Electronic behavior in alloys: Gold-non-transition-metal intermetallics

Chemical effects (charge-transfer and valence-band behavior) upon alloying in Au alloys have been studied in a series of Au-Cd, Au-In, Au-Sn, Au-Sb, and Au-Te intermetallics by ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ Mössbauer and x-ray photoemission measurements. For these alloys, the Mössbauer isomer shifts (2.0-4.7 mm ${\sec }^{-1\mathrm{}}$ relative to Au) indicate significant increases of valence $s$ character at the Au sites, whereas the Au-$4f$ binding-energy shifts (0.2-1.35 eV relative to Au) indicate apparent charge depletion. This study, together with previous observations, confirms that there is little net charge flow between Au and host-metal atoms upon alloying, i.e., that the increase of predominantly $s$-like conduction-electron count, $\Delta n_c$, at a Au site is largely compensated by a depletion in $5d$ count, $\Delta n_d$. It is found from an analysis that the net charge flow at the Au site, $\delta =\Delta n_c+\Delta n_d$, is approximately 0.1 electron and that the ratio $\frac{\Delta n_c}{\Delta n_d}$ is surprisingly constant given the various numerical uncertainties of the analysis. It thus appears that the $d$ bands are actively involved, through hybridization, in alloying, and that such $s-d$ compensation is a general characteristic of Au alloys. Substantial hybridization of the Au $d$ bands with the Au non-$d$ states and with the valence states of main-group metal atoms is observed in the valence-band spectra. The relationship of this hybridization to the $d$ and non-$d$ charge-transfer effects inferred from the core level and Mössbauer shifts is discussed. It is also found that in some cases the isomer shifts of the dilute alloys are similar to those of the intermetallic compounds, indicating that the local order around the Au site in these dilute alloys is similar to that in the intermetallic compounds.