Chemical Pressure-Induced Anion Order–Disorder Transition in LnHO Enabled by Hydride Size Flexibility

Synthesis and Crystal Structure of the Europium(II) Hydride Oxide Iodide Eu5H2O2I4 Showing Blue-Green Luminescence

As the first europium(II) hydride oxide iodide, dark red single crystals of Eu5H2O2I4 could be synthesized from oxygen-contaminated mixtures of EuH2 and EuI2. Its orthorhombic crystal structure (a = 1636.97(9) pm, b = 1369.54(8) pm, c = 604.36(4) pm, Z = 4) was determined via single-crystal X-ray diffraction in the space group Cmcm. Anion-centred tetrahedra [HEu4]7+ and [OEu4]6+ serve as central building blocks interconnected via common edges to infinite ribbons parallel to the c axis. These ribbons consist of four trans-edge connected (Eu2+)4 tetrahedra as repetition unit, two H--centred ones in the inner part, and two O2--centred ones representing the outer sides. They are positively charged, according to ∞1{[Eu5H2O2]4+}, to become interconnected and charge-balanced by iodide anions. Upon excitation with UV light, the compound shows blue-green luminescence with the shortest Eu2+ emission wavelength ever observed for a hydride derivative, peaking at 463 nm. The magnetic susceptibility of Eu5H2O2I4 follows the Curie-Weiss law down to 100 K, and exhibits a ferromagnetic ordering transition at about 10 K.

Exploring Multi-Anion Chemistry in Yttrium Oxyhydrides: Solid-State NMR Studies and DFT Calculations

Rare earth oxyhydrides REO_xH_(3-2x), with RE = Y, Sc, or Gd and a cationic FCC lattice, are reversibly photochromic in nature. It is known that structural details and anion (O^2-:H^-) composition dictate the efficiency of the photochromic behavior. The mechanism behind the photochromism is, however, not yet understood. In this study, we use ¹H, ²H, ¹⁷O, and ⁸⁹Y solid-state NMR spectroscopy and density functional theory (DFT) calculations to study the various yttrium, hydrogen, and oxygen local environments, anion oxidation states, and hydride ion dynamics. DFT models of YO_xH_(3-2x) with both anion-ordered and anion-disordered sublattices are constructed for a range of compositions and show a good correlation with the experimental NMR parameters. Two-dimensional ¹⁷O-¹H and ⁸⁹Y-¹H NMR correlation experiments reveal heterogeneities in the samples, which appear to consist of hydride-rich (x ≈ 0.25) and hydride-poor domains (x ≈ 1) rather than a single composition with homogeneous anion mixing. The compositional variation (as indicated by the different x values in YO_xH_(3-2x)) is determined by comparing static ¹H NMR line widths with calculated ¹H-¹H dipolar couplings of yttrium oxyhydride models. The 1D ¹⁷O MAS spectrum demonstrates the presence of a small percentage of hydroxide (OH^-) ions. DFT modeling indicates a reaction between the protons of hydroxides and hydrides to form molecular hydrogen (H⁺ + H^- → H₂). ¹H MAS NMR indicates the presence of a mobile component that, based on this finding, is attributed to trapped molecular H₂ in the lattice.

Facile formation of barium titanium oxyhydride on a titanium hydride surface as an ammonia synthesis catalyst

Oxyhydrides are promising compounds as supports for ammonia synthesis catalysts because they suppress hydrogen poisoning on the catalyst surface and enhance the ammonia synthesis activity. Herein, we developed a facile method for preparing BaTiO_2.5H_0.5, a perovskite oxyhydride, on a TiH₂ surface via the conventional wet impregnation method using TiH₂ and Ba hydroxide. Scanning electron microscopy and high-angle annular dark-field scanning transmission electron microscopy observations revealed that BaTiO_2.5H_0.5 crystallized as nanoparticles of ca. 100-200 nm on the TiH₂ surface. The Ru-loaded catalyst Ru/BaTiO_2.5H_0.5-TiH₂ exhibited 2.46 times higher ammonia synthesis activity (3.05 mmol-NH₃ g^-1 h^-1 at 400 °C) than the benchmark Ru catalyst Ru-Cs/MgO (1.24 mmol-NH₃ g^-1 h^-1 at 400 °C) because of the suppression of hydrogen poisoning. The analysis of reaction orders showed that the effect of suppressing hydrogen poisoning on Ru/BaTiO_2.5H_0.5-TiH₂ was equivalent to that of the reported Ru/BaTiO_2.5H_0.5 catalyst, thus supporting the formation of BaTiO_2.5H_0.5 perovskite oxyhydride. This study demonstrated that the selection of appropriate raw materials facilitates the formation of BaTiO_2.5H_0.5 oxyhydride nanoparticles on the TiH₂ surface using the conventional synthesis method.

