Superconducting Praseodymium Superhydrides.

Superhydrides have complex hydrogenic sublattices, and are important prototype systems to investigate metallization of hydrogen with potential room-temperature superconductivity. Here we synthesized praseodymium superhydrides (PrH9) via laser heating of metal samples in ammonia borane media at high pressure. Synchrotron X-ray diffraction (XRD) analysis shows the presence of previously predicted F43m-PrH9 and unexpected P63/mmc-PrH9 phases. Moreover, Fm3m-PrH, Fm3m-PrH3 and P4/nmm-PrH3 were found below 52 GPa. P63/mmc-PrH9 is stable at 90-140 GPa, and at pressures above 110 GPa is non-magnetic and superconducting with maximum Tc around 55 K and critical magnetic field Hc(0) up to 15 T. F43m-PrH9 is almost non-magnetic and has Tc = 69 K and Hc(0) < 17 T at 120 GPa. The theoretically predicted stability and superconductivity of discovered praseodymium hydrides are sensitive to magnetism and spin-orbit coupling. Our results highlight the intimate connections among hydrogenic sublattices, density of states, magnetism and superconductivity in Pr-based superhydrides.