  Satterthwaite and Toepke (1970 Phys. Rev. Lett. 25 741) discovered that
Th4H15-Th4D15 superhydrides are superconducting but exhibit no isotope effect.
As the isotope effect is a fundamental prediction of electron-phonon mediated
superconductivity described by Bardeen, Cooper, and Schrieffer (BCS) its
absence alludes to some other mechanism. Soon after this work, Stritzker and
Buckel (1972 Zeitschrift f\"ur Physik A Hadrons and nuclei 257 1-8) reported
that superconductors in the PdHx-PdDx system exhibit the reverse isotope
effect. Yussouff et al (1995 Solid State Communications 94 549) extended this
finding in PdHx-PdDx-PdTx systems. Renewed interest in hydrogen- and
deuterium-rich superconductors is driven by the discovery of
near-room-temperature superconductivity in highly-compressed H3S (Drozdov et
al. 2015 Nature 525 73) and LaH10 (Somayazulu et al 2019 Phys. Rev. Lett. 122
027001) as well as two recently discovered high-pressure hydrogen-rich phases
of ThH9 and ThH10 (Semenok et al 2019 Materials Today, DOI:
10.1016/j.mattod.2019.10.005). We conclude that all known thorium
super-hydrides/deuterides, to date, are unconventional superconductors - along
with the heavy fermions, fullerenes, pnictides, cuprates, where we find they
have Tc/Tf ratios within a range of of 0.008 < Tc/Tf < 0.120, where Tc is the
superconducting transition temperature and Tf is the Fermi temperature.
