The hydride anion in an extended transition metal oxide array: LaSrCoO3H0.7

A new family of anti-perovskite oxyhydrides with tetrahedral GaO4 polyanions

New solid compounds A3-xGaO4H1-y (A = Sr, Ba; x ∼0.15, y ∼0.3), which are the first oxyhydrides containing gallium ions, have been synthesized by high-pressure synthesis. Powder X-ray and neutron diffraction experiments revealed that the series adopts an anti-perovskite structure consisting of hydride-anion-centered HA6 octahedra with tetrahedral GaO4 polyanions, wherein the A- and H-sites show partial defect. Formation energy calculations from the raw materials support that stoichiometric Ba3GaO4H is thermodynamically stable with a wide band gap. Annealing the A = Ba powder under flowing Ar and O2 gas suggests topochemical H- desorption and O2-/H- exchange reactions, respectively.

Geometric frustration of Jahn-Teller order in the infinite-layer lattice

The Jahn-Teller effect, in which electronic configurations with energetically degenerate orbitals induce lattice distortions to lift this degeneracy, has a key role in many symmetry-lowering crystal deformations¹. Lattices of Jahn-Teller ions can induce a cooperative distortion, as exemplified by LaMnO₃ (refs. ^2,3). Although many examples occur in octahedrally⁴ or tetrahedrally⁵ coordinated transition metal oxides due to their high orbital degeneracy, this effect has yet to be manifested for square-planar anion coordination, as found in infinite-layer copper^6,7, nickel^8,9, iron^10,11 and manganese oxides¹². Here we synthesize single-crystal CaCoO₂ thin films by topotactic reduction of the brownmillerite CaCoO_2.5 phase. We observe a markedly distorted infinite-layer structure, with ångström-scale displacements of the cations from their high-symmetry positions. This can be understood to originate from the Jahn-Teller degeneracy of the d_xz and d_yz orbitals in the d⁷ electronic configuration along with substantial ligand-transition metal mixing. A complex pattern of distortions arises in a [Formula: see text] tetragonal supercell, reflecting the competition between an ordered Jahn-Teller effect on the CoO₂ sublattice and the geometric frustration of the associated displacements of the Ca sublattice, which are strongly coupled in the absence of apical oxygen. As a result of this competition, the CaCoO₂ structure forms an extended two-in-two-out type of Co distortion following 'ice rules'¹³.

Compressive-Strain-Facilitated Fast Oxygen Migration with Reversible Topotactic Transformation in La0.5Sr0.5CoOx via All-Solid-State Electrolyte Gating

Modifying the crystal structure and corresponding functional properties of complex oxides by regulating their oxygen content has promising applications in energy conversion and chemical looping, where controlling oxygen migration plays an important role. Therefore, finding an efficacious and feasible method to facilitate oxygen migration has become a critical requirement for practical applications. Here, we report a compressive-strain-facilitated oxygen migration with reversible topotactic phase transformation (RTPT) in La_0.5Sr_0.5CoO_x films based on all-solid-state electrolyte gating modulation. With the lattice strain changing from tensile to compressive strain, significant reductions in modulation duration (∼72%) and threshold voltage (∼70%) for the RTPT were observed, indicating great promotion of RTPT by compressive strain. Density functional theory calculations verify that such compressive-strain-facilitated efficient RTPT comes from significant reduction of the oxygen migration barrier in compressive-strained films. Further, ac-STEM, EELS, and sXAS investigations reveal that varying strain from tensile to compressive enhances the Co 3d band filling, thereby suppressing the Co-O hybrid bond in oxygen vacancy channels, elucidating the micro-origin of such compressive-strain-facilitated oxygen migration. Our work suggests that controlling electronic orbital occupation of Co ions in oxygen vacancy channels may help facilitate oxygen migration, providing valuable insights and practical guidance for achieving highly efficient oxygen-migration-related chemical looping and energy conversion with complex oxides.

Computation-accelerated discovery of the K2NiF4-type oxyhydrides combing density functional theory and machine learning approach

