Silvanite AuAgTe₄: a rare case of gold superconducting material

Gold is one of the most inert metals, forming very few compounds, some with rather interesting properties, and only a few of them are currently known to be superconducting under certain conditions. Compounds of another noble element, Ag, are also relatively rare, and very few of them are superconducting. Finding new superconducting materials containing gold (and silver) is a challenge - especially having in mind that the best high-T_c superconductors under normal conditions are based upon their rather close congener, Cu. Here we report combined X-ray diffraction, Raman, and resistivity measurements, as well as first-principles calculations, to explore the effect of hydrostatic pressure on the properties of the sylvanite mineral, AuAgTe₄. Our experimental results, supported by density functional theory, reveal a structural phase transition at ∼5 GPa from a monoclinic P2/c to P2/m phase, resulting in almost identical coordinations of Au and Ag ions, with rather uniform interatomic distances. Furthermore, resistivity measurements show the onset of superconductivity at ∼1.5 GPa in the P2/c phase, followed by a linear increase of T_c up to the phase transition, with a maximum in the P2/m phase, and a gradual decrease afterwards. Our calculations indicate phonon-mediated superconductivity, with the electron-phonon coupling coming predominantly from the low-energy phonon modes. Thus, along with the discovery of a new superconducting compound of gold/silver, our results advance the understanding of the mechanism behind superconductivity in Au-containing compounds and dichalcogenides of other transition metals.