Anion Exchange Alters Photochromism in Bulk Yttrium Hydride Oxides

Yttrium hydride oxide thin films (YHO) are a rare example of color-neutral photochromic materials, yet the structural origins of this behavior remain poorly understood. In this study, we investigate the anion exchange in bulk YHO using in situ X-ray and neutron powder diffraction, revealing the formation of an intermediate phase, Y2H0.50O2.75, crystallizing in the tetragonal space group I4cm (a = 10.6155(2) Å, c = 10.5973(3) Å). This phase forms upon heating YHO in air and structurally bridges the orthorhombic YHO and bixbyite-type Y2O3. Its mixed-anion substructure features both oxide and hydride ions with partial occupancies and local disorder. Sequential in situ Rietveld refinements allowed us to monitor real-time phase evolution, lattice parameter changes, and anion occupancies, confirming the existence of two compositionally distinct line phases with minimal phase width. According to DFT calculations, Y2H0.50O2.75 is an insulator (3.0 eV ≤ Eg). Optical measurements reveal UV-induced darkening in both YHO and Y2H0.50O2.75, with the latter exhibiting distinct spectral features and suppressed photochromism. These findings provide insight into anion exchange pathways in heteroanionic materials and establish a structural framework for understanding photochromism in the Y–H–O system. This work demonstrates the utility of in situ diffraction for probing dynamic transformations in functional mixed-anion compounds.