The emerging K₂NiF₄-type oxyhydrides with unique hydride ions (H⁻) and O^2- coexisting in the anion sublattice offer superior functionalities for numerous applications. However, the exploration and innovations of the oxyhydrides are challenged by their rarity as a limited number of compounds reported in experiments, owing to the stringent laboratory conditions. Herein, we employed a suite of computations involving ab initio methods, informatics and machine learning to investigate the stability relationship of the K₂NiF₄-type oxyhydrides. The comprehensive stability map of the oxyhydrides chemical space was constructed to identify 76 new compounds with good thermodynamic stabilities using the high-throughput computations. Based on the established database, we reveal geometric constraints and electronegativities of cationic elements as significant factors governing the oxyhydrides stabilities via informatics tools. Besides fixed stoichiometry compounds, mixed-cation oxyhydrides can provide promising properties due to the enhancement of compositional tunability. However, the exploration of the mixed compounds is hindered by their huge quantity and the rarity of stable oxyhydrides. Therefore, we propose a two-step machine learning workflow consisting of a simple transfer learning to discover 114 formable oxyhydrides from thousands of unknown mixed compositions. The predicted high H⁻ conductivities of the representative oxyhydrides indicate their suitability as energy conversion materials. Our study provides an insight into the oxyhydrides chemistry which is applicable to other mixed-anion systems, and demonstrates an efficient computational paradigm for other materials design applications, which are challenged by the unavailable and highly unbalanced materials database.

Recent Advances in Perovskite Catalysts for Efficient Overall Water Splitting

Hydrogen is considered a promising clean energy vector with the features of high energy capacity and zero-carbon emission. Water splitting is an environment-friendly and effective route for producing high-purity hydrogen, which contains two important half-cell reactions, namely, the anodic oxygen evolution reaction (OER) and the cathodic hydrogen evolution reaction (HER). At the heart of water splitting is high-performance electrocatalysts that efficiently improve the rate and selectivity of key chemical reactions. Recently, perovskite oxides have emerged as promising candidates for efficient water splitting electrocatalysts owing to their low cost, high electrochemical stability, and compositional and structural flexibility allowing for the achievement of high intrinsic electrocatalytic activity. In this review, we summarize the present research progress in the design, development, and application of perovskite oxides for electrocatalytic water splitting. The emphasis is on the innovative synthesis strategies and a deeper understanding of structure–activity relationships through a combination of systematic characterization and theoretical research. Finally, the main challenges and prospects for the further development of more efficient electrocatalysts based on perovskite oxides are proposed. It is expected to give guidance for the development of novel non-noble metal catalysts in electrochemical water splitting.

Kinetic Control of Anion Stoichiometry in Hexagonal BaTiO3

The cubic oxyhydride perovskite BaTiO3−xHx, where the well-known ferroelectric oxide BaTiO3 is partially hydridized, exhibits a variety of functions such as being a catalyst and precursor for the synthesis of mixed-anion compounds by utilizing the labile nature of hydride anions. In this study, we present a hexagonal version, BaTi(O3−xHx) (x < 0.6) with the 6H-type structure, synthesized by a topochemical reaction using hydride reduction, unlike reported hexagonal oxyhydrides obtained under high pressure. The conversion of cubic BaTiO3 (150 nm) to the hexagonal phase by heat treatment at low temperature (950~1025 °C) using a Mg getter allows the introduction of large oxygen defects (BaTiO3−x; x − 0.28) while preventing the crystal growth of hexagonal BaTiO3, which has been accessible at high temperatures of ~1500 °C, contributing to the increase of the hydrogen content. Hydride anions in 6H-BaTiO3−xHx preferentially occupy face-sharing sites, as do other oxyhydrides.

Characteristic Resistive Switching of Rare-Earth Oxyhydrides by Hydride Ion Insertion and Extraction

Resistive switching induced by ion migration is promising for applications such as random-access memory (ReRAM) and neuromorphic transistors. Hydride ions (H^-) are an interesting candidate as the migration ion for resistive switching devices because they have fast diffusion in several compounds at room temperature and doping/dedoping can be used effectively to achieve significant changes in the electronic conductivity. Here, we report reversible resistive switching characteristics in rare-earth oxyhydrides (REH_xO_(3-x)/2) induced by field insertion/extraction of H^-. The current-voltage measurements revealed that the resistive switching response, hysteresis, and switching voltage vary greatly with the H^-/O^2- ratio in the films. We fabricated a ReRAM device using Ti/YH_1.3O_0.85/MoO_x structure and confirmed the bipolar-type operation with the resistance switching ratio of 1 order of magnitude over 1000 cycles. The composition gradient of H^-/O^2- in YH_xO_(3-x)/2 films, in addition to the hydrogen-absorbing ability of the top electrode, is essential for effective device operation. Our findings show that hydride-conducting solid-state electrolytes are suitable for resistive switching device development.

