Understanding Chemical Bonding in Alloys and the Representation in Atomistic Simulations

Alloys are widely used in catalysts and structural materials. The nature of chemical bonding and the origin of alloy formation energies, defect energies, and interfacial properties have not been well understood to date but are critical to material performance. In this contribution, we explain the polar nature of chemical bonding and an implementation in classical and reactive atomistic simulations to understand such properties more quantitatively. Electronegativity differences between metal atoms lead to polar bonding, and exothermic alloy formation energies are related to charge transfer between the different elements. These differences can be quantified by atomic charges using pairwise charge increments, determined by matching the computed alloy formation energy to experimentally measured alloy formation energies using pair potentials for the pure metals. The polar character of alloys is comparable to organic molecules and partially ionic minerals, for example, AlNi and AlNi₃ alloys assume significant atomic charges of ±0.40e and +0.60e/-0.20e, respectively. The subsequent analysis of defect sites and defect energies using force-field-based calculations shows excellent agreement with calculations using density functional theory and embedded atom models (EAM). The formation of vacancy and antisite defects is characterized by a redistribution of charge in the first shell of neighbor atoms in the classical models whereby electroneutrality is maintained and charge increments correlate with differences in electronegativity. The proposed atomic charges represent internal dipole and multipole moments, consistent with existing definitions for organic and inorganic compounds and with the extended Born model (Heinz, H.; Suter, U. W. J. Phys. Chem. B 2004, 108 (47), 18341-18352). The method can be applied to any alloy and has a reproducibility of ±10%. In contrast, quantum mechanical charge schemes remain associated with deviations exceeding ±100%. The atomic charges for alloys provide a simple initial measure for the internal electronic structure, surface adsorption of molecules, and reactivity in catalysis and corrosion. The models are compatible with the Interface force field (IFF), CHARMM, AMBER, OPLS-AA, PCFF, CVFF, and GROMOS for reliable atomistic simulations of alloys and their interfaces with minerals and electrolytes from the nanometer scale to the micrometer scale.