Strain and ligand effect on the electronic structure of Ni–Fe and Ni–Cu alloys: Exploring charge redistribution from x-ray spectroscopies and the charge compensation model

Abstract Nickel-based bimetallic alloys are considered thermally and structurally stable, while also possessing desirable catalytic and magnetic functionalities and being highly abundant and affordable. The electronic structure of such alloys is of particular interest from the perspective of atomic size mismatch and elemental crystal structure compatibility. In this study, we utilize x-ray techniques, including x-ray diffraction (XRD), x-ray absorption spectroscopy (XAS), and x-ray photoelectron spectroscopy (XPS), to understand the strain and ligand effects on charge redistribution upon alloying. We investigate the elemental crystal structures for incompatible alloys of Ni (fcc) and Fe (bcc), and the compatible crystal structure of Ni (fcc) and Cu (fcc) for comparison. Emphasis is placed on interpreting the metal 2p XPS binding energy shift in binary alloys, where one element is diluted into the other, based on the framework of strain and ligand effects and the charge compensation model of Watson et al. Of interest are the different “compressibilities” of the 4s and 3d wavefunctions within the Wigner–Seitz volume, V WS , and the volume-strain effect resulting in intra-atomic 4s–3d rehybridizations within the alloy, as well as the chemical intuition of charge transfer based on the ligand effect (electronegativity). These considerations provide perspective on “internal pressure” due to the strain effect and help in understanding the x-ray data and their correlation with the electronic structures and properties of bimetallic alloy systems.