Yttrium Oxyhydride (YHO): Structure & Photochromism

• Yttrium oxyhydride (YHO) is a mixed-anion compound with oxygen and hydrogen anions in a cubic lattice that imparts reversible photochromic and semiconducting behavior.
• Synthesis methods such as reactive sputtering, two-step oxygen incorporation, and high-energy ball milling allow precise control of porosity, oxidation, and defect formation.
• Optical and electrical studies show that YHO undergoes significant light-induced transmittance and resistance changes driven by defect dynamics, oxygen migration, and lattice expansion.

Searching arXiv for Yttrium oxyhydride papers to ground the article in current literature. Yttrium oxyhydride (YHO) is the mixed-anion oxygen-containing yttrium hydride generally written as $\mathrm{YH_xO_y}$, historically reported as “oxygen-containing yttrium hydride” or $\mathrm{YH_x:O}$. In the photochromic literature it denotes yttrium hydride phases with substantial oxygen incorporation, typically obtained by oxidizing a deposited yttrium hydride precursor. Relative to classical yttrium dihydride $\mathrm{YH_2}$ and trihydride $\mathrm{YH_3}$, YHO is distinguished by an expanded cubic lattice, semiconducting transparency in the clear state, and reversible color-neutral photodarkening under visible or UV illumination at room temperature and ambient pressure (Mongstad et al., 2011). Since the initial thin-film reports, the material class has expanded to controlled post-oxidized sputtered films, reactive $e^-$-beam films with in-situ composition tracking, growth-tuned HiPIMS and pulsed-DC variants, vibrationally characterized oxydeuterides, YHO-based bilayers, and photochromic powders produced by reactive ball milling (Montero et al., 2016, Kantre et al., 2020, Zubkins et al., 17 Feb 2025, Baba et al., 8 Aug 2025, Zubkins et al., 26 Feb 2026).

1. Chemical identity and phase relations

YHO is most commonly treated as a mixed-anion compound in which $\mathrm{O^{2-}}$ and $\mathrm{H^-}$ occupy anion sublattices around yttrium. A useful nominal charge-balance relation is $x + 2y \approx 3$, assuming $\mathrm{Y^{3+}}$, $\mathrm{O^{2-}}$, and $\mathrm{YH_x:O}$0 (Montero et al., 2016). This formulation is consistent with the broader rare-earth oxyhydride notation $\mathrm{YH_x:O}$1 cited for thin films (Zubkins et al., 2022).

A recurring misconception is to equate photochromic YHO either with stoichiometric $\mathrm{YH_x:O}$2 or with a simple oxide-hydride mixture. The experimental thin-film literature instead places the photochromic state in an oxygen-stabilized, $\mathrm{YH_x:O}$3-like cubic lattice rather than the hcp structure typical of $\mathrm{YH_x:O}$4. In the 2011 report, transparent sputtered hydrides were fcc and oxygen was implicated in stabilizing this expanded phase; two optically distinct oxygen-containing hydrides were identified, both fcc, with lattice parameters $\mathrm{YH_x:O}$5 Å for the transparent hydride and $\mathrm{YH_x:O}$6 Å for the black hydride (Mongstad et al., 2011). In the two-step oxidation study, the precursor lattice parameter changed from $\mathrm{YH_x:O}$7 Å before oxygen exposure to $\mathrm{YH_x:O}$8 Å after exposure, supporting the interpretation that non-reacted $\mathrm{YH_x:O}$9 was in fact $\mathrm{YH_2}$0 and that oxygen incorporation expands the cubic cell (Montero et al., 2016).

Theoretical work has refined this phase picture by distinguishing highly oxidized YHO from lower-oxidized yttrium oxyhydrides such as $\mathrm{YH_2}$1. HSE06 calculations examined YHO polymorphs in $\mathrm{YH_2}$2, $\mathrm{YH_2}$3, and $\mathrm{YH_2}$4, with calculated lattice constants of $\mathrm{YH_2}$5 Å, $\mathrm{YH_2}$6 Å, and orthorhombic $\mathrm{YH_2}$7 Å, $\mathrm{YH_2}$8 Å, $\mathrm{YH_2}$9 Å, respectively; among these, $\mathrm{YH_3}$0 was the most stable YHO polymorph considered (Strugovshchikov et al., 2020). Experimental films, however, are usually disordered and nanocrystalline rather than long-range ordered realizations of these ideal cells.

