Stability and superconductivity of lanthanum and yttrium decahydrides

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. We find a pressure-induced phase transition from the experimentally observed cubic phase to a hexagonal phase at around 420 GPa. This phase is metastable down to low pressures and could explain experimental observations of hcp impurities in fcc samples. We go on to find that YH₁₀ adopts similar structures and discuss the sensitivity of superconductivity calculations to certain computational parameters.