  Lanthanum hydrides, containing hydrogen framework structures under
compression, display a superconducting state with a high observed critical
temperature. However, this phenomenon has so far only been observed at very
high pressures. Here, we computationally search for superconductors with very
high critical temperatures, but at much lower pressures. We uncover two such
sodalite-type clathrate hydrides, YbH6 and LuH6 (Tc =145 K at P=70 GPa for YbH6
and, especially, Tc =273 K at P=100 GPa for LuH6). These striking properties
are a consequence of the strong interrelationship between the f states present
at the Fermi surface, structural stability and Tc value. For example, TmH6,
with unfilled 4f orbitals, is stable at 50 GPa, while it has a relatively low
value of Tc of 25 K. As for the YbH6 and LuH6 compounds, they have filled
f-shells and the decrease of the f-energy below the Fermi level leads to
formation of the nesting regions on the Fermi surface and, as a result, to
phonon "softening" and an increase in Tc.
