Ultrafast Yttrium Hydride Chemistry at High Pressures\nvia Non-equilibrium States Induced by an X‑ray Free Electron\nLaser

Controlling\nthe formation and stoichiometric content of the desired\nphases of materials has become of central interest for a variety of\nfields. The possibility of accessing metastable states by initiating\nreactions by X-ray-triggered mechanisms over ultrashort time scales\nhas been enabled by the development of X-ray free electron lasers\n(XFELs). Utilizing the exceptionally high-brilliance X-ray pulses\nfrom the EuXFEL, we report the synthesis of a previously unobserved\nyttrium hydride under high pressure, along with nonstoichiometric\nchanges in hydrogen content as probed at a repetition rate of 4.5\nMHz using time-resolved X-ray diffraction. Exploiting non-equilibrium\npathways, we synthesize and characterize a hydride in a Weaire–Phelan\nstructure type at pressures as low as 125 GPa, predicted using a crystal\nstructure search, with a hydrogen content of 4.0–5.75 hydrogens\nper cation, that is enthalpically metastable on the convex hull.