High‐Pressure Stability and Superconductivity of Clathrate Thorium Hydrides

Recently, thorium hydride ThH9 possessing a H‐rich clathrate structure has been experimentally synthesized to exhibit a superconducting transition temperature Tc of 146 K at 170–175 GPa, while the more H‐rich clathrate thorium hydride ThH18 is theoretically predicted to reach a Tc of 296 K at 400 GPa. Using first‐principles calculations, it is found that ThH9 has a more ionic character between Th atoms and H cages than ThH18 and that the latter has a more substantial hybridization of the Th 6p semicore and H 1s states than the former. These different bonding characteristics of ThH9 and ThH18 reflect the very large difference in their stabilization pressures. Furthermore, it is revealed that the H‐derived density of states at the Fermi level EF is about two times larger in ThH18 than in ThH9, which in turn leads to the significant large differences in the electron–phonon coupling (EPC) constant and Tc between the two thorium hydrides. The findings not only present the different bonding and EPC characteristics of ThH9 and ThH18 but also have important implications for the design of H‐rich, high‐Tc clathrate metal hydrides. The high‐pressure stability and superconductivity in H‐rich clathrate thorium hydrides is analyzed. The partial density of states for the Th and H orbitals in ThH9 and ThH18 (left), the calculated superconducting gaps Δ of ThH9 and ThH18 as a function of temperature (right) as well as the H29 and H36 cages surrounding each Th atom in ThH9 and ThH18 are shown.