Anion Exchange Pathways in Half‐Heusler‐Type Holmium Hydride Oxide HoHO

The oxidation of half‐Heusler‐type holmium hydride oxide HoHO in air was investigated by in situ neutron powder diffraction to elucidate the pathway of redox anion exchange in the Ho–H–O system. The data reveal the formation of the oxygen‐rich intermediate Ho 2 H 0.50 O 2.75 , which was isolated after reaction and structurally characterized by Rietveld refinement as isotypic with the corresponding yttrium compound. The starting phase HoHO shows a limited oxygen solubility that can be described by HoH 1− x O 1+ x /2 with 0 ≤ x ≤ 0.1, while the intermediate Ho 2 H 0.50 O 2.75 exhibits rapid ordering of the partially occupied O2 and O4 sites toward occupancies of about one half and no clear evidence for a phase width on the hydrogen‐richer side. Thus, the first anion‐exchange step proceeds closely analogously to that previously observed for YHO. In contrast, the further transformation to Ho 2 O 3 occurs over a comparatively narrow temperature range because the tetragonal metrics of Ho 2 H 0.50 O 2.75 progressively approach cubic symmetry and increasingly overlap with those of holmia. The results establish Ho 2 H 0.50 O 2.75 as the key intermediate in the oxidation of HoHO and highlight both the generality and the methodological limits of tracking anion‐exchange processes by in situ neutron diffraction.