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[Tomoyuki Yamasaki](https://orcid.org/0000-0002-6982-7538), [Keiga Fukui](https://orcid.org/0000-0002-5659-7200), [Soshi Iimura](https://orcid.org/0000-0003-3270-155X), [Shunsuke Tsuda](https://orcid.org/0000-0001-6209-8048), [Hiroshi Mizoguchi](https://orcid.org/0000-0002-0992-7449), [Takahisa Omata](https://orcid.org/0000-0002-6034-4935), [Hideo Hosono](https://orcid.org/0000-0001-9260-6728)

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[Amphoteric Behavior of Hydrogen in Lanthanum Oxyhydrides: Correlation with Electrochemical Properties](https://mdr.nims.go.jp/datasets/311e9f5d-2ed1-4569-ab47-e91218218245)

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Amphoteric Behavior of Hydrogen in Lanthanum Oxyhydrides: Correlation with Electrochemical PropertiesAmphoteric Behavior of Hydrogen in Lanthanum Oxyhydrides:Correlation with Electrochemical PropertiesTomoyuki Yamasaki,* Keiga Fukui, Soshi Iimura, Shunsuke Tsuda, Hiroshi Mizoguchi, Takahisa Omata,and Hideo HosonoCite This: J. Am. Chem. Soc. 2026, 148, 11393−11402 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: Hydrogen incorporated into solids can adopt either protonic (H+)or hydridic (H−) characteristics depending on the position of the Fermi level ofthe host material. In recent years, not only proton-conducting materials but alsohydride-ion-conducting materials have been actively developed, enabling thedevelopment of novel electrochemical devices that exploit the amphoteric natureof hydrogen. At present, however, the relationship between the electrochemicalproperties of hydrogen-ion-conducting materials and the amphoteric nature ofhydrogen remains scarcely explored. Herein, we investigate the electrochemicalwindow (ECW) of the H− conductor LaH3−2xOx and elucidate the underlyingamphoteric behavior of hydrogen. We determine the band alignment of LaH3−2xOxand map the ECW onto an electronic energy scale referenced to the vacuum level. Based on this band alignment, we examine theconduction behavior of LaH3−2xOx in a H2 atmosphere via electromotive force measurements of a hydrogen concentration cell. Thereducing limit of the ECW was governed by hydrogen extraction from LaH3−2xOx, whereas its oxidizing limit was governed by thecapture of positive charge by interstitial H−, leading to H+ formation and electronic conduction. Electromotive force measurementsof a hydrogen concentration cell revealed that, in addition to H− conduction, a finite contribution from H+ conduction emerges neara hydrogen partial pressure of 1 atm. By capturing the amphoteric nature of hydrogen on an electronic energy scale, this paperprovides a unified perspective across different classes of hydrogen-ion conductors.■ INTRODUCTIONHydrogen in solids exhibits various bonding motifs and chargestates, ranging from protonic (H+) to hydridic (H−). Thisamphoteric nature governs various material properties such ascharge-carrier transport, magnetic behavior, and chemicalreactivity.1−8 Hydrogen-ion-conducting materials, includingboth H+ and H− conductors, have been actively developed askey components for fuel cells and other electrochemical energyconversion and storage devices.9−18 Along with the develop-ment of materials exhibiting high ionic conductivity,investigations on the characteristic features of hydrogen ionsin solids beyond mere differences in charge polarity, such astheir distinct chemical reactivity and reaction selectivity, havealso increased.19−21 The availability of a unified descriptor thatcaptures these properties of hydrogen in solids wouldcontribute to the rational design of new functional materialsand electrochemical devices, thereby opening up newpossibilities of hydrogen-based ionics.In semiconductors, hydrogen often occupies interstitial sitesin the host semiconductor lattice, where it can act as a donoror acceptor upon ionization into H+ or H−, thereby altering theelectrical conductivity of the host material.22,23 Hydrogencharge states and donor/acceptor behaviors are often discussedin terms of the theoretically calculated energies of interstitial-hydrogen defect levels within band alignments referenced tothe vacuum level (Evac.) to understand their impact on chargetransport in semiconductors.24,25 The validity of this concepthas been corroborated experimentally, for example, by muonspin spectroscopy.26,27Recently, conceptual frameworks that consider electronicstructure and band-edge positions, established in semi-conductor materials and device research, have increasinglybeen extended to the design of electrochemical devicesemploying ion conductors. In Li-ion battery applications, forexample, the band alignments of Li-conducting materials haveemerged as powerful tools for the design of electrolytes with awide electrochemical window (ECW)28,29�the potentialrange over which an electrolyte functions without electronicleakage�and for optimizing electrode/electrolyte interfa-ces.30,31By contrast, the electrochemical characteristics of hydrogen-ion-conducting materials have not yet been viewed from anelectronic energy scale referenced to the Evac., which isReceived: January 26, 2026Revised: February 26, 2026Accepted: February 27, 2026Published: March 6, 2026Articlepubs.acs.org/JACS© 2026 The Authors. Published byAmerican Chemical Society11393https://doi.org/10.1021/jacs.6c01849J. Am. Chem. Soc. 2026, 148, 11393−11402This article is licensed under CC-BY-NC-ND 4.0Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on March 27, 2026 at 04:06:38 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Tomoyuki+Yamasaki"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Keiga+Fukui"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Soshi+Iimura"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Shunsuke+Tsuda"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hiroshi+Mizoguchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takahisa+Omata"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hideo+Hosono"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hideo+Hosono"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/jacs.6c01849&ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=tgr1&ref=pdfhttps://pubs.acs.org/toc/jacsat/148/10?ref=pdfhttps://pubs.acs.org/toc/jacsat/148/10?ref=pdfhttps://pubs.acs.org/toc/jacsat/148/10?ref=pdfhttps://pubs.acs.org/toc/jacsat/148/10?ref=pdfpubs.acs.org/JACS?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/jacs.6c01849?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://pubs.acs.org/JACS?ref=pdfhttps://pubs.acs.org/JACS?ref=pdfhttps://creativecommons.org/licenses/by-nc-nd/4.0/commonly used in semiconductor research. One reason is thatthe applications of conventional proton-conducting materialshave been largely confined to fuel cells and steam electrolysiscells, where the operating potential range is constrained by theH+/H2 and O2/H2O redox couples. However, recent progressin H− conductors is changing this landscape. It has been shownthat proton-conducting In-doped BaZrO3, when exposed tostrongly reducing atmospheres, undergoes hydrogen reductionto form H−, thereby exhibiting mixed H− and electronconduction.18,32 In addition, electrolytic devices that integrateproton-conducting and hydride-ion-conducting In-dopedBaZrO3 phases have been demonstrated.33 For the design ofnew electrochemical devices that exploit hydrogen amphotericbehavior, it is essential to describe the redox properties ofhydrogen from the viewpoint of electronic energy scale.In this study, we focus on lanthanum oxyhydrides,LaH3−2xOx, which are H− conductors exhibiting fast H−diffusivity.34,35 This class of materials is regarded as a highlypromising candidate for realizing new electrochemical devicesfor CO2 reduction and NH3 synthesis by leveraging the highreducing ability of H−.36,37 Herein, we reveal the amphotericbehavior of hydrogen in LaH3−2xOx under hydrogenatmospheres that are oxidizing conditions for H−. We furthershow that partial protonation of hydrogen under theseconditions defines the ECW of LaH3−2xOx, which isrationalized by an energy band alignment obtained fromphotoemission experiments and defect levels estimated by first-principles calculations.