Metal superhydrides are hydrogen-rich compounds that are generally only stable under high pressures. Since they have a high density of hydrogen atoms, their properties can resemble metallic hydrogen. Metallic hydrogen could be a high-temperature superconductor, but needs even higher pressures for stabilization. Metal superhydrides could show the same effect at lower pressures, in addition to interesting magnetic properties.

Xiaoli Huang, Jilin University, Changchun, China, Alexander G. Kvashnin, Skolkovo Institute of Science and Technology, Moscow, Russia, Artem R. Oganov, Skolkovo Institute of Science and Technology and Northwestern Polytechnical University, Xi’an, China, Tian Cui, Jilin University and Ningbo University, China, and colleagues have found three new neodymium superhydrides: NdH₄, NdH₇, and NdH₉. The compounds were synthesized in a diamond anvil cell from neodymium and NH₃BH₃ at pressures of 85–135 GPa using laser heating. The reactions were monitored using X-ray diffraction.

The team found that NdH₄, NdH₇, and NdH₉ crystallize in the space groups I4/mmm, C2/c, and P6₃/mmc, respectively. The researchers measured the electrical resistance of the compounds and found a sharp drop in resistance for NdH₉ at ca. 4.5 K, which indicates a superconducting transition. Theoretical calculations were used to predict the magnetic properties of the superhydrides. The results indicate that all three compounds show strong magnetism and an antiferromagnetic order.

• High-Pressure Synthesis of Magnetic Neodymium Polyhydrides,
  Di Zhou, Dmitrii V. Semenok, Hui Xie, Xiaoli Huang, Defang Duan, Alex Aperis, Peter M. Oppeneer, Michele Galasso, Alexey I. Kartsev, Alexander G. Kvashnin, Artem R. Oganov, Tian Cui,
  J. Am. Chem. Soc. 2020.
  https://doi.org/10.1021/jacs.9b10439