Electronic structure of Ni-Cu alloys: The d-electron charge distribution

This work investigates charge redistribution in a series of Ni-Cu alloys using x-ray photoemission spectroscopy (XPS) and Ni/Cu L-3,L-2- and K-edge x-ray-absorption near-edge structure (XANES). XPS results show that the constituent d bands are well separated and shifted to a slightly higher binding energy upon dilution into the other host, indicating that the atomic sites in the alloy are not as well screened relative to the pure metal. However, no significant d-band narrowing is observed, suggesting that there is modest d-d interaction in the alloys. In contrast to the XPS observation, XANES results show a reduction in white-line intensity at both edges relative to the pure metal suggesting that both Ni and Cu sites gain d charge. The unoccupied Ni d band is far from fully occupied even at infinite dilution. The discrepancy between the implications of the XPS and XANES results is dealt with using a charge redistribution model in which s-p-d rehybridization takes place at both sites within the framework of electroneutrality and electronegativity considerations. It appears that both Ni and Cu gain a small but measurable amount of d charge in alloy formation through rehybridization (loss of non-d conduction charge). Possible connection between these results and the disappearance of ferromagnetism in Ni1-xCux alloys at x>0.6 is discussed.