Interatomic Potentials Development for Lanthanum Hexaboride 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. Plane-Wave self-consistent field calculations are performed using the suite Quantum Espresso (QE) with ultra-soft Vanderbilt potentials for computing the cohesive per atom in the LaB6 crystal. Instead of sampling the energy surface using grid-based systems for atom positions in the lattice, we use a unique approach where in-silico or fictitious crystal structures than resemble closely the interatomic interactions in LaB6 are used to generate cohesive energy curves that can then be inverted using a Möbius inversion technique to obtain the interatomic potentials as a function of distance for the distinct pairs in the system, namely La-La, B-B, and La-B. In this approach, the cohesive energy is composed of four contributions that includes electrostatics, plus the contributions of the La-La, B-B, and La-B interactions. The functional form for electrostatics is known (Coulomb); there three “pseudo” crystal structures are required to obtain the tree interatomic potentials by inversion of the cohesive energy. Recall that the cohesive energy from DFT calculations is a function of distance usually given in terms of the lattice constant, which is not the same distance use in the interatomic potentials, but they are related. Preliminary results show that the approach can be promising to compute interatomic potentials of different hexaboride materials. The interactomic potential curves can then be easily fitted to classical potential energy models for use in molecular dynamics such as Morse and Buckingham potentials. We present fittings for these in LaB6, which we then use to perform basic molecular dynamics simulations. Fundamental understanding of these materials at the atomic level is crucial for the development of commercial applications.

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