High-Pressure Synthesis of Transition-Metal Oxyhydrides with Double-Perovskite Structures

We report on the high-pressure synthesis, crystal structure, and magnetic properties of four novel transition-metal oxyhydrides─Ba₂NaVO₃H₃, Ba₂NaVO_2.4H_3.6, Ba₂NaCrO_2.2H_3.8, and Ba₂NaTiO₃H₃─crystallizing in the double-perovskite structure. Notably, they have a higher hydride content in their anion sites (50%-63%) than known oxyhydrides with perovskite structures do (≤33%). Vanadium and chromium oxyhydrides exhibited Curie-Weiss magnetic susceptibilities with no magnetic ordering down to 2 K, which may be due to geometrical frustration in their face-centered lattices and weak magnetic interactions. Density functional theory calculations revealed that the transition metal-hydride bonding nature of the prepared oxyhydrides is more covalent than that observed for known perovskite oxyhydrides, as evidenced by the shorter bond lengths of the former. Remarkably, our double-perovskite oxyhydrides with a high hydride content may possess a bonding character intermediate between those of known oxyhydrides and hydrides.

Expanding the hydride chemistry: Antiperovskites A3MO4H (A = Rb, Cs; M = Mo, W) introducing the transition oxometalate hydrides

The four compounds A₃MO₄H (A = Rb, Cs; M = Mo, W) are introduced as the first members of the new material class of the transition oxometalate hydrides. The compounds are accessible via a thermal synthesis route with carefully controlled conditions. Their crystal structures were solved by neutron diffraction of the deuterated analogues. Rb₃MoO₄D, Cs₃MoO₄D and Cs₃WO₄D crystallize in the antiperovskite-like K₃SO₄F-structure type, while Rb₃WO₄D adopts a different orthorhombic structure. ²H MAS NMR, Raman spectroscopy and elemental analysis prove the abundance of hydride ions next to oxometalate ions and experimental findings are supported by quantum chemical calculations. The tetragonal phases are direct and wide band gap semiconductors arising from hydride states, whereas Rb₃WO₄H shows a unique, peculiar valence band structure dominated by hydride states.

The synthesis, structures and electronic properties of the first four heteroanionic compounds containing both hydride and transition oxometalate ions are reported.

Ammonia synthesis on BaTiO2.5H0.5: computational insights into the role of hydrides

Perovskite oxyhydrides such as BaTiO_2.5H_0.5 have been found to be able to catalyze NH₃ synthesis, but the mechanism and the role of the catalyst's lattice hydrides in the catalytic reaction remain unknown. Here we employ first principles density functional theory to investigate the mechanism of ammonia synthesis and the role of lattice hydrides on a prototypical perovskite oxyhydride, BaTiO_2.5H_0.5 (BTOH). Two mechanistic hypotheses, the distal and alternating pathways, have been tested on the Ti₂O₂ termination of the BTOH (210) surface, previously determined to be the most stable surface termination under the reaction conditions considered. In the distal pathway, H atoms hydrogenate N₂ to form the *N-NH_x key intermediates, followed by N-N bond breaking. In the alternating pathway, H atoms hydrogenate N₂ in an alternating fashion to form the *NH_x-NH_y intermediates before N-N bond breaking and formation of co-adsorbed *NH_x/*NH_y on the surface. We find that the subsurface hydride vacancy formed after reaction of *N₂ with the lattice hydride is key to the distal pathway, leading to surface nitride formation after breaking the *N-NH₃ bond, while the neighboring surface Ti sites are key to bridging and stabilizing the *NNH intermediate in the alternating pathway. In both pathways, desorption of NH₃ is the most uphill in energy. Our results provide important insights into the role of hydrides and surface vacancies in hydrogenation reactions over BTOH, which will be useful to guide future spectroscopic experiments such as operando IR and inelastic neutron scattering to verify the key intermediates.

From SmOF to SmH0.78OF0.22: H/F Substitution in Oxide Fluorides as a Synthesis Route to Heteroanionic Compounds

Mixed anionic hydrides of the rare earths are a fascinating class of compounds as potential functional materials, especially in luminescence, as photochromic thin films and for ion conduction. For exploratory studies, the effectiveness of various synthesis methods must be investigated, which is done here for metathesis reactions. The reaction of Sm₂O₃ with PTFE yields SmOF (P2₁/c, a = 5.60133(19) Å, b = 5.65567(19) Å, c = 5.6282(2) Å, β = 90.169(5)°, V = 178.295(11) ų, and Z = 4) in a new, probably metastable, polymorph of the baddeleyite-type structure. Metathesis reactions of SmOF with LiH, NaH, or CaH₂ led to a samarium hydride oxide fluoride, SmH_xOF_1-x; i.e., incomplete H/F exchange occurs. X-ray diffraction and neutron diffraction on a compound with x = 0.78 obtained via NaH reveal hydride, oxide, and fluoride ions to be partially ordered. SmH_0.78OF_0.22 (Ia3̅, a = 10.947(2) Å, V = 1311.7(4) ų, Z = 32) crystallizes in an anti-Li₃AlN₂-type structure with distorted cubic anion coordination for samarium atoms (site symmetry 3̅ and 2) and distorted tetrahedral arrangement of samarium atoms around the anions (site symmetry 1 and 3). It is a fully structurally characterized hydride oxide fluoride and shows a rare crystal chemical feature─the occupation of a crystallographic site by three different anions (0.188 H + 0.667 O + 0.145 F). Interatomic distances between samarium and hydrogen and samarium and the mixed hydrogen/oxygen/fluorine site range from 2.45 to 2.48 Å and 2.29 to 2.42 Å, respectively, and are similar to those in samarium hydride, samarium oxide, and samarium fluoride. Fluoride extraction by reaction with alkali and alkaline earth hydrides has thus proven to be a useful synthesis route to hydride oxides and also hydride oxide halogenides, which might be further exploited in exploratory research on heteroanionic metal hydrides.

