Resolving Redox Anion Exchange and Phase Widths in Yttrium Hydrides by In Situ Neutron Diffraction

Bulk yttrium hydride oxide (YHO) undergoes anion exchange and structural transformation upon oxidation in air. We investigated this behavior using in situ neutron powder diffraction and compared it with that of bulk yttrium trihydride (YH 3 ). YHO was observed to transform from orthorhombic anti ‐LiMgN‐type YHO via Y 2 H 0.50 O 2.75 to bixbyite‐type yttrium oxide (Y 2 O 3 ). Sequential Rietveld refinement identifies Y 2 H 0.50 O 2.75 as the central intermediate and indicates temperature‐dependent oxygen ordering over the O2 and O4 sites. At higher temperatures, the data are consistent with a homogeneity range toward Y 2 O 3− x H 2 x with 0 < x ≤ 0.25 before final conversion to yttria. At elevated temperatures, YHO also shows an order‐disorder phase transformation to the cubic polymorph independent of the presence of oxygen. In contrast, YH 3 does not form bulk hydride oxides under the present conditions but reacts to yttrium dihydride (YH 2 ) and yttrium nitride (YN).