Synthesis and Stability of Lanthanum Superhydrides*

Recent theoretical calculations predict that megabar pressure stabilizes very hydrogen-rich simple compounds having new clathrate-like structures and remarkable electronic properties including room-temperature superconductivity. We report x-ray diffraction and optical studies demonstrating that superhydrides of lanthanum can be synthesized with La atoms in an fcc lattice at 170 GPa upon heating to ~1000 K. The results match the predicted cubic metallic phase of LaH₁₀ having cages of thirty-two hydrogen atoms surrounding each La atom. Upon decompression, the fcc-based structure of LaH₁₀ undergoes a rhombohedral distortion of the La sublattice. The superhydride phases consist of an atomic hydrogen sublattice with H-H distances of ~1.1 Å, which are close to predictions for solid atomic metallic hydrogen at these pressures.