Topotactic transformation of single crystals: From perovskite to infinite-layer nickelates

Topotactic transformations between related crystal structures are a powerful emerging route for the synthesis of novel quantum materials. Whereas most such “soft chemistry” experiments have been carried out on polycrystalline powders or thin films, the topotactic modification of single crystals, the gold standard for physical property measurements on quantum materials, has been studied only sparsely. Here, we report the topotactic reduction of La_1−xCa_xNiO₃ single crystals to La_1−xCa_xNiO_2+δ using CaH₂ as the reducing agent. The transformation from the three-dimensional perovskite to the quasi–two-dimensional infinite-layer phase was thoroughly characterized by x-ray diffraction, electron microscopy, Raman spectroscopy, magnetometry, and electrical transport measurements. Our work demonstrates that the infinite-layer structure can be realized as a bulk phase in crystals with micrometer-sized single domains. The electronic properties of these specimens resemble those of epitaxial thin films rather than powders with similar compositions.

High-Conductive Protonated Layered Oxides from H2 O Vapor-Annealed Brownmillerites

Protonated 3d transition-metal oxides often display low electronic conduction, which hampers their application in electric, magnetic, thermoelectric, and catalytic fields. Electronic conduction can be enhanced by co-inserting oxygen acceptors simultaneously. However, the currently used redox approaches hinder protons and oxygen ions co-insertion due to the selective switching issues. Here, a thermal hydration strategy for systematically exploring the synthesis of conductive protonated oxides from 3d transition-metal oxides is introduced. This strategy is illustrated by synthesizing a novel layered-oxide SrCoO₃ H from the brownmillerite SrCoO_2.5 . Compared to the insulating SrCoO_2.5 , SrCoO₃ H exhibits an unprecedented high electronic conductivity above room temperature, water uptake at 250 °C, and a thermoelectric power factor of up to 1.2 mW K^-2 m^-1 at 300 K. These findings open up opportunities for creating high-conductive protonated layered oxides by protons and oxygen ions co-doping.

Orthogonal chemistry in the design of rare-earth metal oxyhydrides

Inorganic systems containing two or more kinds of anions, such as rare-earth metal oxyhydrides, have a number of interesting properties that can be used in the design and development of new functional materials with desired characteristics. Chemical synthesis of these materials can be accomplished by oxidation of metal hydrides. However, the oxidation process of a metal hydride is directly accompanied by the release of hydrogen; both processes are a combination of two sequential reactions. This is usually not favorable for the formation and crystallization of the ternary oxyhydride composition. One possible way to overcome this problem is to introduce an appropriate amount of oxygen atoms into certain interstitial positions adjacent to the metal sites of the hydride lattice. Guided by the ideas of orthogonality, we have proposed a theoretical model capable of providing a thorough understanding of the chemical processes occurring in a multicomponent system at the molecular level. This model opens the way for predicting a wide range of new, stable multi-anion compounds of different compositions. It can also control functionality by adding noncovalent interactions between different kinds of anions, which can lead to the formation of chiral structures or a significant increase in ferro- and piezoelectric properties.

Low-temperature topotactic oxidation using the solid-state oxidant Zr-doped CeO2

A new topotactic oxidation was developed using the solid-state oxidant Zr-doped CeO₂. For the anti-ThCr₂Si₂ type Y₂O₂Bi, which became superconducting via oxygen intercalation during solid-state oxidation at 1000 °C, the topotactic oxidation enabled not only the oxygen intercalation at a much lower temperature of 200 °C, hampering the segregation of impurity phases, but also the highest superconducting transition temperature for Y₂O₂Bi.