■ EXPERIMENTAL SECTIONSynthesis of Sintered LaH3−2xOx PelletsSintered LaH3−2xOx pellets were prepared for the electrical measure-ments. LaH3 powder was obtained by hydrogenating La ingots(99.9%, Rare Metallic Co., Ltd.) via annealing at 400 °C under a ∼1MPa H2 atmosphere for 10 h. The resulting LaH3 powder was mixedwith La2O3 (99.9%, Kojundo Chemical Laboratory Co., Ltd.) using azirconia mortar and pestle inside an Ar-filled glovebox. The mixedpowders were subsequently annealed at 775 °C under vacuum toobtain LaH3−2xOx. During this process, LaH3 was dehydrogenated toa metallic La, which promoted powder sintering. The samples wererehydrogenated under a hydrogen atmosphere. Subsequent slowcooling yielded crack-free H−-conductive pellets. Structural character-ization was performed via X-ray diffraction (Rigaku Corp.MiniFlex600) in an airtight capsule. Lattice parameters weredetermined by Rietveld analysis using SmartLab Studio II (RigakuCorp.). The dependence of the lattice parameters of LaH3−2xOx on itsoxygen content agreed with the results reported in an earlier work(Figure S1).34 The obtained compositions were consistent with thenominal molar ratio of LaH3 and La2O3, indicating that oxygencontamination during synthesis was negligible. The sinteredLaH3−2xOx pellets were dense, with relative densities exceeding95%, as confirmed by cross-sectional microscopic observations(Figure S2).Preparation of LaH2.8O0.1 Thin FilmsLaH2.8O0.1 thin films were deposited at room temperature by radiofrequency (RF) magnetron sputtering (Kenix Co., Ltd.) with 2-in. Lametal targets (99.9%, Rare Metallic Co., Ltd.). The chamber pressurewas pumped down to 5 × 10−5 Pa prior to deposition. Deposition wasperformed in flowing Ar (99.9999%) with 12.5 vol % H2(99.99999%). The deposition pressure was maintained at 0.6 Pa.The as-deposited films were hydrogen-deficient and, hence, metallic.Immediately after deposition and without air exposure, the films wereexposed to pure H2 gas at room temperature to dose hydrogen andachieve an electronically insulating state (Figure S3). The apparentstoichiometric composition of the films was estimated by comparingthe optical band gap determined from the transmission measurementswith that of powder samples synthesized by a solid-state reaction, thedetails of which are described elsewhere.38 Although oxygen was notintentionally introduced during deposition, the resulting filmsexhibited oxygen incorporation, likely owing to the presence ofresidual oxygen- and/or water-related species in the sputteringchamber.Electrical MeasurementsPrior to the electrical measurements of the sintered LaH2.8O0.1samples (6 mm in diameter and approximately 1 mm in thickness),200 nm Mo or 50 nm Pd electrodes were deposited onto both sides ofthe pellets by RF magnetron sputtering. The LaH2.8O0.1 sample wasplaced on a homemade test jig in an airtight chamber. Alternating-current (AC) impedance spectroscopy was performed from −100 to100 °C under flowing N2 or H2 at ambient pressure, and the bulkLaH2.8O0.1 and electrode interfacial resistances were extracted. Spectrawere recorded from 10 MHz to 1 mHz with an oscillation amplitudeof 100 mV using impedance analyzers (Solartron SI 1260 orNovocontrol α-A equipped with a ZG2 interface). For theLaH2.8O0.1 thin films, a capacitor-like cell, Pd/LaH2.8O0.1/Mo, wasfabricated on a Si wafer. The LaH2.8O0.1 layer thickness was 500 nm. A50 nm Pd top electrode (5 mm in diameter) and 100 nm Mo bottomelectrode were also deposited by RF magnetron sputtering. Sweepvoltammetry was performed by using a potentiostat−galvanostat(Solartron SI 1287A) at 25 °C under a H2 atmosphere with a scanrate of 1 mV s−1.Electromotive Force (EMF) Measurements of a HydrogenConcentration CellEMF measurements were performed using a symmetric Pd/LaH2.8O0.1/Pd cell over 25−100 °C. The pellet was mounted onthe tube-end face, and its periphery was sealed with heat-resistantepoxy to separate the atmospheres on the two faces. A small hydrogenpartial-pressure difference was applied, and the EMF was recordedwith a potentiostat (Solartron SI 1287A).Ultraviolet Photoelectron Spectroscopy (UPS)UPS measurements were conducted in a vacuum chamber with a basepressure below 5 × 10−8 Pa by irradiation with He I emission lines(21.22 eV). The emitted photoelectrons were detected using ahemispherical analyzer (Scienta Omicron DA30-L) with a 10 eV passenergy. Because UPS is highly sensitive to surface contamination, theLaH3−2xOx pellets were fractured in a preparation chamber (∼7 ×10−8 Pa) immediately before measurement. Secondary-electron cutoffspectra were acquired under a negative sample bias to determine theposition of the vacuum level Evac.Photoelectron Yield Spectroscopy (PYS)PYS was carried out with monochromated photons (2.0−4.8 eV)from a Xe lamp (Asahi Spectra MAX-303). The photon flux wascalibrated with an optical power detector (Newport 818-UV). A −50V bias was applied to the sample to extract photoelectrons, and theresulting current to ground was recorded with a pico-ammeter(Keithley 6482).Inverse Photoelectron Spectroscopy (IPES)IPES measurements were conducted in bremsstrahlung isochromatspectroscopy (BIS) mode, using an IPES system (PSP VacuumTechnology, Ltd.). Immediately before each measurement, a sinteredpellet was scraped inside the vacuum chamber to achieve a cleansurface. The kinetic energy of incident electrons was scanned from 5to 20 eV, and emitted photons were detected with a bandpassdetector comprising a fluoride window and a NaCl-coated Ta cone.The energy scale was calibrated by measuring the Fermi edge of Aufoil, thereby determining the instrumental Fermi level (EF).Density Functional Theory (DFT) CalculationsPeriodic DFT calculations were performed as implemented in theVASP code.39,40 The exchange and correlation energies werecalculated by generalized gradient approximation using Perdew−Burke−Ernzerhof parametrization.41 Valence electrons were expandedJournal of the American Chemical Society pubs.acs.org/JACS Articlehttps://doi.org/10.1021/jacs.6c01849J. Am. Chem. Soc. 2026, 148, 11393−1140211394https://pubs.acs.org/doi/suppl/10.1021/jacs.6c01849/suppl_file/ja6c01849_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/jacs.6c01849/suppl_file/ja6c01849_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/jacs.6c01849/suppl_file/ja6c01849_si_001.pdfpubs.acs.org/JACS?ref=pdfhttps://doi.org/10.1021/jacs.6c01849?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asin terms of a plane wave basis set, with the core electrons treatedusing the projected augmented wave approach.42 The plane wavecutoff energy was set to 520 eV for all systems, and the Brillouin zonewas sampled with a k-point grid with a spacing of 0.03 × 2π Å−1. Theconvergence threshold was set to 1.0 × 10−8 eV for electronicrelaxation and 2.0 × 10−5 eV Å−1 for force relaxation. A 2 × 2 × 2supercell of LaH2.5O0.25 with R3̅m symmetry43 (containing 120atoms) was constructed to evaluate the defect behavior. The pydefectcode44 was used to handle and analyze the defect calculations.■ RESULTS AND DISCUSSIONFigure 1a,b show Nyquist plots of the AC impedance for asintered LaH2.8O0.1 pellet measured using Mo electrodesbetween −100 and 100 °C under a N2 atmosphere. At lowertemperatures, a nearly ideal semicircle appears in the high-frequency region, which is attributed to bulk hydride-iontransport in LaH2.8O0.1. This indicates that the contributionfrom grain-boundary conduction is negligibly small, which isconsistent with the high relative density of the sintered sampleand the smooth fracture surface observed in Figure S2. Thelinear impedance response observed in the low-frequencyregion reflects negligible electronic conduction in LaH2.8O0.1and strongly hindered hydrogen transport at the Mo/LaH2.8O0.1 interface, giving rise to a diffusion-limited responsewith pronounced resistive and capacitive components. Theseresults confirm the ion-blocking nature of the Mo electrode. Asthe temperature increases, the semicircle in the high-frequencyregion gradually disappears, leaving only a resistive component.This behavior indicates that ionic transport in the bulkbecomes fast with increasing temperature so that its capacitivecontribution can no longer be resolved within the measuredfrequency range. Ionic conductivity was estimated from thebulk response, which was fitted with an equivalent circuitconsisting of a resistor (R) and constant-phase elements(CPEs) connected in parallel for lower temperatures or with asingle resistor for higher temperatures. The electronicconductivity of LaH2.8O0.1 was subsequently determined fromthe steady-state current under an applied direct-current (DC)bias in this ion-blocking cell (Figure S4). The obtained ionicand electronic conductivities agree well with previouslyreported values for LaH2.8O0.1 (Figure S5).35 The electronicconductivity is several orders of magnitude lower than theionic conductivity, thereby confirming that LaH2.8O0.1 behavesas an ionic conductor with an ionic transport number tion of 1.Figure 1c shows the impedance spectra of a sinteredLaH2.8O0.1 pellet measured at 100 °C under a H2 atmosphere.When Pd electrodes, which are catalytically active for hydrogenand show hydrogen permeability,45,46 are used, the impedancespectra (red