Superconductivity at 7.7 K in new hexagonal bronze Hg$_{x}$ReO$_{3}$

We have successfully synthesized a new rhenium-based hexagonal bronze material, Hg$_{x}$ReO$_{3}$, which exhibits superconductivity with the transition temperature $T_c$ = 7.7 K 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 $($Hg$_{2})^{2+}$ 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.