Dehydration of Electrochemically Protonated Oxide: SrCoO2 with Square Spin Tubes

Controlling oxygen deficiencies is essential for the development of novel chemical and physical properties such as high-T_c superconductivity and low-dimensional magnetic phenomena. Among reduction methods, topochemical reactions using metal hydrides (e.g., CaH₂) are known as the most powerful method to obtain highly reduced oxides including Nd_0.8Sr_0.2NiO₂ superconductor, though there are some limitations such as competition with oxyhydrides. Here we demonstrate that electrochemical protonation combined with thermal dehydration can yield highly reduced oxides: SrCoO_2.5 thin films are converted to SrCoO₂ by dehydration of HSrCoO_2.5 at 350 °C. SrCoO₂ forms square (or four-legged) spin tubes composed of tetrahedra, in contrast to the conventional infinite-layer structure. Detailed analyses suggest the importance of the destabilization of the SrCoO_2.5 precursor by electrochemical protonation that can greatly alter reaction energy landscape and its gradual dehydration (H_1-xSrCoO_2.5-x/2) for the SrCoO₂ formation. Given the applicability of electrochemical protonation to a variety of transition metal oxides, this simple process widens possibilities to explore novel functional oxides.

Enhanced Magnetic Interaction by Face-Shared Hydride Anions in 6H-BaCrO2H

Studies on magnetic oxyhydrides have been almost limited to perovskite-based lattices with corner-sharing octahedra with a M-H-M (M: transition metal) angle of θ ∼ 180°. Using a high-pressure method, we prepared BaCrO₂H with a 6H-type hexagonal perovskite structure with corner- and face-sharing octahedra, offering a unique opportunity to investigate magnetic interactions based on a θ ∼ 90° case. Neutron diffraction for BaCrO₂H revealed an antiferromagnetic (AFM) order at T_N ∼ 375 K, which is higher than ∼240 K in BaCrO_3-xF_x. The relatively high T_N of BaCrO₂H can be explained by the preferred occupancy of H^- at the face-sharing site that provides AFM superexchange in addition to AFM direct exchange interactions. First-principles calculations on BaCrO₂H in comparison with BaCrO₂F and BaMnO₃ further reveal that the direct Cr-Cr interaction is significantly enhanced by shortening the Cr-Cr distance due to the covalent nature of H^-. This study provides a useful strategy for the extensive control of magnetic interactions by exploiting the difference in the covalency of multiple anions.

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.

Novel bismuth oxy hydride chromate (HBi3(CrO4)O3) nano-sheets/rods synthesized by one step one pot wet chemical method

A novel inorganic hydride material, hydrogen bismuth chromium oxide (HBi3(CrO4)O3), with 2D nano sheets and 1D nanorods were prepared for the first time using a simple, green, hazard free hydrothermal method. The X-ray diffraction (XRD) results confirms that the as-formed sample (HBi3(CrO4)O3) has monoclinic crystal system with a space group of P21/a. The field emission scanning electron microscope (FESEM) analysis clearly reveal that the new material contains large quantity of 2D nanosheets of thickness <10 nm and spread over >1000 nm and with small amounts of micro-rods of width in the range of 1 to 5 μm and lengths in the range of 40 to 100 μm. The EDS analysis confirms the presence of ‘Bi’, ‘Cr’, and ‘O’ and it further evidences the purity of the sample. The fourier transform infra-red (FT-IR) spectra evidences that the sample has Bi-H, Bi-O and Cr-O bonds as expected for HBi3(CrO4)O3. This material has a potential to find its place in hydrogen storage material, photo/electro catalysis, fuel cells, optoelectronics and rechargeable batteries, therefore it needs materials researcher attention immediately.

Diffusional Dynamics of Hydride Ions in the Layered Oxyhydride SrVO2H

Perovskite-type oxyhydrides are hydride-ion-conducting materials of promise for several types of technological applications; however, the conductivity is often too low for practical use and, on a fundamental level, the mechanism of hydride-ion diffusion remains unclear. Here, we, with the use of neutron scattering techniques, investigate the diffusional dynamics of hydride ions in the layered perovskite-type oxyhydride SrVO₂H. By monitoring the intensity of the elastically scattered neutrons upon heating the sample from 100 to 430 K, we establish an onset temperature for diffusional hydride-ion dynamics at about 250 K. Above this temperature, the hydride ions are shown to exhibit two-dimensional diffusion restricted to the hydride-ion sublattice of SrVO₂H and that occurs as a series of jumps of a hydride ion to a neighboring hydride-ion vacancy, with an enhanced rate for backward jumps due to correlation effects. Analysis of the temperature dependence of the neutron scattering data shows that the localized jumps of hydride ions are featured by a mean residence time of the order of 10 ps with an activation energy of 0.1 eV. The long-range diffusion of hydride ions occurs on the timescale of 1 ns and with an activation energy of 0.2 eV. The hydride-ion diffusion coefficient is found to be of the order of 1 × 10^-6 cm² s^-1 in the temperature range of 300-430 K, which is similar to other oxyhydrides but higher than for proton-conducting perovskite analogues. Tuning of the hydride-ion vacancy concentration in SrVO₂H thus represents a promising gateway to improve the ionic conductivity of this already highly hydride-ion-conducting material.