circles) exhibit an inductive feature above 1 MHz,followed by two semicircles that converge toward the real axisat low frequencies. These semicircles are likely associated withFigure 1. Nyquist plots of a sintered LaH2.8O0.1 pellet. (a) Nyquist plot measured with Mo electrodes at −100 °C under a N2 atmosphere. Thegreen line represents the fitting result obtained using an equivalent circuit consisting of a resistor, R, and constant phase elements, CPEs, connectedin parallel along with an additional CPE accounting for ion-blocking polarization. (b) Temperature dependence of the Nyquist plots measured withMo electrodes under a N2 atmosphere. The purple, blue, green, and red open circles correspond to temperatures of 0, 25, 50, and 100 °C,respectively. (c) Nyquist plots measured at 100 °C under a H2 atmosphere. The blue and red open circles represent data obtained with Mo (ion-blocking) and Pd (ion-reversible) electrodes, respectively.Figure 2. Measurement of the ECW of a LaH2.8O0.1 thin film. (a) Schematic illustration of an asymmetric Pd/LaH2.8O0.1/Mo electrochemical cellfabricated on a Si substrate. A DC bias was applied to the cell under 1 atm H2. The Pd electrode was taken to be in equilibrium with hydrogen andwas used as a nonpolarizable reference electrode. (b, c) I−V curves obtained by linear sweep voltammetry of the cell shown in (a) at a scan rate of 1mV s−1 plotted on a (b) logarithmic and (c) linear current scale. A cathodic polarization was applied to the Mo electrode with respect to the Pdelectrode by sweeping the potential in the range 0 V → −1 V → 0 V vs Pd, extracting hydrogen from the thin film. The first and second cycles areshown in sky blue and blue, respectively. An anodic polarization was applied by sweeping the potential in the range 0 V → 0.2 V → 0 V vs Pd,injecting hydrogen into the thin film. The first and second cycles are shown in pink and red, respectively. The I−V curves measured underpotentials above 0.2 V are shown in gray. After a current kink at approximately 0.2 V, the film becomes irreversibly low-resistive, which is attributedto H2 evolution at the bottom Mo electrode and the destruction of the capacitor-like structure.Journal of the American Chemical Society pubs.acs.org/JACS Articlehttps://doi.org/10.1021/jacs.6c01849J. Am. Chem. Soc. 2026, 148, 11393−1140211395https://pubs.acs.org/doi/suppl/10.1021/jacs.6c01849/suppl_file/ja6c01849_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/jacs.6c01849/suppl_file/ja6c01849_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/jacs.6c01849/suppl_file/ja6c01849_si_001.pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig2&ref=pdfpubs.acs.org/JACS?ref=pdfhttps://doi.org/10.1021/jacs.6c01849?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ascharge-transfer reactions at the Pd interface and changes in thesurface coverage of adsorbed hydrogen species.47 Replacing Pdwith Mo on the same specimen leaves the high-frequencyresponse essentially unchanged; by contrast, in the low-frequency region, the impedance measured with Mo electrodesshows a linear response characteristic of ion-blocking (bluecircles). These results indicate that Pd electrodes do notinduce additional electronic conduction in LaH2.8O0.1, butenable ion transport across the Pd/LaH2.8O0.1 interfaces andact as ion-reversible electrodes.Figure 2a shows a schematic illustration of an asymmetricPd/LaH2.8O0.1/Mo cell fabricated on a Si substrate. Theasymmetric cell was polarized under a H2 atmosphere toinvestigate the ECW of LaH2.8O0.1. Because hydrogen can bereversibly exchanged between the gas phase and the Pdelectrode, the hydrogen chemical potential μH° at the Pd/LaH2.8O0.1 interface is fixed. Under a zero net H− current(Hebb−Wagner condition),48 the applied bias determines thehydrogen chemical potential μH at the Mo side, as described ineq 1.= ° + ° = ° +RTp p FE2ln( / )H H H H H2 2 (1)where R, T, pHd2, pHd2° , F, and E denote the gas constant, absolutetemperature, hydrogen partial pressure at the Mo/LaH2.8O0.1interface, hydrogen partial pressure at the Pd/LaH2.8O0.1interface (1 atm), Faraday constant, and electric potentialapplied to Mo relative to Pd, respectively. Figure 2b,c show thecurrent−voltage (I−V) curves obtained from the linear sweepvoltammograms of the asymmetric cell measured at 25 °C.Upon the cathodic polarization of the Mo side (i.e., decreasingμH at the Mo/LaH2.8O0.1 interface), the current rises sharply ataround −0.4 V. This onset is attributed to hydrogen releasefrom LaH3−2xOx through the Pd electrode, accompanied by theformation of hydrogen vacancies and electrons at the Mo side,according to eq 2.+ +× •H12H V eH 2 H (2)where the defects are denoted according to the Kröger−Vinknotation.49 Since the hydrogen vacancies VH× act as shallowdonors, an increase in their density leads to spatial overlap ofthe associated electronic states, resulting in the formation of adefect band near the conduction band minimum (CBM).38,50This steep increase in current indicates that electronicconduction prevails over ionic conduction in LaH2.8O0.1,which is defined as the reduction limit of the ECW ofLaH2.8O0.1 (blue curve in Figure 2c). When the electricpotential is swept back to values more positive than −0.4 V,the current retraces nearly the same curve as in the forwardsweep, and LaH2.8O0.1 returns to a high-resistance state.Because hydrogen is incorporated into LaH2.8O0.1 through thetop Pd electrode, this behavior is attributed to the refilling ofhydrogen vacancies and the accompanying decrease in electroncarrier density.Conversely, under anodic polarization (i.e., increasing μH atthe Mo/LaH2.8O0.1 interface), the current increases from ∼0 V,again indicating the appearance of electronic conduction. Thisbehavior defines the oxidation limit of the ECW of LaH2.8O0.1(Figure 2c). When H− ions are driven toward the bottom Moelectrode by the applied field, H2 evolution at the Mo electrodemay generate the resulting electronic current. Once the appliedvoltage exceeds approximately 0.2 V, the current begins to risesteeply, and, in the reverse sweep, it no longer retraces theforward curve, indicating the occurrence of an irreversiblechange in the cell (gray curve in Figure 2b). This trend isascribed to an electrical short between the top and bottomelectrodes caused by the destruction of the capacitor-likestructure owing to H2 evolution at the Mo electrode, as shownin Figure S6. Thus, the overpotential for the H2 evolutionreaction at the Mo electrode is estimated to be around 0.2 V.Below this overpotential, where H2 evolution does not occur,the current in the reverse sweep retraces the forward I−Vcurve, and this behavior remains unchanged even afterrepeated cycling (red curve). Thus, the increase in currentFigure 3. Energy band alignment for LaH3−2xOx. (a) UPS and IPES spectra (solid lines) and PYS data (dots) for LaH3−2xOx. All energy levels arereferenced to the vacuum level (Evac.). The spectra for x = 0, 0.1, and 0.2 are shown in red, green, and blue, respectively. The CBM and VBM arerepresented by dotted and chain lines, respectively, and EF is marked with a black arrow. (b) Oxygen-content dependence of the CBM and VBMenergies of LaH3−2xOx. The CBM and VBM are represented by blue and red triangles, respectively, and the corresponding EF values are shown asblack solid lines. Schematic illustrations of the arrangements of La and H atoms primarily comprising each band edge, viewed along the [100]direction, are also given.Journal of the American Chemical Society pubs.acs.org/JACS Articlehttps://doi.org/10.1021/jacs.6c01849J. Am. Chem. Soc. 2026, 148, 11393−1140211396https://pubs.acs.org/doi/suppl/10.1021/jacs.6c01849/suppl_file/ja6c01849_si_001.pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig3&ref=pdfpubs.acs.org/JACS?ref=pdfhttps://doi.org/10.1021/jacs.6c01849?