  Recently, several research groups announced reaching the point of
metallization of hydrogen above 400 GPa. Following the mainstream of extensive
investigations of compressed polyhydrides, in this work we demonstrate that
small (4 atom %) doping of molecular hydrogen by strontium leads to a dramatic
reduction in the metallization pressure to about 200 GPa. Studying the
high-pressure chemistry of the Sr-H system at 56-180 GPa, we observed the
formation of several previously unknown compounds: C2/m-Sr$_3$H$_{13}$,
pseudocubic SrH$_6$, SrH$_9$ with cubic F-43m Sr sublattice, and
pseudotetragonal P1-SrH$_{22}$, the metal hydride with the highest hydrogen
content discovered so far. Unlike Ca and Y, strontium forms molecular
semiconducting polyhydrides, whereas calcium and yttrium polyhydrides are
high-Tc superconductors with an atomic H sublattice. The latter phase,
SrH$_{22}$ or Sr$_{0.04}$H$_{0.96}$, may be considered as a convenient model of
the consistent bandgap closure and metallization of hydrogen. Using the
impedance measurements in diamond anvil cells at 300-440 K, we estimated the
direct bandgap of the Pm-3n-like compound P1-SrH$_6$ to be 0.44-0.51 eV at 150
GPa, and its metallization pressure to be 220 GPa. Together with the machine
learning interatomic potentials, the impedance spectroscopy allowed us to
estimate the diffusion coefficients of hydrogen D$_H$ = 1.0-2.8 E-10 m$^2$/s in
SrH$_6$ and 1.2-2.1 E-9 m$^2$/s in P1-SrH$_{22}$ at 500-600 K.