Splitting of Hydrogen Atoms into Proton-Electron Pairs at BaO-Ru Interfaces for Promoting Ammonia Synthesis under Mild Conditions

Ru catalysts promoted with alkali and alkaline earth have shown superior ammonia (NH₃) synthesis activities under mild conditions. Although these promoters play a vital role in enhancing catalytic activity, their function has not been clearly understood. Here, we synthesize a series of Ba-Ru/MgO catalysts with an optimal Ru particle size (∼2.3 nm) and tailored BaO-Ru interfacial structures. We discover that the promoting effect is created through the separate storage of H⁺/e^- pairs at the BaO-Ru interface. Chemisorbed H atoms on Ru dissociate into H⁺/e^- pairs at the BaO-Ru interface, where strongly basic, nonreducible BaO selectively captures H⁺ while leaving e^- on Ru. The resulting electron accumulation in Ru facilitates N₂ activation via enhanced π-backdonation and inhibits hydrogen poisoning during NH₃ synthesis. Consequently, the formation of intimate BaO-Ru interface without an excessive loss of accessible Ru sites enables the synthesis of highly active catalysts for NH₃ synthesis.

Hidden chemical order in disordered Ba7Nb4MoO20 revealed by resonant X-ray diffraction and solid-state NMR

The chemical order and disorder of solids have a decisive influence on the material properties. There are numerous materials exhibiting chemical order/disorder of atoms with similar X-ray atomic scattering factors and similar neutron scattering lengths. It is difficult to investigate such order/disorder hidden in the data obtained from conventional diffraction methods. Herein, we quantitatively determined the Mo/Nb order in the high ion conductor Ba₇Nb₄MoO₂₀ by a technique combining resonant X-ray diffraction, solid-state nuclear magnetic resonance (NMR) and first-principle calculations. NMR provided direct evidence that Mo atoms occupy only the M2 site near the intrinsically oxygen-deficient ion-conducting layer. Resonant X-ray diffraction determined the occupancy factors of Mo atoms at the M2 and other sites to be 0.50 and 0.00, respectively. These findings provide a basis for the development of ion conductors. This combined technique would open a new avenue for in-depth investigation of the hidden chemical order/disorder in materials.

Influence of Crystal Structure, Encapsulation, and Annealing on Photochromism in Nd Oxyhydride Thin Films

Thin films of rare earth metal oxyhydrides show a photochromic effect, the precise mechanism of which is yet unknown. Here, we made thin films of NdH_3-2x O _x and show that we can change the band gap, crystal structure, and photochromic contrast by tuning the composition (O^2-:H^-) via the sputtering deposition pressure. To protect these films from rapid oxidation, we add a thin ALD coating of Al₂O₃, which increases the lifetime of the films from 1 day to several months. Encapsulation of the films also influences photochromic bleaching, changing the time dependency from first-order kinetics. As well, the partial annealing which occurs during the ALD process results in a dramatically slower bleaching speed, revealing the importance of defects for the reversibility (bleaching speed) of photochromism.

Emerging Materials and Methods toward Ammonia-Based Energy Storage and Conversion

Efficient storage and conversion of renewable energies is of critical importance to the sustainable growth of human society. With its distinguishing features of high hydrogen content, high energy density, facile storage/transportation, and zero-carbon emission, ammonia has been recently considered as a promising energy carrier for long-term and large-scale energy storage. Under this scenario, the synthesis, storage, and utilization of ammonia are key components for the implementation of ammonia-mediated energy system. Being different from fossil fuels, renewable energies normally have intermittent and variable nature, and thus pose demands on the improvement of existing technologies and simultaneously the development of alternative methods and materials for ammonia synthesis and storage. The energy release from ammonia in an efficient manner, on the other hand, is vital to achieve a sustainable energy supply and complete the nitrogen circle. Herein, recent advances in the thermal-, electro-, plasma-, and photocatalytic ammonia synthesis, ammonia storage or separation, ammonia thermal/electrochemical decomposition and conversion are summarized with the emphasis on the latest developments of new methods and materials (catalysts, electrodes, and sorbents) for these processes. The challenges and potential solutions are discussed.