2. Synthesis routes and oxidation strategies

The established thin-film route begins with yttrium hydride deposition and then relies on controlled oxygen incorporation. The original photochromic films were grown by reactive magnetron sputtering from a metallic Y target in mixed Ar and $\mathrm{YH_3}$1 with a working pressure of $\mathrm{YH_3}$2 Pa and gas-flow ratio $\mathrm{YH_3}$3; residual water vapor was identified as a likely oxygen source, and the films were photochromic in the as-deposited state under ambient air without a capping layer (Mongstad et al., 2011). A more explicit two-step synthesis subsequently deposited oxygen-free $\mathrm{YH_3}$4 and then admitted oxygen from air through an Al capping layer with “a low but non-zero oxygen permeability,” enabling in-situ observation of the transformation from dark, opaque $\mathrm{YH_3}$5-like precursor to transparent yellowish YHO (Montero et al., 2016).

Reactive $\mathrm{YH_3}$6-beam evaporation provided a complementary route in which Y was deposited in $\mathrm{YH_3}$7 at $\mathrm{YH_3}$8 Pa and then oxidized by controlled $\mathrm{YH_3}$9 doses. In that study, in-situ ion-beam analysis tracked O and H during oxidation, and one representative sample reached the onset of photochromism at $e^-$0 after an oxygen dose of $e^-$1 Langmuir (Kantre et al., 2020). The same work emphasized that the photo-state did not measurably alter composition within its analytical sensitivity, implying that photodarkening is not a simple stoichiometric conversion.

A 2026 comparison of reactive HiPIMS and reactive pulsed-DC magnetron sputtering showed that deposition energetics strongly affect the precursor microstructure and thereby the later YHO state. Both methods used post-oxidation by venting to air, but the critical working pressure $e^-$2 needed to obtain transparent and photochromic films was higher for HiPIMS, $e^-$3 Pa, than for pulsed-DCMS, $e^-$4 Pa, because the more energetic HiPIMS discharge produced denser films at a given pressure (Zubkins et al., 26 Feb 2026).

A major recent extension is powder synthesis. Reactive high-energy planetary ball milling of yttrium metal under $e^-$5 bar hydrogen for up to $e^-$6 h, followed by controlled oxidation in ultra-dry technical air with $e^-$7 ppm and $e^-$8, produced nanostructured photochromic YHO powders with predominantly sub-500 nm particle sizes (Baba et al., 8 Aug 2025).

Route	Representative conditions	Result
Reactive sputtering	Ar/$e^-$9, $\mathrm{O^{2-}}$0 Pa, $\mathrm{O^{2-}}$1	As-deposited ambient-photochromic thin films
Two-step sputter + air oxidation	$\mathrm{O^{2-}}$2 precursor, Al cap with low O permeability	Gradual $\mathrm{O^{2-}}$3 YHO conversion
Reactive $\mathrm{O^{2-}}$4-beam + $\mathrm{O^{2-}}$5 dosing	$\mathrm{O^{2-}}$6 Pa, photochromism at $\mathrm{O^{2-}}$7	In-situ composition-controlled YHO
Reactive ball milling + dry-air oxidation	$\mathrm{O^{2-}}$8 bar $\mathrm{O^{2-}}$9, up to $\mathrm{H^-}$0 h, ultra-dry air	Photochromic YHO powders

