Face-centered-cubic yttrium trihydride high-pressure phase stabilized at ambient pressures by mechanical milling

Abstract

The crystal structure of yttrium trihydride (YH3) is usually characterized by hexagonal close-packed (HCP) metal ordering at ambient conditions and transforms into an FCC structure (FCC-YH3−δ) at pressures greater than 8 GPa. FCC-YH3−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 YH3(HCP-YH3) in this study. The lattice constant of the FCC-“YH3−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 YH3 from 2.2 eV to 2.45 eV measured by UV–vis spectrometry. FCC-YH3 was dehydrogenated to an FCC “YH2” dihydride phase at 520 K, which is known to be conventionally present at ambient pressures. However, the YH2 generated from FCC-YH3−δ maintained its FCC structure upon re-hydrogenation to trihydride, unlike how YH2 usually changes its structure from FCC to HCP. The volume change after the dehydrogenation of FCC-YH3−δ was approximately 5%, which is smaller than that of HCP-YH3 (approximately 10%).