Ab-initio study of Al-doping on electron density and charge distribution in boron carbide clusters

Boron carbide (B₄C) is a high-performance material valued for its hardness, yet it is susceptible to local amorphization under high-stress conditions, degrading its mechanical strength. Aluminum doping has been proposed to mitigate this phenomenon. This study investigates the electronic effects of incorporating aluminum into the intericosahedral chain using isolated cluster models: undoped B₇₂(C-C-C) and Al-doped B₇₂(C^Al^C). Results demonstrate that aluminum substitution induces pronounced electron density redistribution and strong electronic polarization. The aluminum atom loses electron density, becoming positively charged (+ 0.611 e⁻), while neighboring carbon atoms accumulate density. This asymmetry creates a significant uncompensated dipole moment of approximately 2.04 D in the doped cluster, which is entirely absent in the undoped structure. This polarization effect likely influences the spatial distribution of Al atoms in the solid phase and, consequently, affects the mechanical, thermal, and electronic properties of aluminum-modified boron carbide.

Calculations were performed using Density Functional Theory (DFT) with the hybrid B3LYP functional and the STO-3G basis set. The molecular orbital method, using a linear combination of atomic orbitals (LCAO-MO), was employed to analyze the three-dimensional electron density distribution. To represent bulk material behavior, atomic coordinates were adopted from prior periodic boundary condition calculations without further optimization. Mulliken population analysis was used to determine atomic charges and electron populations. All computational procedures and electronic property calculations were conducted using the GAMESS'09 software package and the Atomic Simulation Environment (ASE), while visualizations were performed using the ChemCraft software package.