  The study of superconductivity in compressed hydrides is of great interest
due to measurements of high critical temperatures (Tc) in the vicinity of room
temperature, beginning with the observations of LaH10 at 170-190 GPa. However,
the pressures required for synthesis of these high Tc superconducting hydrides
currently remain extremely high. Here we show the investigation of crystal
structures and superconductivity in the La-B-H system under pressure with
particle-swarm intelligence structure searches methods in combination with
first-principles calculations. Structures with six stoichiometries, LaBH,
LaBH3, LaBH4, LaBH6, LaBH7 and LaBH8, were predicted to become stable under
pressure. Remarkably, the hydrogen atoms in LaBH8 were found to bond with B
atoms in a manner that is similar to that in H3S. Lattice dynamics calculations
indicate that LaBH7 and LaBH8 become dynamically stable at pressures as low as
109.2 and 48.3 GPa, respectively. Moreover, the two phases were predicted to be
superconducting with a critical temperature (Tc) of 93 K and 156 K at 110 GPa
and 55 GPa, respectively. Our results provide guidance for future experiments
targeting new hydride superconductors with both low synthesis pressures and
high Tc.
