auger photoelectron spectroscopy and other techniques, particularly for calcium hexaboride. We also characterize the energetics of hydrogen adsorption using DFT by estimating the difference in energies of the materials with and without hydrogen on the surface. These simulation methods and experimental results provide very valuable insights on the physical and chemical mechanisms of hydrogen adsorption in metal hexaboride materials.

Barcelona, Spain