Electric field gradients in Au-Te alloys and charge transfer in some intermetallic compounds of nontransition metals with gold

This work is an extension of an earlier study of charge transfer occurring when Au is alloyed with nontransition (main-group) elements. Au Mössbauer-isomer-shift and quadrupole-splitting results are reported for a series of Au-Te alloys: Au${\mathrm{Te}}_2$, ${\mathrm{Au}}_{0.12}$ ${\mathrm{Te}}_{0.88}$, ${\mathrm{Au}}_{0.052}$ ${\mathrm{Te}}_{0.948}$, and ${\mathrm{Au}}_{0.01}$ ${\mathrm{Te}}_{0.99}$. Asymmetric doublets are observed in the ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ Mössbauer spectra of these alloys and are attributed to the Goldanskii-Karyagin effect. The sign of the electric field gradient is deduced to be negative. The quadrupole splittings, substantial relative to those ordinarily encountered in metals, are consistent with the presence of an amount of bonding charge along nearest-neighbor Te-Au-Te lines which exceeds by about $0.1e$ the charge along the two longer Te-Au-Te axes defining the equatorial positions in the distorted Te octahedron. The net charge transfer, $\delta$, in Au${\mathrm{Te}}_2$ and in related $\mathrm{Au}{M}_2$ compounds—compounds of common composition but not common structure—has been evaluated with a modified model which includes photoelectron binding-energy shifts, isomer shifts, and volume-correction terms at both the Au and $M$ sites. With an extension of this model, an attempt is made to reduce the extent of uncertainty arising from reference-level shifts in the analysis of photoelectron data: the net charge transfer is assumed to be a linear function of the chemical-potential difference between the alloying species. As has been found previously, the analysis provides evidence for $d$—non-$d$ charge compensation. That is, at Au sites a decrease in $d$ count accompanies a net gain, $\delta$, in charge. The extended model yields $\delta$ values which vary more smoothly than those obtained previously and show a trend consistent with general chemical behavior. Finally, while the full analysis indicates a gain of charge at Au sites and charge loss at the metalloid sites in Au${\mathrm{Sn}}_2$, Au${\mathrm{Sb}}_2$, and Au${\mathrm{Te}}_2$, the Mössbauer isomer-shift data for the metalloids actually indicate that $s$ charge increases in Te, though it decreases in the cases of Sn and Sb. This, taken with the netcharge-flow estimates, suggests a systematic variation in the relative loss of $s$ vs $p$ charge across the Au${\mathrm{Sn}}_2$-Au${\mathrm{Sb}}_2$-Au${\mathrm{Te}}_2$ sequence. Energy differences associated with these $s$,$p$ effects are significant in relation to the heats of formation of these compounds.