3. Structure, disorder, and vibrational signatures

Experimentally, photochromic YHO films are usually polycrystalline cubic materials with substantial disorder and small crystallites. XRD measurements on YHO and YDO films showed peaks near $\mathrm{H^-}$1, $\mathrm{H^-}$2, $\mathrm{H^-}$3, and $\mathrm{H^-}$4, indexed as $\mathrm{H^-}$5, $\mathrm{H^-}$6, $\mathrm{H^-}$7, and $\mathrm{H^-}$8; relative to $\mathrm{H^-}$9-$x + 2y \approx 3$0, the peaks shift to lower angles, consistent with lattice expansion by oxygen incorporation, and the extracted lattice parameters fall in the $x + 2y \approx 3$1 Å range (Zubkins et al., 17 Feb 2025). Scherrer analysis gave average crystallite sizes of $x + 2y \approx 3$2 nm, while SEM indicated fibrous, V-shaped grains and porosity typical of Thornton zone 1/T growth (Zubkins et al., 17 Feb 2025). These observations align with optical and compositional studies that report porosity gradients and refractive-index gradients in sputtered films, especially above the pressure range where oxidation during or after growth becomes facile (Zubkins et al., 2022).

Vibrational spectroscopy provides a structural diagnostic that is unusually informative for YHO because isotopic substitution separates hydrogen-dominated from lattice-dominated modes. FTIR spectra of YHO showed broad bands at approximately $x + 2y \approx 3$3, $x + 2y \approx 3$4, $x + 2y \approx 3$5, and $x + 2y \approx 3$6; upon deuteration, the high-frequency band shifted to about $x + 2y \approx 3$7, whereas the lower-frequency bands remained unchanged (Zubkins et al., 17 Feb 2025). The unshifted low-frequency bands were assigned to Y/O-dominated lattice vibrations, while the $x + 2y \approx 3$8 band was interpreted as H/D-dominated but not purely local hydride motion. The observed ratio $x + 2y \approx 3$9, and the corresponding $\mathrm{Y^{3+}}$0, deviates from the $\mathrm{Y^{3+}}$1 limit expected for a pure H oscillator, indicating mixed vibrational character and substantial coupling to the Y–O framework (Zubkins et al., 17 Feb 2025).

Solid-state $\mathrm{Y^{3+}}$2 NMR on YDO further emphasized heterogeneity. The spectra resolved a narrow site assigned to trapped molecular $\mathrm{Y^{3+}}$3, a broad Gaussian hydride site, and a very broad low-symmetry site with quadrupole coupling constants $\mathrm{Y^{3+}}$4 kHz under MAS, consistent with highly disordered local environments (Zubkins et al., 17 Feb 2025). No clear evidence for long-range anion order was found, and the combination of broad FTIR bands, XRD peak broadening, and NMR line shapes supports a disordered mixed-anion lattice rather than an ideally ordered stoichiometric crystal.

4. Optical, electrical, and kinetic behavior

The defining property of YHO is reversible photochromism. In the original thin-film report, exposure to visible and UV light at $\mathrm{Y^{3+}}$5 triggered a color-neutral decrease in optical transmission across the visible and near-IR; for a $\mathrm{Y^{3+}}$6 nm film, the average transmission in the $\mathrm{Y^{3+}}$7 nm interval decreased by $\mathrm{Y^{3+}}$8 after 1 h, and a $\mathrm{Y^{3+}}$9 nm film showed a reduction from $\mathrm{O^{2-}}$0 to $\mathrm{O^{2-}}$1 under repeated illuminations (Mongstad et al., 2011). The darkening was predominantly absorptive: the optical density $\mathrm{O^{2-}}$2 increased broadly, reflection also decreased, and the band-gap position remained unchanged during photodarkening (Mongstad et al., 2011).

Excitation-energy dependence supported an electronic threshold. Blue illumination at $\mathrm{O^{2-}}$3 eV produced the strongest resistivity change, green at $\mathrm{O^{2-}}$4 eV a weaker change, and red at $\mathrm{O^{2-}}$5 eV a substantially weaker one, suggesting that band-to-band carrier excitation is central to the response when the experimental gap is near $\mathrm{O^{2-}}$6 eV (Mongstad et al., 2011). Electrical measurements tracked this optical behavior: the resistance dropped by about $\mathrm{O^{2-}}$7 during one illumination sequence, and longer exposures could yield up to about $\mathrm{O^{2-}}$8 reduction (Mongstad et al., 2011).

