Prediction of ambient superconductivity in ternary thorium-silicon superhydrides with a breathing kagome lattice

A major focus of current research on superconducting superhydrides is to achieve superconductivity at lower pressures, especially ambient pressure. Here we present the prediction of a class of high-temperature ternary hydrogen-rich superconductors, which are metastable at mild pressure and dynamically stable at ambient pressure using crystal structure searching. Our findings reveal the existence of a hexagonal $\mathrm{ThSi}{\mathrm{H}}_7$ compound with a superconducting $T_{\mathrm{c}}$ of 61.5 K at ambient pressure, as estimated by electron-phonon coupling calculations. Interestingly, the H sites in $\mathrm{ThSi}{\mathrm{H}}_7$ materialize a breathing kagome network, equivalent to a triangular lattice of trimer plaquettes, which is entirely different from previously known hydrogen clathrate structures. By substituting Th with lanthanide elements, we discovered that $\mathrm{LaSi}{\mathrm{H}}_7$ and $\mathrm{CeSi}{\mathrm{H}}_7$ are also dynamically stable at 0 GPa, while $\mathrm{PrSi}{\mathrm{H}}_7$ and $\mathrm{NdSi}{\mathrm{H}}_7$ are dynamically stable at 10 and 15 GPa with high $T_{\mathrm{c}}$ values of 63.9 and 65.9 K, respectively. Our results provide a system for superconducting hydrides and open a promising avenue for studying ambient pressure high-temperature superconductors.