Hydroxyl Defects in LiFePO4 Cathode Material: DFT+U and an Experimental Study

Lithium iron phosphate, LiFePO₄, a widely used cathode material in commercial Li-ion batteries, unveils a complex defect structure, which is still being deciphered. Using a combined computational and experimental approach comprising density functional theory (DFT)+U and molecular dynamics calculations and X-ray and neutron diffraction, we provide a comprehensive characterization of various OH point defects in LiFePO₄, including their formation, dynamics, and localization in the interstitial space and at Li, Fe, and P sites. It is demonstrated that one, two, and four (five) OH groups can effectively stabilize Li, Fe, and P vacancies, respectively. The presence of D (H) at both Li and P sites for hydrothermally synthesized deuterium-enriched LiFePO₄ is confirmed by joint X-ray and neutron powder diffraction structure refinement at 5 K that also reveals a strong deficiency of P of 6%. The P occupancy decrease is explained by the formation of hydrogarnet-like P/4H and P/5H defects, which have the lowest formation energies among all considered OH defects. Molecular dynamics simulation shows a rich structural diversity of these defects, with OH groups pointing both inside and outside vacant P tetrahedra creating numerous energetically close conformers, which hinders their explicit localization with diffraction-based methods solely. The discovered conformers include structural water molecules, which are only by 0.04 eV/atom H higher in energy than separate OH defects.

A combinatory ferroelectric compound bridging simple ABO3 and A-site-ordered quadruple perovskite

The simple ABO₃ and A-site-ordered AA′₃B₄O₁₂ perovskites represent two types of classical perovskite functional materials. There are well-known simple perovskites with ferroelectric properties, while there is still no report of ferroelectricity due to symmetry breaking transition in A-site-ordered quadruple perovskites. Here we report the high pressure synthesis of an A-site-ordered perovskite PbHg₃Ti₄O₁₂, the only known quadruple perovskite that transforms from high-temperature centrosymmetric paraelectric phase to low-temperature non-centrosymmetric ferroelectric phase. The coordination chemistry of Hg²⁺ is changed from square planar as in typical A-site-ordered quadruple perovskite to a rare stereo type with 8 ligands in PbHg₃Ti₄O₁₂. Thus PbHg₃Ti₄O₁₂ appears to be a combinatory link from simple ABO₃ perovskites to A-site-ordered AA′₃Ti₄O₁₂ perovskites, sharing both displacive ferroelectricity with former and structure coordination with latter. This is the only example so far showing ferroelectricity due to symmetry breaking phase transition in AA′₃B₄O₁₂-type A-site-ordered perovskites, and opens a direction to search for ferroelectric materials.

There are few reports of ferroelectricity due to symmetry breaking transition in A-site-ordered quadruple perovskites. Here, the authors find one with phase transition from a high-temperature centrosymmetric paraelectric phase to a low-temperature non-centrosymmetric ferroelectric phase in a high pressure synthesized compound.

Exploring the anion chemical space of Ln2OF2-xClxH2 (Ln = Y, La, Gd): a model of electroelastic material with high mechanical sensitivity and energy harvesting

The main result of our investigation is the prediction of a new family of multianion compounds - Ln₂OF_2-xCl_xH₂ (Ln = Y, La, Gd) which due to anomalous elastic behaviour could present interest for the design and development of electromechanical devices. The composition Ln₂OF_2-xCl_xH₂ utilizes a complex heteroatomic anion [OF_2-xCl_xH₂]^6-; in a solid state, as it follows from the DFT calculations, the system crystallizes into a columnar-type layered structure of P3m1 or R3m trigonal symmetries in which the LnO(F,Cl)H and Ln(F,Cl)FH layers are uniformly stacked in an alternating order along the high-symmetry c axis. In the trigonal lattice without an inversion center, the resulting two-layer geometry puts groups of the anionic species together in a way that gives rise to a strong localization of valence charge density. We showed that being globally stable, such specific crystal architecture may lead to a high asymmetry of mechanical and electrical responses with respect to imposed loads. Moreover, small dynamic changes of the equilibrium charge and bonding configurations may cause rather enhanced structural sensitivity of the elastic responses at low pressures. Comparison of electromechanical characteristics showed that the predicted materials can serve as direct successors of the line of polyvinylidene fluoride (PVDF) piezopolymers.

