  We conducted a joint experimental-theoretical investigation of the
high-pressure chemistry of europium polyhydrides at pressures of 86-130 GPa. We
discovered several novel magnetic Eu superhydrides stabilized by anharmonic
effects: cubic $EuH_{9}$, hexagonal $EuH_{9}$, and an unexpected cubic (Pm-3n)
clathrate phase, $Eu_{8}H_{46}$. Monte Carlo simulations indicate that cubic
$EuH_{9}$ has antiferromagnetic ordering with T(Neel) up to 24 K, whereas
hexagonal $EuH_{9}$ and Pm-3n-$Eu_{8}H_{46}$ possess ferromagnetic ordering
with T(Curie) = 137 and 336 K, respectively. The electron-phonon interaction is
weak in all studied europium hydrides, and their magnetic ordering excludes
s-wave superconductivity, except, perhaps, for distorted pseudohexagonal
$EuH_{9}$. The equations of state predicted within the DFT+U approach (the
Hubbard corrections were found within linear response theory) are in close
agreement with the experimental data. This work shows the great influence of
the atomic radius on symmetry-breaking distortions of the crystal structures of
superhydrides and on their thermodynamic stability.