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asobserved in this voltage range is attributed to the resistivechange of LaH2.8O0.1 associated with the onset of electronicconduction. The appearance of electronic conduction whenhydrogen is injected into the film under anodic polarizationcan be explained by two possible scenarios: (i) injectedinterstitial hydrogen is ionized into H−, releasing holes+× •h12H H Hi i2 (3)or (ii) interstitial hydrogen is protonated and acts as anelectron donor+× •12H H H ei i2 (4)We investigated the origin of electronic conduction inLaH3−2xOx by hydrogen injection based on the band alignmentdetermined from the photoemission experiments and thedefect energy levels evaluated by DFT calculations. Figure 3apresents the combined UPS and IPES spectra for LaH3−2xOx(x = 0, 0.1, and 0.2), showing the energies of the valence bandmaximum (VBM), CBM, and EF referenced to the Evac. and theoptical band gap (Eg). The EF were determined from thesecondary-electron cutoff in the He I (21.22 eV) UPS spectra.As shown in previous work, the VBM energy of LaH3 cannotbe reliably determined from the photoemission spectraobtained by irradiation with vacuum ultraviolet (VUV) raysbecause the VBM is mainly composed of H 1s states with avery small density of states, and the H-1s photoionization crosssection in the VUV range is small, resulting in an extremelyweak photoemission intensity near the VBM.51 Accordingly,the VBM energies for the oxyhydrides were determined fromthe onset of the total PYS profiles obtained by excitation withphotons in the visible-to-ultraviolet light range (2.0−4.8 eV),in which the photoionization cross section of H 1s states isrelatively large.52 The resulting PYS are plotted as dots inFigure 3a. The CBM energies were determined by adding theEg reported in an earlier study38 to the VBM energiesdetermined from PYS. The resulting CBMs are in goodagreement with the onset of the IPES spectra.Figure 3b shows the oxygen-content dependence of theCBM and VBM energies of LaH3−2xOx (x = 0, 0.1, and 0.2). Asthe oxygen content increases, the band gap widens, primarilybecause the La-5d-derived CBM shifts upward, whereas theVBM slightly shifts downward. Consequently, the EF, whichlies at nearly the same energy relative to the Evac., movestoward the middle of the gap. This trend is consistent with theoxygen-content dependence of the observed electricalcharacteristics: LaH3 behaves as an n-type electronicconductor,53 while in the oxyhydrides, electronic conductionis suppressed and ion conduction becomes predominant.34,35The upward shift of the CBM with increasing oxygen contentreflects the formation of La−O ionic bonds. This is supportedby the core-level X-ray photoemission spectra for La 3d, whichexhibit a downward shift relative to the Evac. with increasingoxygen content (Figure S7). On the other hand, theinsensitivity of VBM energy to oxygen content suggests thatthe bonding character of La−H, and thus the nature of hydrideions, is not substantially altered by oxygen doping. Oxygen-undoped LaH3 adopts a structure in which hydrogen denselyoccupies both the tetrahedral and octahedral interstitial sites ofthe fcc La lattice. As a result, hydride ions in LaH3 exhibitappreciable H−H interactions, and the VBM is dominated bythe antibonding states between neighboring hydride ions,which significantly elevate the VBM relative to the energy ofisolated hydride ions. Consequently, the H 1s-derived VBMlies close to the EF, indicating that hydride ions in LaH3 areFigure 4. Calculated defect and equilibrium Fermi levels in LaH2.5O0.25. (a) Plots of the formation enthalpies (ΔH) of point defects in LaH2.5O0.25as a function of the EF position relative to the energy of VBM (EV), calculated for (a) pHd2= 1 atm. (b) Hypothetical plot for pHd2≫ 1 atm at 300 K.The formation enthalpies calculated at pHd2= 1 atm are shown as faint lines for comparison. Hi, VH(Oc.), and OH(T) denote an interstitial hydrogenoccupying a vacant octahedral site (Oc.) created for charge compensation upon O doping, a hydrogen vacancy at an Oc. site, and an oxygen at atetrahedral hydrogen site (T), respectively. The ε (±) represents the charge-neutrality level (CNL) of Hi. Relaxed structure upon the formation of(c) Hi′ and (d) Hi• in LaH2.5O0.25. La, O(T), H(T), and Hi are shown as green, red, pink, and black spheres, respectively. A dihydrogen defect isformed between Hi• and a neighboring H(T), and its side-on interaction with an adjacent La atom is indicated by red dashed lines. (e) Schematicillustration of the bonding/antibonding interactions between Hi× and a surrounding H−, accounting for the upward shift of the CNL.Journal of the American Chemical Society pubs.acs.org/JACS Articlehttps://doi.org/10.1021/jacs.6c01849J. Am. Chem. Soc. 2026, 148, 11393−1140211397https://pubs.acs.org/doi/suppl/10.1021/jacs.6c01849/suppl_file/ja6c01849_si_001.pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig4&ref=pdfpubs.acs.org/JACS?ref=pdfhttps://doi.org/10.1021/jacs.6c01849?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asbetter described as being close to a neutral hydrogen thanthose in ionic alkali and alkaline-earth metal hydrides.51,54These characteristic features of hydride ions are largelyretained in the oxygen-doped oxyhydrides.Figure 4a plots the calculated defect formation energies ofpoint defects against the EF in LaH2.5O0.5 under pHd2= 1 atm at300 K. As demonstrated in Figures 2b, S8, and S9, LaH3−2xOxcan switch between ionic and electronic conductive states bythe electrochemical insertion and extraction of H− andvariations in pHd2or μH in the gas phase. In the temperaturerange of our experiments (≤100 °C), only hydrogen speciesexhibit appreciable mobility, whereas La and O are essentiallyimmobile.35 Accordingly, the equilibrium EF was estimated byimposing charge neutrality on the defects whose formationdoes not involve La or O diffusion. Under pHd2= 1 atm or μH =μH° , the equilibrium EF is determined by charge compensationbetween positively charged VH(Oc.)• and negatively chargedOH(T)′ and Hi′. Here, the interstitial site is an octahedral anionsite that is left vacant upon oxygen substitution to maintaincharge neutrality. The resultant EF lies approximately 0.6 eVbelow the CBM, which is in good agreement with theexperimentally observed energy separation between the EF andCBM in LaH2.6O0.2 shown in Figure 3.Under pHd2≫ 1 atm or μH ≫ μH° , VH(Oc.) formation becomesless favorable, whereas Hi formation becomes more favorable.Consequently, the EF, which is determined by the chargeneutrality between these defects, shifts to a lower level. Thistrend is confirmed by calculations at pHd2= 100 atm, as shownin Figure S10. Thus, upon anodic polarization, as shown inFigure 2b, provided that polarization is not limited by the H2evolution reaction, the EF can be lowered to the chargeneutrality level (CNL), ε (±), of Hi22,26 and does not decreasefurther because it is pinned at the CNL by the redoxequilibrium between negatively charged Hi′ and positivelycharged Hi•. For this reason, EF never drops into the valenceband, and the hole-doping reaction described by eq 3, in whichHi× acts as an acceptor, does not occur. Even when a morepositive potential is applied beyond this CNL to further lowerthe EF, Hi is oxidized according to eq 4, generating electrons.The released electrons populate an impurity band in the gap,leading to electronic conduction, as observed upon anodicpolarization of the oxyhydride (Figure 2b). The impurity bandformed during this process may be analogous to that generatedby the photoinduced protonation of H− in rare-earthoxyhydride thin films, a process proposed as the origin oftheir photochromic behavior.55,56 Electronic conduction isobserved upon only a slight anodic polarization with respect tothe Pd electrode equilibrated with 1 atm of H2. This findingsuggests that the experimental EF in equilibrium with thehydrogen atmosphere is located very close to the CNL of Hi,which represents the oxidation limit of H−.The relaxed structures of LaH2.5O0.5 containing Hi′ and Hi•are presented in Figure 4c,d, respectively. Hi• is electrostati-cally attracted to hydrogen at the nearest tetrahedral site, and adihydrogen defect, which can be denoted as H2i• in Kröger−Vink notation, is formed. The resulting H−H distance of 0.84Å is significantly longer than that in a H2 molecule (0.74 Å).57Such a dihydrogen defect may be regarded as analogous to thatin transition-metal dihydrogen complexes in organometallicchemistry.58 In these complexes, an H2 molecule can bind side-on to the metal center by donating its two σ electrons to aFigure 5. EMF measurements of a hydrogen concentration cell. (a) Schematic illustration of the hydrogen concentration cell setup for LaH2.8O0.1.(b) Time evolution of the EMF at 25 °C when pHd2′ is fixed at 1 atm while pHd2″ is varied. The dashed lines in red, orange, yellow, green, and blue serveas eye guides for pHd2″ = 1.0, 0.88, 0.75, 0.63, and 0.50 atm, respectively. (c) EMF at 25 °C plotted as a function of p pln( / )RTF2 H H2 2. The red trianglesdenote the EMF values obtained while pHd2″ is decreased, whereas the blue triangles denote those obtained while pHd2″ is increased. The correspondinglinear fit, shown as a green line, and slope are also presented. The black dashed line represents a slope of 1. (d) Temperature dependence of theapparent transport numbers of H− (red) and H+ (blue) determined from the EMF measurements, together with the ionic transport number fromthe DC polarization of a Mo ion-blocking cell (black).Journal of the American Chemical Society pubs.acs.org/JACS Articlehttps://doi.org/10.1021/jacs.6c01849J. Am. Chem. Soc. 2026, 148, 11393−1140211398https://pubs.acs.org/doi/suppl/10.1021/jacs.6c01849/suppl_file/ja6c01849_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/jacs.6c01849/suppl_file/ja6c01849_si_001.pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig5&ref=pdfpubs.acs.org/JACS?ref=pdfhttps://doi.org/10.1021/jacs.6c01849?