Topotactic Hydrogen in Nickelate Superconductors and Akin Infinite-Layer Oxides ABO_{2}

Superconducting nickelates appear to be difficult to synthesize. Since the chemical reduction of ABO_{3} [rare earth (A), transition metal (B)] with CaH_{2} may result in both ABO_{2} and ABO_{2}H, we calculate the topotactic H binding energy by density functional theory (DFT). We find intercalating H to be energetically favorable for LaNiO_{2} but not for Sr-doped NdNiO_{2}. This has dramatic consequences for the electronic structure as determined by DFT+dynamical mean field theory: that of 3d^{9} LaNiO_{2} is similar to (doped) cuprates, 3d^{8} LaNiO_{2}H is a two-orbital Mott insulator. Topotactic H might hence explain why some nickelates are superconducting and others are not.

On Prediction of a Novel Chiral Material Y2H3O(OH): A Hydroxyhydride Holding Hydridic and Protonic Hydrogens

Examination of possible pathways of how oxygen atoms can be added to a yttrium oxyhydride system allowed us to predict new derivatives such as hydroxyhydrides possessing the composition M₂H₃O(OH) (M = Y, Sc, La, and Gd) in which three different anions (H^-, O²-, and OH^-) share the common chemical space. The crystal data of the solid hydroxyhydrides obtained on the base of DFT modeling correspond to the tetragonal structure that is characterized by the chiral space group P 4 1 . The analysis of bonding situation in M₂H₃O(OH) showed that the microscopic mechanism governing chemical transformations is caused by the displacements of protons which are induced by interaction with oxygen atoms incorporated into the crystal lattice of the bulk oxyhydride. The oxygen-mediated transformation causes a change in the charge state of some adjacent hydridic sites, thus forming protonic sites associated with hydroxyl groups. The predicted materials demonstrate a specific charge ordering that is associated with the chiral structural organization of the metal cations and the anions because their lattice positions form helical curves spreading along the tetragonal axis. Moreover, the effect of spatial twisting of the H^- and H⁺ sites provides additional linking via strong dihydrogen bonds. The structure-property relationships have been investigated in terms of structural, mechanical, electron, and optical features. It was shown that good polar properties of the materials make them possible prototypes for the design of nonlinear optical systems.

Trigonal-Planar Low-Spin Co2+ in a Layered Mixed-Polyhedral Network from Topotactic Reduction

Topotactic reduction of the perovskite oxide TbBaCo₂O_5.5 with CaH₂ leads to a new crystalline phase TbBaCo₂O_4.5, adopting a 2 × 2 × 1 superstructure compared to TbBaCo₂O_5.5. The structure consists of a corner-shared network of square pyramidal CoO₅ and trigonal planar CoO₃ units. Magnetic susceptibility and variable temperature neutron diffraction data reveal that TbBaCo₂O_4.5 adopts a G-type antiferromagnetically ordered structure (T_N ∼ 322 K). The ordered moments are consistent with the presence of low-spin Co²⁺ (S = ¹/₂) in trigonal-planar coordination and high-spin Co²⁺ centers in square pyramidal coordination. TbBaCo₂O_4.5 shows lower conductivity than TbBaCo₂O_5.5, which is consistent with the p-type conduction behavior. The unique anion vacancy arrangements in TbBaCo₂O_4.5 further complement the role of A-cations in controlling the oxygen vacancy distribution in LnBaCo₂O_5+δ series and demonstrate more opportunity to tune the structural and physical properties based on cationic and anionic lattice coupling.

Electric Field-Controlled Multistep Proton Evolution in H x SrCoO2.5 with Formation of H-H Dimer

Ionic evolution-induced phase transformation can lead to wide ranges of novel material functionalities with promising applications. Here, using the gating voltage during ionic liquid gating as a tuning knob, the brownmillerite SrCoO2.5 is transformed into a series of protonated H x SrCoO2.5 phases with distinct hydrogen contents. The unexpected electron to charge-neutral doping crossover along with the increase of proton concentration from x = 1 to 2 suggests the formation of exotic charge neutral H-H dimers for higher proton concentration, which is directly visualized at the vacant tetrahedron by scanning transmission electron microscopy and then further supported by first principles calculations. Although the H-H dimers cause no change of the valency of Co2+ ions, they result in clear enhancement of electronic bandgap and suppression of magnetization through lattice expansion. These results not only reveal a hydrogen chemical state beyond anion and cation within the complex oxides, but also suggest an effective pathway to design functional materials through tunable ionic evolution.

