Molecular Dynamics and Interatomic Potentials Development for Metal Hexaborides From Density Functional Theory


Hexaboride materials have a unique crystal structure that make them very attractive for applications related to gas storage including hydrogen and other light gases. The lattice is simple cubic with boron octahedra at each corner of the cube bonded at the apexes. The octahedra consist of the boron atoms, with four adjacent neighbors in every octahedron for every boron atom, and one on the main axes of the cube. The metal atom is located in the middle of the unit cell and can donate electrons to the structure, imparting a metallic character to hexaborides with metal ions of +3 charge, and semiconductor character to hexaborides with metal ions of +2 charge. In this work, we discuss the development of interatomic potentials for lanthanum hexaboride (LaB6) using density functional theory methods (Quantum Espresso) and their implementation in molecular dynamics (MD) simulations (DL_POLY) to model both self-diffusion and diffusion under the influence of external electric field—assisted diffusion. Preliminary MD results show important mechanistic aspects of the diffusion of metal ions inside the metal hexaboride framework. We will discuss the pair-potentials methodology including their inversion from cohesive energy surfaces and further optimization at the molecular dynamics level. The pair-potentials developed reproduce very well the cohesive energies as well as the atom mean square displacements at the MD level. Our results show that the approach can be promising to developing interatomic potentials of different hexaboride materials such as Ca, Sm, and Ba, which are currently under development. Fundamental modeling of these materials at the atomic level is crucial for the development of commercial applications.

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