Electronic structure of disordered Au-Pd alloys studied by electron spectroscopies

The occupied and unoccupied electronic structures of disordered ${\mathrm{Au}}_x{\mathrm{Pd}}_{1-x}$ alloys are studied by valence-band photoemission, bremsstrahlung isochromat spectroscopy (BIS), and x-ray absorption near-edge spectroscopy (XANES). The occupied partial spectral weights (PSW’s) of Au $5d$ and Pd $4d$ states are obtained from the valence-band photoemission spectra using synchrotron radiation by taking the matrix-element effect into account. We use the Cooper minimum phenomenon of the Pd $4d$ states with the measured ratios of photoionization cross sections. The Pd $4d$ PSW’s are found to form a virtual bound state in the Pd-diluted alloy but become broader as the Pd concentration increases due to the $\mathrm{Pd}\hspace{0.5em}4d–\mathrm{Pd}\hspace{0.5em}4d$ hybridization. On the other hand, Au ${5d}_{5/2}$ states show the common-band behavior due to the appreciable mixing with Pd ${4d}_{5/2}$ states, while Au ${5d}_{3/2}$ states retain their sharp structure and show the split-band behavior. These experimental PSW’s of Au-Pd alloys are in good qualitative agreement with the results of recent self-consistent-field coherent-potential-approximation calculations. The comparison of the experimental Pd PSW of Au-Pd with those of other Pd–noble-metal alloys clearly shows that in noble-metal-rich alloys, the mixing of Pd $4d$ states with host $d$ bands increases in the order of Ag-Pd, Au-Pd, Cu-Pd systems. This trend results in the split-band for Au-Pd and Ag-Pd in Pd diluted alloys, but gives the common band for Cu-Pd. The unoccupied Pd $4d$ states of disordered ${\mathrm{Au}}_x{\mathrm{Pd}}_{1-x}$ alloys obtained from BIS and XANES spectra show the gradual filling of Pd $4d$ states as the Au concentration is increased, but it is not completely filled even in the Pd-diluted alloy.