Recovery occurs spontaneously in darkness, but bleaching is typically slower than darkening and depends on temperature and prior exposure. Moderate heating, such as $\mathrm{O^{2-}}$9, accelerates bleaching, though recovery also proceeds at room temperature (Mongstad et al., 2011). A “memory effect” is characteristic: previously exposed regions darken faster during later illumination, and imprinted patterns disappear upon bleaching but reappear on re-illumination, persisting for weeks in thin films (Mongstad et al., 2011).

A thicker-film study made the environmental dependence explicit. For a $\mathrm{YH_x:O}$00 nm YHO film, luminous transmittance dropped from $\mathrm{YH_x:O}$01 to $\mathrm{YH_x:O}$02 under visible-light illumination; in air, recovery occurred within a few hours, whereas in an $\mathrm{YH_x:O}$03 glove box with $\mathrm{YH_x:O}$04 ppm the same sample showed minimal recovery over 24 h and progressively lost transparency over repeated cycles (Baba et al., 2019). This result tied bleaching kinetics to ambient oxygen availability rather than to a purely internal relaxation process.

Powders display the same qualitative phenomena, although reflectance replaces transmittance as the observable. Under $\mathrm{YH_x:O}$05 nm illumination at $\mathrm{YH_x:O}$06 for $\mathrm{YH_x:O}$07 min, YHO powders showed approximately $\mathrm{YH_x:O}$08 reflectance decrease at $\mathrm{YH_x:O}$09 nm; after 5 min in darkness most of the contrast recovered, and nine on/off cycles showed the same memory effect previously associated with thin films (Baba et al., 8 Aug 2025).

5. Mechanistic models and unresolved questions

The microscopic mechanism of YHO photochromism remains unsettled, but several explanations are now constrained by experiment. First, the photochromic change is not well described as a simple band-edge shift: the 2011 study reported that the gap near the $\mathrm{YH_x:O}$10 value remained unchanged during darkening, even though absorption increased across the visible and near-IR (Mongstad et al., 2011). Second, it is not well described as hydroxyl formation: FTIR under $\mathrm{YH_x:O}$11 eV illumination revealed increased absorbance extending into the mid-IR up to approximately $\mathrm{YH_x:O}$12, but no measurable phase transformation and no increase in OH-band intensity (Zubkins et al., 17 Feb 2025).

An early structural-electronic hypothesis proposed a light-induced localized structural change within the fcc oxygen-stabilized hydride, but an effective-medium model based on a transparent matrix plus metallic hydride inclusions treated with the Bruggeman approximation did not reproduce the spectra (Mongstad et al., 2011). Later work reintroduced metallic-domain ideas in a more specific form. The 2019 “breathing” study argued that illumination drives some oxygen atoms toward the surface, leaving an oxygen-deficient bulk responsible for photodarkening and lattice contraction; in this picture, YHO reversibly exchanges oxygen with its environment during illumination/darkness cycling (Baba et al., 2019). XPS showed surface oxygen enrichment after illumination, and DFT associated the effect with light-induced weakening of the Y–O bond through a pseudo–Jahn–Teller-type electronic instability (Baba et al., 2019).

The 2025 vibrational study converged on a closely related but more explicitly defect-driven interpretation. Because UV illumination increased visible and mid-IR absorption without altering lattice vibrational signatures, the authors concluded that the process is primarily electronic/defect-driven rather than a chemically driven OH process or a large-scale crystallographic phase transition (Zubkins et al., 17 Feb 2025). That interpretation is broadly compatible with the 2026 growth-controlled study, which framed the literature in terms of competing models involving metallic domains, anion-vacancy-mediated electronic transitions with local charge and lattice relaxation, and broader defect-based mechanisms, while noting that the relative roles of O/H defect migration and local electronic trapping remain under debate (Zubkins et al., 26 Feb 2026).

A plausible synthesis of these results is that oxygen mobility, defect-state formation, and local electronic restructuring are coupled rather than mutually exclusive. The literature does not yet resolve whether metallic nanodomains, oxygen-deficient oxyhydride regions, or trapped-carrier/defect-center absorption dominate under all growth conditions.

