Ambient pressure recovery of the structurally unconventional hydride Y₃Fe₄H₂₀.

Abstract

Reaching pressures in the 100 GPa range enables the synthesis of hydrogen-rich compounds with non-traditional hydrogen stoichiometries and hydrogen sublattices. The remarkable properties of such polyhydrides, including superconductivity, have attracted great interest. A crucial next step is their recovery under ambient conditions. Here we report the synthesis of Y₃Fe₄H₂₀ at pressures starting from 60 GPa by compressing a hydrogenated Y-Fe compound, embedded in hydrogen in a laser-heated diamond anvil cell. Single-crystal X-ray diffraction resolves the Y₃Fe₄ lattice framework, while a constrained ab initio structural search is used to determine the hydrogen positions. FeH₈ units form the building blocks and are connected edge-to-edge by sharing two hydrogen atoms, creating a framework that hosts Y cations. Y₃Fe₄H₂₀ can be recovered at ambient conditions, where it stays metastable for tens of hours.