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asvacant metal d orbital. The elongated H−H bond observed inLaH2.5O0.25 is attributable to bonding interactions with theneighboring La cations, as indicated by the red dashed line inFigure 4d. A similar configuration has been reported forcrystalline yttrium oxyhydrides, YH3−2xOx, in relation to theirphotochromic mechanism, in which such dihydrogen defectshave been proposed to act as electron donors.56,59Our static DFT calculations indicate that the formationenergy of Hi• is rather larger than that of VH(Oc.)•, implying thatHi• is unlikely to contribute to determining the EF positionunder pHd2= 1 atm at 300 K. This assessment, however, doesnot fully reflect the actual hydrogen environment inLaH3−2xOx. As described above, in LaH3−2xOx, most of thetetrahedral and octahedral interstitial sites of the fcc Lasublattice are occupied by hydrogen, resulting in a denselypacked hydrogen sublattice. In such an environment, Hi canreadily engage in bonding−antibonding interactions with thesurrounding H−, giving rise to a shift in the CNL to higherenergies, thereby making Hi′ more susceptible to oxidation andlowering the formation energy of Hi• (Figure 4e). This isreasonable in light of the fact that hydrogen exhibits highdiffusivity in LaH3−2xOx even at room temperature. Under suchconditions, the positive charge need not remain staticallylocalized within a single H−H pair but can be shared by alarger number of surrounding mobile H−. Such collectiveinteractions within the hydrogen network stabilize Hi•compared with the static-defect picture. As a consequence,LaH3−2xOx is expected to exhibit a characteristic high CNL,which narrows the oxidizing limit of the ECW. Accordingly, atequilibrium under pHd2= 1 atm, EF is located just above theelevated CNL, so that a slight anodic polarization, correspond-ing to a small downward shift of EF, would be sufficient toreach the CNL.The EMF of a hydrogen concentration cell was measuredunder conditions near 1 atm of H2 using the experimentalsetup illustrated in Figure 5a. A symmetric Pd/LaH2.8O0.1/Pdcell was bonded to a Pyrex tube with epoxy adhesive, whichserved as a gastight seal separating the two electrodes. Thecross-sectional scanning electron microscopy image ofLaH2.8O0.1 in Figure S2 reveals a dense microstructure withoutobservable voids, indicating that mechanical gas leakagethrough the pellet is negligible when a hydrogen partial-pressure gradient is applied across the sample.Figure 5b shows the EMF relaxation behavior recorded at 25°C following stepwise changes in the hydrogen partial pressure,pHd2″ , at one electrode, while the hydrogen partial pressure at theother electrode, pHd2′ , was fixed at 1 atm. Owing to the bipolarityof hydrogen, two types of electrode reactions can beconsidered= ++12H H e2 (5)and+ =12H e H2 (6)The corresponding theoretical magnitudes of the EMF forthese reactions are given by eqs 7 and 8, respectively.=+ +tRTFp pEMF2ln( / )H H H H2 2 (7)= tRTFp pEMF2ln( / )H H H H2 2 (8)where tH+ and tH− are the transport numbers of H+ and H−,respectively. Thus, the negative EMF on the lower pHd2″ sideclearly indicates that eq 6 is dominant for Pd/LaH2.8O0.1. TheEMF exhibits fast relaxation and attains a steady state within∼10 min, indicating that the electrode reactions have reachedequilibrium. This is attributed to rapid H2 dissociation andassociation at Pd electrodes.As shown in Figure 5c, the measured EMF is proportional top pln( / )RTF2 H H2 2, but its magnitude is slightly smaller than thevalue for tH− = 1, indicating that tH− is somewhat less than 1.However, as shown in Figure S5, under these conditions, thetransport number of electronic carriers tel is very small (<10−4),and tion is essentially 1. Thus, the reduced EMF cannot beascribed to electronic leakage.Considering that the EMF is measured under conditions inwhich Hi can be partially protonated, the EMF may beexpected to include contributions from not only the reactiondescribed by eq 8 but also that described by eq 7. The EMFdetermined by the competition between these reactions isgiven by eq 9= +RTFt t p pEMF2( )ln( / )H H H H2 2 (9)The slope of the experimental EMF in Figure 5c correspondsto the value of tH− − tH+. The apparent individual values of tH−and tH+ can be determined using tion = tH− + tH+ = 1, assummarized in Figure 5d. We emphasize that the EMF dataonly quantify the net number of charges transported perhydrogen transported and, therefore, do not identify themicroscopic mechanism of the mixed conduction of H− andH+. Nevertheless, the observed EMF behavior suggests that, asdiscussed above, the EF approaches the CNL of the Hi under aH2 atmosphere.Here, we take an overview of the amphoteric behavior ofhydrogen across H+ and H− conductors from the perspectiveof an electronic energy scale. Figure 6 summarizes the energyband alignments of LaH3−2xOx (left) and a typical perovskite-type proton-conducting oxide electrolyte, that is, an acceptor-doped BaZrO3 (Acc-doped BZO; right).60 The ECW ofLaH3−2xOx is also shown, and all energy levels are referencedFigure 6. Energy band alignments of LaH2.8O0.1 and a proton-conducting acceptor-doped BaZrO3 (Acc-doped BZO), together withthe electrochemical window (ECW). The band positions and ECW ofLaH2.8O0.1 were determined experimentally in this work. The bandalignment of Acc-doped BZO was constructed based on valuesreported in previous studies,60 with the EF drawn schematically at anarbitrary level below the midgap under oxidizing conditions. Uponreduction of Acc-doped BZO, the EF shifts toward the CNL of Hi,leading to the conversion of H+ into H−.Journal of the American Chemical Society pubs.acs.org/JACS Articlehttps://doi.org/10.1021/jacs.6c01849J. Am. Chem. Soc. 2026, 148, 11393−1140211399https://pubs.acs.org/doi/suppl/10.1021/jacs.6c01849/suppl_file/ja6c01849_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/jacs.6c01849/suppl_file/ja6c01849_si_001.pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?fig=fig6&ref=pdfpubs.acs.org/JACS?ref=pdfhttps://doi.org/10.1021/jacs.6c01849?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asto the Evac.. At the reduction limit of LaH3−2xOx (i.e., low pHd2),VH• formation gives rise to a defect band just below theconduction band. At the oxidation limit (i.e., high pHd2), Hioxidation leads to electron doping. Thus, the CNL of the Hicorresponds to this oxidation limit. The EF of LaH3−2xOx inequilibrium with 1 atm of H2 lies close to this oxidation limit.As revealed by the EMF measurements, hydrogen transportunder these conditions involves not only H− conduction butalso a partial contribution from H+ conduction. However,because the EF remains within the ECW, LaH3−2xOx does notexhibit electronic conduction.The VBM of Acc-doped BZO, which is mainly composed ofO 2p states, lies nearly 2 eV lower in energy than the H 1s-derived VBM of LaH3−2xOx. In Acc-doped BZO, oxygenvacancies (VO••) form to maintain charge neutrality uponacceptor doping. Protons are introduced via the hydrationreaction of VO•• and conduct as ionic charge carriers.9 As theEF of Acc-doped BZO is located closer to the VBM than to themiddle of the band gap under oxidizing conditions,61 this EF ispositioned at a significantly lower energy than that ofLaH3−2xOx, illustrating that LaH3−2xOx has much strongerreducing power than proton-conducting oxides such as Acc-BZO.19Under strongly reducing conditions, perovskite-type oxidescan accommodate H−, leading to mixed H− and electronicconduction.6,18,32 This may be understood as the EF crossingthe CNL of Hi from the oxidizing to the reducing side, therebychanging from H+ to H−. Thus, the CNL of Hi defines thereducing limit of Acc-doped BZO. To the best of ourknowledge, however, the reduction behavior of these oxideshas not yet been examined in terms of electronic energy on anabsolute scale referenced to the Evac.. Nevertheless, theoreticalcalculations indicate that, among various oxides, the CNL of Hiis typically located at around −4 eV relative to the Evac..24−26This provides a clear contrast to LaH3−2xOx, in which the CNLis located at an exceptionally high energy owing to strong H−H interactions.Taken together, it can be seen that, among materials thataccommodate what are referred to as “hydride ions”, theposition of the EF can vary widely. Although “hydride ions” arecommonly regarded as strong reducing species based on anelectrode potential of E(H2/H−) = −2.25 V versus thestandard hydrogen electrode in aqueous systems,62 the factthat their redox potential in solids, E(H+/H−)�correspondingto the CNL position�is strongly dependent on the hostmaterial has largely been overlooked. Describing hydrogen insolids in terms of an electronic energy scale is expected toprovide valuable insight into its chemical reactivity and charge-transport properties. Future studies that systematicallyorganize band alignments across a wide range of hydrogen-ion conductors will help achieve a comprehensive under-standing of the characteristics of hydrogen in solids and willprovide a rational basis for designing new electrochemicaldevices that exploit the amphoteric nature of hydrogen.