6. Growth control, derivative systems, and applications

YHO is unusually sensitive to deposition conditions because growth mode sets the later oxidation pathway. In reactive pulsed-DC sputtering, increasing working pressure promotes more porous growth, faster oxygen ingress, and a refractive-index gradient perpendicular to the substrate; films deposited below about $\mathrm{YH_x:O}$13 Pa remain dense and metallic $\mathrm{YH_x:O}$14, whereas films deposited at or above roughly $\mathrm{YH_x:O}$15 Pa become partly transparent and, after post-oxidation, photochromic (Zubkins et al., 2022). The same study reported that transmittance-rise time constants after oxygen dosing at $\mathrm{YH_x:O}$16 nm decreased from about $\mathrm{YH_x:O}$17 s at $\mathrm{YH_x:O}$18 Pa to $\mathrm{YH_x:O}$19 s at $\mathrm{YH_x:O}$20 Pa and $\mathrm{YH_x:O}$21 s at $\mathrm{YH_x:O}$22 Pa, directly linking porosity to oxidation kinetics (Zubkins et al., 2022).

The HiPIMS versus pulsed-DC comparison demonstrated that composition alone does not predict performance. Near the respective critical pressures, both methods produced films with solar transmittance near $\mathrm{YH_x:O}$23 and lattice parameters of $\mathrm{YH_x:O}$24 Å, but the pulsed-DCMS film had $\mathrm{YH_x:O}$25 eV and photochromic contrast $\mathrm{YH_x:O}$26, whereas the HiPIMS film had $\mathrm{YH_x:O}$27 eV and contrast $\mathrm{YH_x:O}$28. The pulsed-DCMS film also had lower $\mathrm{YH_x:O}$29 ratio, $\mathrm{YH_x:O}$30 versus $\mathrm{YH_x:O}$31, and a pronounced $\mathrm{YH_x:O}$32 out-of-plane orientation rather than the largely random orientation of the HiPIMS film (Zubkins et al., 26 Feb 2026). This established microstructure and discharge energetics as central control parameters.

YHO has also been integrated into composite and bilayer architectures. In YHO/$\mathrm{YH_x:O}$33 bilayers, hydrogen intercalation from YHO into X-ray-amorphous $\mathrm{YH_x:O}$34 forms $\mathrm{YH_x:O}$35, increasing contrast at $\mathrm{YH_x:O}$36 nm to about $\mathrm{YH_x:O}$37 after 20 h of UVA-violet exposure, compared with $\mathrm{YH_x:O}$38 for single-layer YHO, but at the cost of incomplete bleaching and chemical instability (Strods et al., 20 Jun 2025). This is not intrinsic YHO photochromism in the narrow sense; rather, it is a hydrogen-coupled derivative system in which YHO supplies mobile hydrogen to a second chromic layer.

Applications repeatedly proposed across the literature are smart windows and adaptive glazing, optical memory and rewritable media, sensors, and patternable coatings (Mongstad et al., 2011, Baba et al., 8 Aug 2025). Powder synthesis substantially broadens processing options because the material can be dispersed into polymer composites; YHO–polystyrene cast films have already been patterned by masked $\mathrm{YH_x:O}$39 nm illumination, demonstrating spatially resolved rewritability (Baba et al., 8 Aug 2025). At the same time, several limitations remain consistent across studies: bleaching is slower than darkening in standalone YHO, the microscopic mechanism is unresolved, over-oxidation suppresses photochromism, and powder precursors are highly sensitive to uncontrolled oxidation before stabilization (Mongstad et al., 2011, Baba et al., 8 Aug 2025).

YHO therefore occupies a distinctive position within inorganic photochromics: it is an oxygen-stabilized yttrium hydride derivative whose clear-state transparency, room-temperature ambient operation, visible-light activation, and coupled optical-electrical response arise from a microstructure- and composition-sensitive mixed-anion lattice. The central research problems are now less about demonstrating the phenomenon than about controlling oxygen and hydrogen distributions, clarifying the operative defect physics, and translating thin-film behavior into stable large-area or particulate formats.