Alkali-Rich Antiperovskite M3FCh (M = Li, Na; Ch = S, Se, Te): The Role of Anions in Phase Stability and Ionic Transport

To improve ionic conductivity, solid-state electrolytes with polarizable anions that weakly interact with mobile ions have received much attention, a recent example being lithium/sodium-rich antiperovskite M₃HCh (M = Li, Na; Ch = S, Se, Te). Herein, in order to clarify the role of anions in antiperovskites, the M₃FCh family, in which the polarizable H^- anion at the octahedral center is replaced by the ionic F^- anion, is investigated theoretically and experimentally. We unexpectedly found that the stronger attractive interaction between F^- and M⁺ ions does not slow down the M⁺ ion diffusion, with the calculated energy barrier being as low as that of M₃HCh. This fact suggests that the low-frequency rotational phonon modes of the octahedron of cubic M₃FCh (and M₃HCh) are intrinsic to facilitate the fast ionic diffusion. A systematic analysis further reveals a correlation between the tolerance factor t and the ionic transport: as t decreases within the cubic phase, the rotational mode becomes softer, resulting in the reduction of the migration energy. The cubic iodine-doped Li₃FSe has a room-temperature ionic conductivity of 5 × 10^-5 S/cm with a bulk activation energy of 0.18 eV.

Computational Chemistry-Guided Syntheses and Crystal Structures of the Heavier Lanthanide Hydride Oxides DyHO, ErHO, and LuHO

Heteroanionic hydrides offer great possibilities in the design of functional materials. For ternary rare earth hydride oxide REHO, several modifications were reported with indications for a significant phase width with respect to H and O of the cubic representatives. We obtained DyHO and ErHO as well as the thus far elusive LuHO from solid-state reactions of RE2O3 and REH3 or LuH3 with CaO and investigated their crystal structures by neutron and X-ray powder diffraction. While DyHO, ErHO, and LuHO adopted the cubic anion-ordered half-Heusler LiAlSi structure type (F4¯3m, a(DyHO) = 5.30945(10) Å, a(ErHO) = 5.24615(7) Å, a(LuHO) = 5.171591(13) Å), LuHO additionally formed the orthorhombic anti-LiMgN structure type (Pnma; LuHO: a = 7.3493(7) Å, b = 3.6747(4) Å, c = 5.1985(3) Å; LuDO: a = 7.3116(16) Å, b = 3.6492(8) Å, c = 5.2021(7) Å). A comparison of the cubic compounds’ lattice parameters enabled a significant distinction between REHO and REH1+2xO1−x (x < 0 or x > 0). Furthermore, a computational chemistry study revealed the formation of REHO compounds of the smallest rare earth elements to be disfavored in comparison to the sesquioxides, which is why they may only be obtained by mild synthesis conditions.

Formation of PbCl2-type AHF (A = Ca, Sr, Ba) with partial anion order at high pressure

The high-pressure structures of alkaline earth metal hydride-fluorides (AHFs) (A = Ca, Sr, Ba) were investigated up to 8 GPa. While AHF adopts the fluorite-type structure (Fm3[combining macron]m) at ambient pressure without anion ordering, the PbCl2-type (cotunnite-type) structure (Pnma) is formed by pressurization, with a declining trend of critical pressure as the ionic radius of the A2+ cation increases. In contrast to PbCl2-type LaHO and LaOF whose anions are fully ordered, the H-/F- anions in the high-pressure polymorph of SrHF and BaHF are partially ordered, with a preferential occupation of H- at the square-pyramidal site (vs. tetrahedral site). First-principles calculations partially support the preferential anion occupation and suggest occupation switching at higher pressure. These results provide a strategy for controlling the anion ordering and local structure in mixed-anion compounds.

Interplay of Alkali, Transition Metals, Nitrogen, and Hydrogen in Ammonia Synthesis and Decomposition Reactions