■ CONCLUSIONIn this work, we investigated the ECW of LaH3−2xOx andpositioned it on an absolute electronic energy scale referencedto the Evac.. The ECW of the LaH2.8O0.1 thin film determinedby Hebb−Wagner polarization ranged from approximately−0.4 to 0 V versus Pd in equilibrium with a 1 atm H2atmosphere. From the photoemission experiments, wedetermined the positions of the VBM and CBM of LaH3−2xOx(x = 0, 0.1, and 0.2) from the Evac.; the VBM lies atapproximately −4 eV, while the CBM is located at −2.5 to −2eV, depending on the oxygen content of the oxyhydride. Wefound that the EF under 1 atm of H2 is determined by thecharge neutrality condition among Hi′, VH•, and OH′, and liesabove the midgap, close to the CBM. Based on this EFposition, we examined the redox reactions that determine thereducing and oxidizing limits of the ECW of LaH3−2xOx. Thereducing limit of the ECW is governed by the hydrogenextraction reaction from LaH3−2xOx, which creates VH• andthereby forms a defect band near the CBM, giving rise toelectronic conduction. On the other hand, the oxidizing limitof the ECW is not governed by H2 evolution from theoxidation of H− but by a reaction in which interstitial H− ionscapture positive charges to form H+, thereby giving rise toelectronic conduction. This implies that the EF approaches theCNL of Hi. H+ can be stabilized in the H− lattice throughbonding−antibonding interactions with the surrounding H−ions, enabled by the short H−H separations in LaH3−2xOx.When the EMF of a hydrogen concentration cell is measurednear pHd2= 1 atm, which is close to the onset of Hi oxidation,both H− conduction and a partial contribution from H+conduction are observed. Such behavior reflects the ampho-teric nature of hydrogen. Capturing this amphoteric nature ofhydrogen on an electronic energy scale provides a unifiedperspective on the chemical reactivity and charge-transportproperties across different classes of hydrogen-ion conductors.Our findings can contribute to the rational design andunderstanding of new materials and devices that effectivelyexploit the amphoteric character of hydrogen.■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/jacs.6c01849.Details of structural and compositional analyses forprepared LaH3−2xOx, UPS spectra of LaH2.8O0.1 thinfilms, mechanical failure induced by hydrogen evolutionat the Mo/LaH2.8O0.1 interface, ion blocking measure-ments for LaH2.8O0.1, XPS spectra of LaH3−2xOx,electrical properties of LaH2.6O0.2 dependent on thehydrogen chemical potential in the atmosphere, andcalculated defect formation energy in LaH2.5O0.25 (PDF)■ AUTHOR INFORMATIONCorresponding AuthorTomoyuki Yamasaki − Institute of Multidisciplinary Researchfor Advanced Materials (IMRAM), Tohoku University,Sendai 980-8577, Japan; orcid.org/0000-0002-6982-7538; Email: tomoyuki.yamasaki.a1@tohoku.ac.jpAuthorsKeiga Fukui − Graduate Faculty of Interdisciplinary Research,University of Yamanashi, Kofu, Yamanashi 400-8511,Japan; orcid.org/0000-0002-5659-7200Soshi Iimura − National Institute for Materials Science(NIMS), Tsukuba, Ibaraki 305−0047, JapanShunsuke Tsuda − National Institute for Materials Science(NIMS), Tsukuba, Ibaraki 305−0047, JapanJournal of the American Chemical Society pubs.acs.org/JACS Articlehttps://doi.org/10.1021/jacs.6c01849J. Am. Chem. Soc. 2026, 148, 11393−1140211400https://pubs.acs.org/doi/10.1021/jacs.6c01849?goto=supporting-infohttps://pubs.acs.org/doi/suppl/10.1021/jacs.6c01849/suppl_file/ja6c01849_si_001.pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Tomoyuki+Yamasaki"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-6982-7538https://orcid.org/0000-0002-6982-7538mailto:tomoyuki.yamasaki.a1@tohoku.ac.jphttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Keiga+Fukui"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-5659-7200https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Soshi+Iimura"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Shunsuke+Tsuda"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hiroshi+Mizoguchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfpubs.acs.org/JACS?ref=pdfhttps://doi.org/10.1021/jacs.6c01849?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asHiroshi Mizoguchi − National Institute for Materials Science(NIMS), Tsukuba, Ibaraki 305−0047, Japan; orcid.org/0000-0002-0992-7449Takahisa Omata − Institute of Multidisciplinary Research forAdvanced Materials (IMRAM), Tohoku University, Sendai980-8577, Japan; orcid.org/0000-0002-6034-4935Hideo Hosono − National Institute for Materials Science(NIMS), Tsukuba, Ibaraki 305−0047, Japan; InternationalResearch Frontiers initiative MDX Research Center forElement Strategy, Institute of Science Tokyo, Yokohama226−8503, JapanComplete contact information is available at:https://pubs.acs.org/10.1021/jacs.6c01849NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThis work was supported by the MEXT Element StrategyInitiative to Form Core Research Center (Grant NumberJPMXP0112101001). T.Y. was supported in part by JSPSKAKENHI (Grant Numbers 23K19174 and 24K17762). Thiswork was also partly supported by the Material SolutionsCenter (MaSC), Tohoku University, Japan.■ REFERENCES(1) Huiberts, J. N.; Griessen, R.; Rector, J. H.; Wijngaarden, R. J.;Dekker, J. P.; de Groot, D. G.; Koeman, N. J. Yttrium and LanthanumHydride Films with Switchable Optical Properties. Nature 1996, 380(6571), 231−234.(2) Hayashi, K.; Matsuishi, S.; Kamiya, T.; Hirano, M.; Hosono, H.Light-Induced Conversion of an Insulating Refractory Oxide into aPersistent Electronic Conductor. Nature 2002, 419 (6906), 462−465.(3) Mongstad, T.; Platzer-Björkman, C.; Maehlen, J. P.; Mooij, L. P.A.; Pivak, Y.; Dam, B.; Marstein, E. S.; Hauback, B. C.; Karazhanov, S.Zh. A New Thin Film Photochromic Material: Oxygen-ContainingYttrium Hydride. Sol. Energy Mater. Sol. Cells 2011, 95 (12), 3596−3599.(4) Hayward, M. A.; Cussen, E. J.; Claridge, J. B.; Bieringer, M.;Rosseinsky, M. J.; Kiely, C. J.; Blundell, S. J.; Marshall, I. M.; Pratt, F.L. The Hydride Anion in an Extended Transition Metal Oxide Array:LaSrCoO3H0.7. Science 2002, 295 (5561), 1882−1884.(5) Montero, J.; Svedlindh, P.; Österlund, L. Photo-InducedReversible Modification of the Curie−Weiss Temperature inParamagnetic Gadolinium Compounds. Solid State Commun. 2024,378, No. 115419.(6) Kobayashi, Y.; Hernandez, O. J.; Sakaguchi, T.; Yajima, T.;Roisnel, T.; Tsujimoto, Y.; Morita, M.; Noda, Y.; Mogami, Y.; Kitada,A.; Ohkura, M.; Hosokawa, S.; Li, Z.; Hayashi, K.; Kusano, Y.; Kim, J.eun.; Tsuji, N.; Fujiwara, A.; Matsushita, Y.; Yoshimura, K.;Takegoshi, K.; Inoue, M.; Takano, M.; Kageyama, H. An Oxyhydrideof BaTiO3 Exhibiting Hydride Exchange and Electronic Conductivity.Nat. Mater. 2012, 11 (6), 507−511.(7) Bang, J.; Matsuishi, S.; Hiraka, H.; Fujisaki, F.; Otomo, T.; Maki,S.; Yamaura, J.; Kumai, R.; Murakami, Y.; Hosono, H. HydrogenOrdering and New Polymorph of Layered Perovskite Oxyhydrides:Sr2VO4−xHx. J. Am. Chem. Soc. 2014, 136 (20), 7221−7224.(8) Takagi, S.; Orimo, S. Recent Progress in Hydrogen-RichMaterials from the Perspective of Bonding Flexibility of Hydrogen.Scr. Mater. 2015, 109, 1−5.(9) Kreuer, K. D. Proton-Conducting Oxides. Annu. Rev. Mater. Res.2003, 33 (33, 2003), 333−359.(10) Tsujikawa, K.; Hyodo, J.; Fujii, S.; Takahashi, K.; Tomita, Y.;Shi, N.; Murakami, Y.; Kasamatsu, S.; Yamazaki, Y. Mitigating ProtonTrapping in Cubic Perovskite Oxides via ScO6 Octahedral Networks.Nat. Mater. 2025, 24, 1949−1956.(11) Haile, S. M.; Chisholm, C. R. I.; Sasaki, K.; Boysen, D. A.; Uda,T. Solid Acid Proton Conductors: From Laboratory Curiosities toFuel Cell Electrolytes. Faraday Discuss. 2007, 134 (0), 17−39.(12) Fop, S.; Vivani, R.; Masci, S.; Casciola, M.; Donnadio, A.Anhydrous Superprotonic Conductivity in the Zirconium AcidTriphosphate ZrH5(PO4)3. Angew. Chem., Int. Ed. 2023, 62 (18),No. e202218421.(13) Yamaguchi, T.; Tsukuda, S.; Ishiyama, T.; Nishii, J.; Yamashita,T.; Kawazoe, H.; Omata, T. Proton-Conducting Phosphate Glass andIts Melt Exhibiting High Electrical Conductivity at IntermediateTemperatures. J. Mater. Chem. A 2018, 6 (46), 23628−23637.