Synthesis of Laves phase hydrides YFe2H6 and YFe2H7 at high pressure: Reaching a limit of interstitial hydrogen uptake

The C15 Laves phase compound YFe2 has been extensively studied for its significant hydrogen absorption in its tetrahedral interstitial sites. It can stably hold up to five hydrogen atoms per formula unit under ambient conditions. However, the maximum hydrogen capacity in such Laves phase materials remains an open question. The use of high pressure is a tool that facilitates the formation of polyhydrides with high hydrogen-to-metal ratios. By subjecting YFe2H4.2 to hydrogen pressures up to 50 GPa in a diamond anvil cell without any heating, two new interstitial compounds, YFe2H6 and YFe2H7, were synthesized. Synchrotron X-ray diffraction and ab initio calculations reveal that these new compounds have ordered hydrogen atoms occupying two types of tetrahedral interstitial sites, A2B2 and B4 (for YFe2H7), and a new triangular interstitial site AB2, in an orthorhombic structure distorted from the C15 Laves phase. The magnetic properties of YFe2Hx hydrides have to be taken into account to reproduce experimental volumes, identify stable compounds on the convex hull of the YFe2-H system, and explain deviations from Vegard’s law regarding volume expansion with hydrogen concentration. These compounds cannot be retrieved at ambient pressure upon pressure release.