ConspectusThe fixation of dinitrogen to ammonia is critically important for the biogeochemical cycle on earth. Ammonia also holds promise as a sustainable energy carrier. Tremendous effort has been devoted to the development of green processes and advanced materials for ammonia synthesis and decomposition under milder conditions, and encouraging progress has been made.The reduction of dinitrogen to ammonia needs electrons and protons, which hydridic hydrogen H^- could supply. Polarized, electron-rich N_xH_y intermediates, on the other hand, can be stabilized by alkali or alkaline earth metal cations to lower kinetic barriers in the transformation. The inherent properties of alkali/alkaline earth metal hydrides (denoted as AH) endow them with a unique function in ammonia synthesis.In this Account, recent efforts in the exploration of alkali or alkaline earth metal hydrides (denoted as AH), amides, and imides (denoted as ANH hereafter) for ammonia synthesis and decomposition reactions will be summarized and discussed. We begin with an introduction to the chemistry of A with N₂, NH₃, and H₂, highlighting the interconversion between AH and ANH that has profound implications on the formation and decomposition of NH₃. We then present our finding on the strong synergistic effect between ANH and transition metals (TM) in ammonia decomposition catalysis, which stimulated our subsequent research on AH for ammonia synthesis. We discuss the effect and function mechanism of AH in the thermocatalytic and chemical looping ammonia synthesis processes. In the thermocatalytic process, AH cooperates with both early and late TM forming either composite catalysts with two active centers or complex metal hydride catalysts with electron- and hydrogen-rich ionic centers facilitating ammonia synthesis with high activities at lower temperatures. Very interestingly, AH levels the catalytic performances of TMs and intervenes in the energy-scaling relations of TM-only catalysts. Moreover, ANH serves as a new type nitrogen carrier effectively mediating ammonia synthesis via a low-temperature chemical looping process, in which N₂ is fixed by AH forming ANH. Subsequently, ANH is hydrogenated to ammonia and AH. Late TMs have a strong catalytic effect on the chemical looping process. The unique interplay of A, N, TM, and H^- offers plenty of opportunities for achieving dinitrogen conversion under mild conditions, while further efforts are needed to address the challenges in the fundamental understanding and practical application.

HoHO: A Paramagnetic Air-Resistant Ionic Hydride with Ordered Anions

The substitution of hydrogen for oxygen atoms in metal oxides provides opportunities for influencing the solid-state properties. Such hydride oxides (or oxyhydrides) are potential functional materials and scarce. Here, we present the synthesis and characterization of holmium hydride oxide with the stoichiometric composition HoHO. It was prepared by the reaction of Ho₂O₃ with either HoH₃ or CaH₂ as a powder of light-yellow color in sunlight and pink color in artificial light (Alexandrite effect), which is commonly observed for ionic Ho(III) compounds. HoHO crystallizes with an ordered fluorite superstructure (F4̅3m, a = 5.27550(13) Å, half-Heusler LiAlSi type), as evidenced by powder X-ray and neutron powder diffraction on both hydride and deuteride and supported by quantum-mechanical calculations. HoHO is the first representative with considerable ionic bonding for this structure type. The thermal stability and inertness toward air are remarkably high for a hydride because it reacts only above 540 K to form Ho₂O₃. At 294(1) K and 25(3)% relative humidity, HoHO is stable for at least 3 months. HoHO is paramagnetic with μ_eff(Ho³⁺) = 10.41(2) μ_B without any sign of magnetic ordering down to 2 K.

Hydride-based antiperovskites with soft anionic sublattices as fast alkali ionic conductors

Most solid-state materials are composed of p-block anions, only in recent years the introduction of hydride anions (1s²) in oxides (e.g., SrVO₂H, BaTi(O,H)₃) has allowed the discovery of various interesting properties. Here we exploit the large polarizability of hydride anions (H^-) together with chalcogenide (Ch^2-) anions to construct a family of antiperovskites with soft anionic sublattices. The M₃HCh antiperovskites (M = Li, Na) adopt the ideal cubic structure except orthorhombic Na₃HS, despite the large variation in sizes of M and Ch. This unconventional robustness of cubic phase mainly originates from the large size-flexibility of the H^- anion. Theoretical and experimental studies reveal low migration barriers for Li⁺/Na⁺ transport and high ionic conductivity, possibly promoted by a soft phonon mode associated with the rotational motion of HM₆ octahedra in their cubic forms. Aliovalent substitution to create vacancies has further enhanced ionic conductivities of this series of antiperovskites, resulting in Na_2.9H(Se_0.9I_0.1) achieving a high conductivity of ~1 × 10^-4 S/cm (100 °C).

Positive thermal expansion facilitates the formation of argentophilic forces following an order–disorder phase transition

Variable-temperature studies of the 0D silver-based complex [Ag₂(CF₃SO₃)₂(4-SB)₄] reveal formation of argentophilic forces upon warming due to a phase transition.

