Rare-earth hydrides can exhibit high-temperature superconductivity under high
pressure. Here, we apply a crystal structure prediction method to the current
record-holding Tc material, LaH10, and a candidate for even higher
Tc, YH10. We find a pressure-induced phase transition from the
experimentally observed cubic LaH10 phase to a new hexagonal phase at
around 420 GPa. This hexagonal phase could explain experimental observations of
hcp impurities in fcc samples. We find that YH10 forms similar structures
to LaH10 and discuss the sensitivity of superconductivity calculations to
the computational parameters used