Superconducting Phase at 7.7 K in the Hg_xReO₃ Compound with a Hexagonal Bronze Structure

We have successfully synthesized a new rhenium-based hexagonal bronze material, Hg_xReO₃, which exhibits superconductivity with the transition temperature T_c=7.7K at ambient pressure and 11.1 K at 4 GPa. This compound is a superconductor with the highest T_c among hexagonal bronzes. Moreover, it presents the novel crystallographic feature that (Hg₂)2⁺ polycations, in contrast to monatomic cations in known hexagonal bronzes, are incorporated into open channels. There is evidence that conducting electrons tightly couple with Hg-related phonons. Our results inspire detailed studies on the role of the rattling phonon in the occurrence of superconductivity in the hexagonal bronzes.