High T c Superconductivity in Heavy Rare Earth Hydrides Supported by the National Natural Science Foundation of China (Grant Nos. 12122405, 51632002, and 11974133), the Program for Changjiang Scholars and Innovative Research Team in Universities (Grant No. IRT 15R23). C.J.P. acknowledges financial support from the Engineering and Physical Sciences Research Council (Grant No. EP/P022596/1)

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, YbH₆ and LuH₆. These hydrides are metastable and are predicted to superconduct with T _c ∼ 145 K at 70 GPa and T _c ∼ 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 _c value. For example, TmH₆, with unfilled 4 f orbitals, is stable at 50 GPa, but has a relatively low value of T _c of 25 K. The YbH₆ and LuH₆ 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 _c. [ABSTRACT FROM AUTHOR]