Ionic conduction mechanism in Ca-doped lanthanum oxychloride

The mechanism of ionic conduction in Ca-doped lanthanum oxychloride (LaOCl) was investigated using first-principles calculations based on density functional theory. The calculations of the point defect formation energies suggest that Cl^- ion vacancies and substituted Ca²⁺ ions at La sites were dominant point defects. Although the migration energy of an O^2- ion is 0.95 eV, the migration energy of a Cl^- ion was calculated to be 0.44 eV, which is consistent with the reported experimental value. These results imply that the main carrier in Ca-doped LaOCl is Cl^- ions and ionic conduction occurs by a Cl^- ion vacancy mechanism.

One Structure, Two Elements-LuGe2 Superconductor vs Ordinary Metallic Conductor LuSn2. A Case Study on How Site-Selective Germanium for Tin Atom Substitution Leads to Modulating of the Charge Distribution

The substitution of chemically similar elements in a given crystal structure is an effective way to enhance physical properties, but the understanding on such improvements is usually impeded because the substitutions are random, and the roles of the different atoms cannot be distinguished by crystallographic symmetry. Herein, we provide a detailed crystallographic analysis and property measurements for the continuous solid solutions LuGe_xSn_2-x (0 < x < 2). The results show that there is no apparent change of the global symmetry, with the end-members LuGe₂ and LuSn₂, as well as the intermediate LuGe_xSn_2-x compositions adopting the ZrSi₂ type structure (space group Cmcm, Pearson index oC12). Yet, the refinements of the crystal structures from single-crystal X-ray diffraction data show that Ge-Sn atom substitutions are not random, but occur preferentially at the zigzag chain. The patterned distribution of two group 14 elements leads to a significant variation in chemical bonding and charge ordering within the other structural fragment, the 2D square nets, thereby resulting in tuned electron transport. The enhancement is greater than that for the typical Bloch-Gruneisen model and more akin to that for the parallel-resistor model. Magnetization measurements on single crystals show bulk superconductivity in all LuGe_xSn_2-x samples with shielding fractions as high as 90%. Specific heat data confirm the effect to originate from residual metallic tin in the material, indicating that Sn atom substitutions in the 2D square nets cause disruptions of the hypervalent bonding and local anisotropy, which ultimately leads to vanishing of the superconducting state in the end-member LuGe₂. This work sheds light on how the complexity in chemical interactions by two different carbon congeners leads to changes in the physical properties and how they can be correlated with the induced charge distribution. These studies also provide a general approach to modulation of charge density and. thus, of emerging physical properties in other classes of intermetallic systems based on the main-group elements of groups 13 to 15.

Preferential Orientation of Photochromic Gadolinium Oxyhydride Films

We report preferential orientation control in photochromic gadolinium oxyhydride (GdHO) thin films deposited by a two-step process. Gadolinium hydride (GdH_2-x) films were grown by reactive magnetron sputtering, followed by oxidation in air. The preferential orientation, grain size, anion concentrations and photochromic response of the films were strongly dependent on the deposition pressure. The GdHO films showed a preferential orientation along the [100] direction and exhibited photochromism when synthesized at deposition pressures of up to 5.8 Pa. The photochromic contrast was larger than 20% when the films were deposited below 2.8 Pa with a 0.22 H₂/Ar flow ratio. We argue that the relation of preferential orientation and the post deposition oxidation since oxygen concentration is known to be a key parameter for photochromism in rare-earth oxyhydride thin films. The experimental observations described above were explained by the decrease of the grain size as a result of the increase of the deposition pressure of the sputtering gas, followed by a higher oxygen incorporation.

Pressure-induced phase transitions in DL-glutamic acid monohydrate crystal

DL-glutamic acid monohydrate crystal was synthesized from an aqueous solution by slow evaporation technique. The crystal was submitted to high-pressure (1 atm-14.3 GPa) to investigate its vibrational behavior and the occurrence of phase transitions. We performed Raman spectroscopy as probe and through the analysis of the spectra we discovered three structural phase transitions. The first one occurs around 0.9 GPa. In this phase transition, glutamic acid molecules suffer modifications in their conformations while water molecules are less affected. The second phase transition at 4.8 GPa involves conformational changes related to CO₂^-, NH₃⁺ units and the water molecules, while the third one, between 10.9 and 12.4 GPa, involves motions of several parts of the glutamic acid as well as the water molecules. Considering the dynamic of high pressure, the second phase of DL-glutamic acid monohydrate crystal presented a better stability compared with the second phase of its polymorphs α and β L-glutamic acid. In addition, water molecules seem to play important role on this structural stability. All changes are reversible.