(14) Verbraeken, M. C.; Cheung, C.; Suard, E.; Irvine, J. T. S. HighH− Ionic Conductivity in Barium Hydride. Nat. Mater. 2015, 14 (1),95−100.(15) Papac, M.; Stevanovic,́ V.; Zakutayev, A.; O’Hayre, R. TripleIonic−Electronic Conducting Oxides for next-Generation Electro-chemical Devices. Nat. Mater. 2021, 20 (3), 301−313.(16) Shiraiwa, T.; Yamasaki, T.; Kushimoto, K.; Kano, J.; Omata, T.Enhanced Proton Transport in Nb-Doped Rutile TiO2: A HighlyUseful Class of Proton-Conducting Mixed Ionic Electronic Con-ductors. J. Am. Chem. Soc. 2025, 147 (34), 30757−30767.(17) Kura, C.; Kunisada, Y.; Tsuji, E.; et al. Hydrogen Separation byNanocrystalline Titanium Nitride Membranes with High Hydride IonConductivity. Nat. Energy 2017, 2, 786−794.(18) Toriumi, H.; Kobayashi, G.; Saito, T.; Kamiyama, T.; Sakai, T.;Nomura, T.; Kitano, S.; Habazaki, H.; Aoki, Y. Barium Indate−Zirconate Perovskite Oxyhydride with Enhanced Hydride Ion/Electron Mixed Conductivity. Chem. Mater. 2022, 34 (16), 7389−7401.(19) Ooya, K.; Li, J.; Fukui, K.; Iimura, S.; Nakao, T.; Ogasawara, K.;Sasase, M.; Abe, H.; Niwa, Y.; Kitano, M.; Hosono, H. RutheniumCatalysts Promoted by Lanthanide Oxyhydrides with High Hydride-Ion Mobility for Low-Temperature Ammonia Synthesis. Adv. EnergyMater. 2021, 11 (4), No. 2003723.(20) Clark, D.; Malerød-Fjeld, H.; Budd, M.; Yuste-Tirados, I.;Beeaff, D.; Aamodt, S.; Nguyen, K.; Ansaloni, L.; Peters, T.; Vestre, P.K.; Pappas, D. K.; Valls, M. I.; Remiro-Buenamañana, S.; Norby, T.;Bjørheim, T. S.; Serra, J. M.; Kjølseth, C. Single-Step HydrogenProduction from NH3, CH4, and Biogas in Stacked Proton CeramicReactors. Science 2022, 376 (6591), 390−393.(21) Hirose, T.; Matsui, N.; Itoh, T.; Hinuma, Y.; Ikeda, K.; Gotoh,K.; Jiang, G.; Suzuki, K.; Hirayama, M.; Kanno, R. High-Capacity,Reversible Hydrogen Storage Using H−-Conducting Solid Electro-lytes. Science 2025, 389 (6766), 1252−1255.(22) Van de Walle, C. G.; Neugebauer, J. HYDROGEN INSEMICONDUCTORS. Annu. Rev. Mater. Res. 2006, 36 (1), 179−198.(23) McCluskey, M. D.; Tarun, M. C.; Teklemichael, S. T.Hydrogen in Oxide Semiconductors. J. Mater. Res. 2012, 27 (17),2190−2198.(24) Kılıç, Ç.; Zunger, A. N-Type Doping of Oxides by Hydrogen.Appl. Phys. Lett. 2002, 81 (1), 73−75.(25) Van de Walle, C. G.; Neugebauer, J. Universal Alignment ofHydrogen Levels in Semiconductors, Insulators and Solutions. Nature2003, 423 (6940), 626−628.(26) Kadono, R.; Hosono, H. Ambipolarity of Hydrogen in MatterRevealed by Muons. Adv. Phys. 2023, 72 (4), 409−476.(27) Chaykina, D.; de Krom, T.; Colombi, G.; Schreuders, H.; Suter,A.; Prokscha, T.; Dam, B.; Eijt, S. Structural Properties and AnionDynamics of Yttrium Dihydride and Photochromic Oxyhydride ThinFilms Examined by in Situ μ+ SR. Phys. Rev. B 2021, 103 (22),No. 224106.(28) Bai, C.; Li, Y.; Xiao, G.; Chen, J.; Tan, S.; Shi, P.; Hou, T.; Liu,M.; He, Y.-B.; Kang, F. Understanding the Electrochemical Windowof Solid-State Electrolyte in Full Battery Application. Chem. Rev. 2025,125 (14), 6541−6608.(29) Thompson, T.; Yu, S.; Williams, L.; Schmidt, R. D.; Garcia-Mendez, R.; Wolfenstine, J.; Allen, J. L.; Kioupakis, E.; Siegel, D. J.;Journal of the American Chemical Society pubs.acs.org/JACS Articlehttps://doi.org/10.1021/jacs.6c01849J. Am. Chem. Soc. 2026, 148, 11393−1140211401https://orcid.org/0000-0002-0992-7449https://orcid.org/0000-0002-0992-7449https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takahisa+Omata"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-6034-4935https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hideo+Hosono"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/doi/10.1021/jacs.6c01849?ref=pdfhttps://doi.org/10.1038/380231a0https://doi.org/10.1038/380231a0https://doi.org/10.1038/nature01053https://doi.org/10.1038/nature01053https://doi.org/10.1016/j.solmat.2011.08.018https://doi.org/10.1016/j.solmat.2011.08.018https://doi.org/10.1126/science.1068321https://doi.org/10.1126/science.1068321https://doi.org/10.1016/j.ssc.2023.115419https://doi.org/10.1016/j.ssc.2023.115419https://doi.org/10.1016/j.ssc.2023.115419https://doi.org/10.1038/nmat3302https://doi.org/10.1038/nmat3302https://doi.org/10.1021/ja502277r?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/ja502277r?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/ja502277r?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1016/j.scriptamat.2015.07.024https://doi.org/10.1016/j.scriptamat.2015.07.024https://doi.org/10.1146/annurev.matsci.33.022802.091825https://doi.org/10.1038/s41563-025-02311-whttps://doi.org/10.1038/s41563-025-02311-whttps://doi.org/10.1039/B604311Ahttps://doi.org/10.1039/B604311Ahttps://doi.org/10.1002/anie.202218421https://doi.org/10.1002/anie.202218421https://doi.org/10.1039/C8TA08162Jhttps://doi.org/10.1039/C8TA08162Jhttps://doi.org/10.1039/C8TA08162Jhttps://doi.org/10.1038/nmat4136https://doi.org/10.1038/nmat4136https://doi.org/10.1038/s41563-020-00854-8https://doi.org/10.1038/s41563-020-00854-8https://doi.org/10.1038/s41563-020-00854-8https://doi.org/10.1021/jacs.5c05805?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/jacs.5c05805?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/jacs.5c05805?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1038/s41560-017-0002-2https://doi.org/10.1038/s41560-017-0002-2https://doi.org/10.1038/s41560-017-0002-2https://doi.org/10.1021/acs.chemmater.2c01467?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acs.chemmater.2c01467?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acs.chemmater.2c01467?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1002/aenm.202003723https://doi.org/10.1002/aenm.202003723https://doi.org/10.1002/aenm.202003723https://doi.org/10.1126/science.abj3951https://doi.org/10.1126/science.abj3951https://doi.org/10.1126/science.abj3951https://doi.org/10.1126/science.adw1996https://doi.org/10.1126/science.adw1996https://doi.org/10.1126/science.adw1996https://doi.org/10.1146/annurev.matsci.36.010705.155428https://doi.org/10.1146/annurev.matsci.36.010705.155428https://doi.org/10.1557/jmr.2012.137https://doi.org/10.1063/1.1482783https://doi.org/10.1038/nature01665https://doi.org/10.1038/nature01665https://doi.org/10.1080/00018732.2024.2413342https://doi.org/10.1080/00018732.2024.2413342https://doi.org/10.1103/PhysRevB.103.224106https://doi.org/10.1103/PhysRevB.103.224106https://doi.org/10.1103/PhysRevB.103.224106https://doi.org/10.1021/acs.chemrev.4c01012?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acs.chemrev.4c01012?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-aspubs.acs.org/JACS?ref=pdfhttps://doi.org/10.1021/jacs.6c01849?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asSakamoto, J. Electrochemical Window of the Li-Ion Solid ElectrolyteLi7La3Zr2O12. ACS Energy Lett. 2017, 2 (2), 462−468.(30) Tateyama, Y.; Gao, B.; Jalem, R.; Haruyama, J. TheoreticalPicture of Positive Electrode−Solid Electrolyte Interface in All-Solid-State Battery from Electrochemistry and Semiconductor PhysicsViewpoints. Curr. Opin. Electrochem. 2019, 17, 149−157.(31) Hikima, K.; Shimizu, K.; Kiuchi, H.; Hinuma, Y.; Suzuki, K.;Hirayama, M.; Matsubara, E.; Kanno, R. Operando Analysis ofElectronic Band Structure in an All-Solid-State Thin-Film Battery.Commun. Chem. 2022, 5 (1), No. 52.(32) Takahashi, T.; Toriumi, H.; Kobayashi, G.; Saito, T.; Mori, K.;Jeong, S.; Habazaki, H.; Aoki, Y. Mechanistic Insights into HydrideIncorporation in BaZr1−xInxO3−δ-Based Perovskite Oxyhydrides.Chem. Mater. 2025, 37 (19), 7834−7845.(33) Maeda, R.; Toriumi, H.; Takahashi, T.; Ariga-Miwa, H.; Uruga,T.; Jeong, S.; Aoki, Y. Bipolar Electrolysis Cells with Hydride Ion-Proton Conductor Heterojunctions. Cell Rep. Phys. Sci. 2025, 6 (10),No. 102839.(34) Fukui, K.; Iimura, S.; Tada, T.; Fujitsu, S.; Sasase, M.;Tamatsukuri, H.; Honda, T.; Ikeda, K.; Otomo, T.; Hosono, H.Characteristic Fast H− Ion Conduction in Oxygen-SubstitutedLanthanum Hydride. Nat. Commun. 2019, 10 (1), No. 2578.(35) Fukui, K.; Iimura, S.; Iskandarov, A.; Tada, T.; Hosono, H.Room-Temperature Fast H− Conduction in Oxygen-SubstitutedLanthanum Hydride. J. Am. Chem. Soc. 2022, 144 (4), 1523−1527.(36) Abdellah, A. M.; Ismail, F.; Siig, O. W.; Yang, J.; Andrei, C. M.;DiCecco, L.-A.; Rakhsha, A.; Salem, K. E.; Grandfield, K.; Bassim, N.;Black, R.; Kastlunger, G.; Soleymani, L.; Higgins, D. Impact ofPalladium/Palladium Hydride Conversion on Electrochemical CO2Reduction via in-Situ Transmission Electron Microscopy andDiffraction. Nat. Commun. 2024, 15 (1), No. 938.(37) Guo, J.; Cai, Y.; Gao, W.; Chen, P. Hydrides for DinitrogenConversion. ACS Catal. 2025, 15 (17), 14805−14812.