Face-Centered-Cubic Yttrium Trihydride High-Pressure Phase Stabilized at Ambient Pressures by Mechanical Milling

Abstract The crystal structure of yttrium trihydride (YH 3 ) is usually characterized by hexagonal close-packed (HCP) metal ordering at ambient conditions and transforms into an FCC structure (FCC-YH 3 − δ ) at pressures greater than 8 GPa. FCC-YH 3 − δ was previously considered to only be stable under high pressures, but we found that it can be stabilized even at ambient pressures by the mechanical milling of hexagonal YH 3 (HCP-YH 3 ) in this study. The lattice constant of the FCC-“YH 3 − δ ” sample was evaluated to be a = 0.52801(2) nm, which corresponds to the expected value calculated in other studies. This structural change from HCP to FCC enlarged band gap of the YH 3 from 2.2 eV to 2.45 eV measured by UV-vis spectrometry. FCC-YH 3 was dehydrogenated to an FCC “YH 2 ” dihydride phase at 520 K, which is known to be conventionally present at ambient pressures. However, the YH 2 generated from FCC-YH 3 − δ maintained its FCC structure upon re-hydrogenation to trihydride, unlike how YH 2 usually changes its structure from FCC to HCP. The volume change after the dehydrogenation of FCC-YH 3 − δ was approximately 5%, which is smaller than that of HCP-YH 3 (approximately 10%).