Ambient pressure recovery of the structurally unconventional hydride Y3Fe4H20

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. A hydrogen-rich material, Y3Fe4H20, is synthesized at more than 60 GPa and remains metastable at ambient conditions for days. The combination of single-crystal x-ray diffraction and ab initio structural search allows to determine its structure.