High T_\rm c Superconductivity in Heavy Rare Earth Hydrides

Sulfur and lanthanum hydrides under compression display superconducting states with high observed critical temperatures. It has been recently demonstrated that carbonaceous sulfur hydride displays room temperature superconductivity. However, this phenomenon has been observed only at very high pressure. Here, we theoretically search for superconductors with very high critical temperatures, but at much lower pressures. We describe two of such sodalite-type clathrate hydrides, YbH_6 and LuH_6. These hydrides are metastable and are predicted to superconduct with T_\rm c \sim 145 K at 70 GPa and T_\rm c \sim 273 K at 100 GPa, respectively. This striking result is a consequence of the strong interrelationship between the f states present at the Fermi level, structural stability, and the final T_\rm c value. For example, TmH_6, with unfilled 4f orbitals, is stable at 50 GPa, but has a relatively low value of T_\rm c of 25 K. The YbH_6 and LuH_6 compounds, with their filled f-shells, exhibit prominent phonon “softening”, which leads to a strong electron-phonon coupling, and as a result, an increase in T_\rm c.