High-Pressure Synthesis of Magnetic Neodymium Polyhydrides

The current search for room-temperature superconductivity is inspired by the unique properties of the electron-phonon interaction in metal superhydrides. Encouraged by the recently found highest-TC superconductor fcc-LaH10, here we discover several superhydrides of another lanthanide — neodymium. We identify three novel metallic Nd-H phases at pressures range from 85 to 135 GPa: I4/mmm-NdH4, C2/с-NdH7, and P63/mmc-NdH9, synthesized by laser-heating metal samples in NH3BH3 media for in situ generation of hydrogen. A lower trihydride Fm3 m-NdH3 is found at pressures from 2 to 52 GPa. I4/mmm-NdH4 and C2/с-NdH7 are stable from 135 down to 85 GPa, and P63/mmc-NdH9 from 110 to 130 GPa. Measurements of the electrical resistance of NdH9 demonstrate a possible superconducting transition at ~ 4.5 K in P63/mmc-NdH9. Our theoretical calculations predict that all the neodymium hydrides have antiferromagnetic order at pressures below 150 GPa and represent one of the first discovered examples of strongly correlated sup...