  Recently, the thorium hydride ThH9 possessing an 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 was theoretically predicted to reach a Tc of 296 K at 400 GPa.
Using first-principles calculations, we find 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 are
associated with their stability at very different pressures. Furthermore, we
reveal that (i) the H-derived density of states at the Fermi level Ef is about
two times larger in ThH18 than in ThH9. (ii) the average squared phonon
frequency of H atoms is about 29% higher in ThH18 than in ThH9, and (iii) the
Fermi surface average squared electron-phonon matrix element is similar between
the two hydrides. Consequently, the electron-phonon coupling constant of ThH18
becomes much greater than that of ThH9, leading to a significant Tc difference
between the two thorium hydrides. Our findings not only provide an explanation
for the very large differences in the stabilization pressure and
superconducting transition temperature between ThH9 and ThH18 but also have
important implications for the design of H-rich, high-Tc clathrate metal
hydrides.