YHO, an Air-Stable Ionic Hydride

Metal hydride oxides are an emerging field in solid-state research. While some lanthanide hydride oxides (LnHO) were known, YHO has only been found in thin films so far. Yttrium hydride oxide, YHO, can be synthesized as bulk samples by a reaction of Y₂O₃ with hydrides (YH₃, CaH₂), by a reaction of YH₃ with CaO, or by a metathesis of YOF with LiH or NaH. X-ray and neutron powder diffraction reveal an anti-LiMgN type structure for YHO (Pnma, a = 7.5367(3) Å, b = 3.7578(2) Å, and c = 5.3249(3) Å) and YDO (Pnma, a = 7.5309(3) Å, b = 3.75349(13) Å, and c = 5.3192(2) Å); in other words, a distorted fluorite type with ordered hydride and oxide anions was observed. Bond lengths (average 2.267 Å (Y-O), 2.352 Å (Y-H), 2.363 Å (Y-D), >2.4 Å (H-H and D-D), >2.6 Å (H-O and D-O), and >2.8 Å (O-O)) and quantum-mechanical calculations on density functional theory level (band gap 2.8 eV) suggest yttrium hydride oxide to be a semiconductor and to have considerable ionic bonding character. Nonetheless, YHO exhibits a surprising stability in air. An in situ X-ray diffraction experiment shows that decomposition of YHO to Y₂O₃ starts at only above 500 K and is still not complete after 14 h of heating to a final temperature of 1000 K. YHO hydrolyzes in water very slowly. The inertness of YHO in air is very beneficial for its potential use as a functional material.

The rare earth metal hydride tellurides REHTe (RE=Y, La–Nd, Gd–Er)

The synthesis and crystal structure of a series of rare earth metal hydride tellurides with the composition REHTe (RE = Y, La–Nd, Gd–Er) is reported. These compounds have been obtained by the reaction of rare earth metal dihydrides (REH2) with elemental tellurium in sealed tantalum capsules at T = 700°C using cesium chloride (CsCl) as fluxing agent, which can be washed away with water due to the astonishing insensitivity of these hydride tellurides (REHTe) against hydrolysis. All of the compounds crystallize in the hexagonal space group P6̅m2 with a filled WC-type crystal structure, exhibiting a mutual trigonal-prismatic coordination of the heavy ions (RE 3+ and Te2−), while the hydride anions reside in the trigonal prismatic voids surrounded by three rare earth metal cations expanding their coordination pattern to a tricapped trigonal prism. This 1H-type crystal structure is compared with the 1H- and 2H-type structures of the respective hydride selenides (REHSe, RE = Y, La–Nd, Gd–Tm, Lu). Both hexagonal basic crystal structures can be derived from the AlB2-type structure as demonstrated in a Bärnighausen tree by group-subgroup relationships.

Oxyhydride Nature of Rare-Earth-Based Photochromic Thin Films

Thin films of rare-earth (RE)-oxygen-hydrogen compounds prepared by reactive magnetron sputtering show a unique color-neutral photochromic effect at ambient conditions. While their optical properties have been studied extensively, the understanding of the relationship between photochromism, chemical composition, and structure is limited. Here we establish a ternary RE-O-H composition-phase diagram based on chemical composition analysis by a combination of Rutherford backscattering and elastic recoil detection. The photochromic films are identified as oxyhydrides with a wide composition range described by the formula REO _xH_{3-2 x} where 0.5 ≤ x ≤ 1.5. We propose an anion-disordered structure model based on the face-centered cubic unit cell where the O^2- and H^- anions occupy tetrahedral and octahedral interstices. The optical band gap varies continuously with the anion ratio, demonstrating the potential of band gap tuning for reversible optical switching applications.

Ba2ScHO3: H- Conductive Layered Oxyhydride with H- Site Selectivity

Hydride (H^-) conduction is a new frontier related to hydrogen transport in solids. Here, a new H^- conductive oxyhydride Ba₂ScHO₃ was successfully synthesized using a high-pressure technique. Powder X-ray and neutron diffraction experiments investigated the fact that Ba₂ScHO₃ adopts a K₂NiF₄-type structure with H^- ions preferentially occupying the apical sites, as supported by theoretical calculations. Electrochemical impedance spectra showed that Ba₂ScHO₃ exhibited H^- conduction and a conductivity of 5.2 × 10^-6 S cm^-1 at 300 °C. This value is much higher than that of BaScO₂H, which has an ideal perovskite structure, suggesting the advantage of layered structures for H^- conduction. Tuning site selectivity of H^- ions in layered oxyhydrides might be a promising strategy for designing fast H^- conductors applicable for novel electrochemical devices.