(38) Yamasaki, T.; Takaoka, R.; Iimura, S.; Kim, J.; Hiramatsu, H.;Hosono, H. Characteristic Resistive Switching of Rare-Earth Oxy-hydrides by Hydride Ion Insertion and Extraction. ACS Appl. Mater.Interfaces 2022, 14 (17), 19766−19773.(39) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for AbInitio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys.Rev. B 1996, 54 (16), 11169−11186.(40) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total EnergyCalculations for Metals and Semiconductors Using a Plane-WaveBasis Set. Comput. Mater. Sci. 1996, 6 (1), 15−50.(41) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized GradientApproximation Made Simple. Phys. Rev. Lett. 1996, 77 (18),No. 3865.(42) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to theProjector Augmented-Wave Method. Phys. Rev. B 1999, 59 (3),No. 1758.(43) Colombi, G.; Stigter, R.; Chaykina, D.; Banerjee, S.; Kentgens,A. P. M.; Eijt, S. W. H.; Dam, B.; de Wijs, G. A. Energy, Metastability,and Optical Properties of Anion-Disordered ROxH3−2x (Y, La)Oxyhydrides: A Computational Study. Phys. Rev. B 2022, 105 (5),No. 054208.(44) Kumagai, Y.; Tsunoda, N.; Takahashi, A.; Oba, F. Insights intoOxygen Vacancies from High-Throughput First-Principles Calcula-tions. Phys. Rev. Mater. 2021, 5 (12), No. 123803.(45) Nakatsuji, H.; Hada, M. Interaction of a Hydrogen Moleculewith Palladium. J. Am. Chem. Soc. 1985, 107 (26), 8264−8266.(46) Flanagan, T. B.; Oates, W. A. The Palladium-Hydrogen System.Annu. Rev. Mater. Res. 1991, 21, 269−304.(47) Lin, D.; Lasia, A. Electrochemical Impedance Study of theKinetics of Hydrogen Evolution at a Rough Palladium Electrode inAcidic Solution. J. Electroanal. Chem. 2017, 785, 190−195.(48) Maier, J. Physical Chemistry of Ionic Materials: Ions and Electronsin Solids; John Wiley & Sons, 2004.(49) Kröger, F.; Vink, H. J. Relations between the Concentrations ofImperfections in Solids. J. Phys. Chem. Solids 1958, 5 (3), 208−223.(50) Ng, K. K.; Zhang, F. C.; Anisimov, V. I.; Rice, T. M. Theory forMetal Hydrides with Switchable Optical Properties. Phys. Rev. B 1999,59 (8), No. 5413.(51) Yamasaki, T.; Iimura, S.; Kim, J.; Hosono, H. ExtremelyShallow Valence Band in Lanthanum Trihydride. J. Am. Chem. Soc.2023, 145 (1), 560−566.(52) Yeh, J. J.; Lindau, I. Atomic Subshell Photoionization CrossSections and Asymmetry Parameters: 1 ⩽ Z ⩽ 103. At. Data Nucl.Data Tables 1985, 32 (1), 1−155.(53) Libowitz, G. G. Electronic Properties of the Rare EarthHydrides. Ber. Bunsengesellschaft Phys. Chem. 1972, 76 (8), 837−845.(54) Misemer, D. K.; Harmon, B. N. Self-Consistent ElectronicStructure of Lanthanum Dihydride and Lanthanum Trihydride. Phys.Rev. B 1982, 26 (10), No. 5634.(55) Komatsu, Y.; Shimizu, R.; Sato, R.; Wilde, M.; Nishio, K.;Katase, T.; Matsumura, D.; Saitoh, H.; Miyauchi, M.; Adelman, J. R.;McFadden, R. M. L.; Fujimoto, D.; Ticknor, J. O.; Stachura, M.;McKenzie, I.; Morris, G. D.; MacFarlane, W. A.; Sugiyama, J.;Fukutani, K.; Tsuneyuki, S.; Hitosugi, T. Repeatable PhotoinducedInsulator-to-Metal Transition in Yttrium Oxyhydride Epitaxial ThinFilms. Chem. Mater. 2022, 34 (8), 3616−3623.(56) Banerjee, S.; Chaykina, D.; Stigter, R.; Colombi, G.; Eijt, S. W.H.; Dam, B.; de Wijs, G. A.; Kentgens, A. P. M. Exploring Multi-Anion Chemistry in Yttrium Oxyhydrides: Solid-State NMR Studiesand DFT Calculations. J. Phys. Chem. C 2023, 127 (29), 14303−14316.(57) Huber, K. P.; Herzberg, G. Constants of Diatomic Molecules.In Molecular Spectra and Molecular Structure: IV. Constants of DiatomicMolecules; Huber, K. P.; Herzberg, G., Eds.; Springer US: Boston, MA,1979; pp 8−689 DOI: 10.1007/978-1-4757-0961-2_2.(58) Kubas, G. J. Fundamentals of H2 Binding and Reactivity onTransition Metals Underlying Hydrogenase Function and H2Production and Storage. Chem. Rev. 2007, 107 (10), 4152−4205.(59) Chai, J.; Shao, Z.; Wang, H.; Ming, C.; Oh, W.; Ye, T.; Zhang,Y.; Cao, X.; Jin, P.; Zhang, S.; Sun, Y.-Y. Ultrafast Processes inPhotochromic Material YHxOy Studied by Excited-State DensityFunctional Theory Simulation. Sci. China Mater. 2020, 63 (8), 1579−1587.(60) Saeed, S. W.; Norby, T.; Bjørheim, T. S. Charge-CarrierEnrichment at BaZrO3/SrTiO3 Interfaces. J. Phys. Chem. C 2019, 123(34), 20808−20816.(61) Rowberg, A. J. E.; Li, M.; Ogitsu, T.; Varley, J. B. Polarons andElectrical Leakage in BaZrO3 and BaCeO3. Phys. Rev. Mater. 2023, 7(1), No. 015402.(62) Pourbaix, M.; Franklin, J. A. Atlas of electrochemical equilibria inaqueous solutions, 2nd ed.; National Association of CorrosionEngineers: Houston, TX, 1974.Journal of the American Chemical Society pubs.acs.org/JACS Articlehttps://doi.org/10.1021/jacs.6c01849J. Am. Chem. Soc. 2026, 148, 11393−1140211402https://doi.org/10.1021/acsenergylett.6b00593?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acsenergylett.6b00593?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1016/j.coelec.2019.06.003https://doi.org/10.1016/j.coelec.2019.06.003https://doi.org/10.1016/j.coelec.2019.06.003https://doi.org/10.1016/j.coelec.2019.06.003https://doi.org/10.1038/s42004-022-00664-whttps://doi.org/10.1038/s42004-022-00664-whttps://doi.org/10.1021/acs.chemmater.5c01482?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acs.chemmater.5c01482?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1016/j.xcrp.2025.102839https://doi.org/10.1016/j.xcrp.2025.102839https://doi.org/10.1038/s41467-019-10492-7https://doi.org/10.1038/s41467-019-10492-7https://doi.org/10.1021/jacs.1c11353?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/jacs.1c11353?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1038/s41467-024-45096-3https://doi.org/10.1038/s41467-024-45096-3https://doi.org/10.1038/s41467-024-45096-3https://doi.org/10.1038/s41467-024-45096-3https://doi.org/10.1021/acscatal.5c03953?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acscatal.5c03953?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acsami.2c03483?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acsami.2c03483?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1103/PhysRevB.54.11169https://doi.org/10.1103/PhysRevB.54.11169https://doi.org/10.1016/0927-0256(96)00008-0https://doi.org/10.1016/0927-0256(96)00008-0https://doi.org/10.1016/0927-0256(96)00008-0https://doi.org/10.1103/PhysRevLett.77.3865https://doi.org/10.1103/PhysRevLett.77.3865https://doi.org/10.1103/PhysRevB.59.1758https://doi.org/10.1103/PhysRevB.59.1758https://doi.org/10.1103/PhysRevB.105.054208https://doi.org/10.1103/PhysRevB.105.054208https://doi.org/10.1103/PhysRevB.105.054208https://doi.org/10.1103/PhysRevMaterials.5.123803https://doi.org/10.1103/PhysRevMaterials.5.123803https://doi.org/10.1103/PhysRevMaterials.5.123803https://doi.org/10.1021/ja00312a078?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/ja00312a078?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1146/annurev.ms.21.080191.001413https://doi.org/10.1016/j.jelechem.2016.12.037https://doi.org/10.1016/j.jelechem.2016.12.037https://doi.org/10.1016/j.jelechem.2016.12.037https://doi.org/10.1016/0022-3697(58)90069-6https://doi.org/10.1016/0022-3697(58)90069-6https://doi.org/10.1103/PhysRevB.59.5398https://doi.org/10.1103/PhysRevB.59.5398https://doi.org/10.1021/jacs.2c10927?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/jacs.2c10927?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1016/0092-640X(85)90016-6https://doi.org/10.1016/0092-640X(85)90016-6https://doi.org/10.1002/bbpc.19720760854https://doi.org/10.1002/bbpc.19720760854https://doi.org/10.1103/PhysRevB.26.5634https://doi.org/10.1103/PhysRevB.26.5634https://doi.org/10.1021/acs.chemmater.1c03450?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acs.chemmater.1c03450?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acs.chemmater.1c03450?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acs.jpcc.3c02680?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acs.jpcc.3c02680?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acs.jpcc.3c02680?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1007/978-1-4757-0961-2_2https://doi.org/10.1007/978-1-4757-0961-2_2?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/cr050197j?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/cr050197j?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/cr050197j?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1007/s40843-020-1343-xhttps://doi.org/10.1007/s40843-020-1343-xhttps://doi.org/10.1007/s40843-020-1343-xhttps://doi.org/10.1021/acs.jpcc.9b06296?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acs.jpcc.9b06296?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1103/PhysRevMaterials.7.015402https://doi.org/10.1103/PhysRevMaterials.7.015402pubs.acs.org/JACS?ref=pdfhttps://doi.org/10.1021/jacs.6c01849?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-as