Interaction of Hydrogen with MB₆ (M = Ba, Ca, La, and Sr) Surfaces from First Principles

Abstract

We show results of basic energetics and interacting behavior of hydrogen with metal hexaboride surfaces using a combination of self-consistent density functional calculations and dynamics based on the Car–Parrinello method. Our results show that hydrogen is strongly attracted to localized exposed boron atoms and interactions with the terminal cations are strictly repulsive. From these, preliminary local adsorption energy calculations suggest that a single hydrogen molecule per surface unit-cell is possible (one ML). Strongest bonds are found when hydrogen is above the terminal boron atoms affected by reduced coordination and dangling bonds. This location serves to restore the hexaboride unit to a more stable structure by providing electronic density to the deficient surface octahedra. Additionally, trajectories from dynamic simulations provide insight into how hydrogen recombination reactions occur on the surface through dissociative adsorption and the method of travel prior to recombination to be along the octahedral face and bridging sites connecting separate unit cells on the surface. Upon adsorption, a single hydrogen atom becomes localized at the dangling bond site while the second interacts with the surface along a weaker potential energy path. Desorption at lower temperatures occurs when migrating atoms from separate adsorption sites intersect to form a new pair.

Aromatic Hydrocarbons as Molecular Propellants for Electric Propulsion Thrusters

“…The materials commonly used for thermionic emission are typically fed with very pure and inert xenon, and the performance is very sensitive to changes in their surface chemical composition. Extensive exploration of the literature strongly indicates that hydrocarbons are not compatible with most common types of thermionic emitter materials [36][37][38][39][40][41][42][43]. While research on the poisoning behavior of most emitters is scarce, emitters are likely to be inoperable in non-inert atmospheres because of their fragile and easily disrupted emission capacities.…”

Pressure-induced evolution of structures and promising superconductivity of ScB6

“…Moreover, since the discovery of the highest superconducting critical temperature (T c ) (B39 K) in MgB 2 [1][2][3][4][5] under ambient pressure, the metal borides have been investigated extensively to design new potential high-temperature superconductors. 6,7 The alkaline-earth-metal hexaborides MB 6 (M = Ca, Mg, Ba, and Sr) show semi-conductivity and unusual weak ferromagnetic-like properties [5][6][7][8][9][10][11][12] possibly because of their low-density free-electron gas. CaB 6 with a vertex-connected B 6 octahedral network exhibits weak ferromagnetism at high temperatures.…”

“…The alkaline-earth-metal hexaborides MB 6 (M = Ca, Mg, Ba, and Sr) show semi-conductivity and unusual weak ferromagnetic-like properties 5–12 possibly because of their low-density free-electron gas. CaB 6 with a vertex-connected B 6 octahedral network exhibits weak ferromagnetism at high temperatures.…”

Morphology Control of Tantalum Carbide Nanoparticles through Dopant Additions

“…Strain effects have also been explored as parameters for morphological modification of copper and nickel particles, and doping has been applied for obtaining nanorods of magnesium . In addition, modeling − and experimental techniques − have been applied for exploring the behavior of borides of cubic and other morphologies. Moreover, experimental efforts have investigated shape change in cubic carbides as a function of carbon stoichiometry, which influences the relative growth rate of the dominant {111} and {100} facets. − …”