Charge redistribution and electronic behavior in a series of Au-Cu alloys

Abstract

A series of Au-Cu alloys of various stoichiometries and order have been studied using x-ray photoemission spectroscopy (XPS) and x-ray absorption spectroscopy (XAS). Significant electronic changes are associated with alloying and with changes in the local environment. The Au 5d electron charge depletion has been determined independently from XPS core-level and Mössbauer isomer shifts and from x-ray absorption near-edge structure (XANES) measurements. Strong correlations were found using both methods in the parameters investigated here, especially in the elucidation of the charge-transfer mechanism. It is found that at the Au site there is a loss of d charge, upon alloying, which increases as Au becomes more dilute in Cu. This along with XANES evidence of d charge gain at the Cu site, conduction (primarily 6s) charge gain at the Au site, and overall charge gain at the Au site verifies the charge compensation model in which Au loses d charge but is overcompensated by a gain of conduction charge. This observation is in line with electronegativity (Au is more electronegative than Cu) and electroneutrality (the overall gain of charge is small) arguments. It is also found that there is a strong linear correlation between the 5d hole count at the Au site and (1) the Mössbauer isomer shift, (2) alloy d-band width, (3) ‘‘Au’’ apparent spin-orbit splitting in the alloy d band. By comparing the ordered and disordered species (which have different local environments) it is found that the charge transfer depends primarily on the local environment (coordination number of like and unlike atoms) and not on atomic separation. The charge transfer (d-band depletion) was larger in the ordered species and can be explained by the number of Cu and Au nearest neighbors as compared to the disordered phases.