A Dimeric Hydride-Bridged Complex with Geometrically Distinct Iron Centers Giving Rise to an S = 3 Ground State

Structural and spectroscopic characterization of the dimeric iron hydride complex [Ph₂B(^tBuIm)₂FeH]₂ reveals an unusual structure in which a tetrahedral iron(II) site (S = 2) is connected to a square planar iron(II) site (S = 1) by two bridging hydride ligands. Magnetic susceptibility reveals strong ferromagnetic coupling between iron centers, with a coupling constant of J = +110(12) cm^-1, to give an S = 3 ground state. High-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy confirms this model. A qualitative molecular orbital analysis of the electronic structure, as supported by electronic structure calculations, reveals that the observed spin configuration results from the orthogonal alignment of two geometrically distinct four-coordinate iron fragments held together by highly covalent hydride ligands.

Freestanding Cubic ZrN Single-Crystalline Films with Two-Dimensional Superconductivity

The successful fabrication of freestanding two-dimensional (2D) crystals that exhibit unprecedented high crystal quality and macroscopic continuity renovates the conventional cognition that 2D long-range crystalline order cannot stably exist at finite temperatures. Current progresses are primarily limited to van der Waals (vdW) layered materials, while studies on how to obtain 2D materials from nonlayered bulk crystals remain sparse. Herein, we report the experimental realization of vdW-like cubic ZrN single crystal and emphasize the significant role of confined electrons in stabilizing the atomic structure at the 2D limit. Furthermore, the exfoliated ZrN single-crystal films with a few nanometers thick exhibit dimensional crossover effect of emerging 2D superconductivity with the unconventional upper critical field beyond Pauli paramagnetic limit, which suggests a dimensional effect in the pairing mechanism of dimensionally confined superconductors.

Advances in the LiCl salt flux method and the preparation of phase pure La2-xNdxLiHO3 (0 ≤ x ≤ 2) oxyhydrides

The LiCl salt flux method is an established aid in oxyhydride synthesis. By operating the flux below its melting point, we have obtained phase pure Nd2LiHO3 for the first time. Further, the suitability of the flux method is shown to be dictated by a delicate balance between the thermal stability of the oxyhydride in question and the ionic mobility of the reactants.

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.

Neutron Scattering Investigations of Hydride Species in Heterogeneous Catalysis

In heterogeneous catalysis, hydrides on the surface or in the bulk play a critical role as either active components or reaction intermediates in many hydrogen-involving reactions, but characterization of the nature and structure of these hydride species remains challenging. Neutron scattering, which is extremely sensitive to light elements, such as hydrogen, has shown great potential in meeting this challenge. In this Minireview, recent advances in neutron studies of hydride species, mainly over the two most typical classes of catalysts-metals and oxides-are surveyed. Findings on catalysts outside these categories are raised if they are considered to be relevant for contextualization in the present Minireview. The adsorption, dissociation, spillover, and reactivity of hydrogen, especially hydride species over supported metal and oxide catalysts, have been successfully investigated, mostly by means of neutron vibrational spectroscopy. Insights from these neutron studies, which are otherwise not possible with other techniques, shed light on the interaction mechanism of hydrogen with solid surfaces and reaction mechanisms in which hydrogen is involved. Future research challenges on neutron scattering studies of hydrides, as well as catalysis in general, are also highlighted, and more operando-type neutron studies need be conducted to advance the field.

Rhodium-containing oxide-hydrides: covalently stabilized mixed-anion solids

The first rhodium-containing oxide-hydride phases, LaSrCo0.5Rh0.5O3H and La0.5Sr1.5Mn0.5Rh0.5O3H, have been prepared via topochemical anion exchange. This clearly demonstrates the ability of rhodium, a late 4d transition metal, to kinetically stabilize oxide-hydride lattices, reinforcing the paradigm of covalent stabilization of transition-metal oxide-hydride phases.

Progress toward Solid State Synthesis by Design

Ages of history are defined by the underlying materials that promoted human development: stone, bronze, and iron ages. Since the middle of the last century, humanity has lived in a silicon age, where the development of the transistor ushered in new technologies previously thought inconceivable. But as technology has advanced, so have the requirements for new materials to sustain increasing physical demands. The field of solid state chemistry is dedicated to the discovery of new materials and phenomena, and though most materials discoveries in history have been through serendipity rather than careful reaction design, the last few decades have seen an increase in the number of materials discovered through a consideration of chemical reaction kinetics and thermodynamics. Materials by design have changed the way solid state chemists approach the synthesis of possible materials with interesting and useful properties. Unlike other chemistry subfields such as organic chemistry and biochemistry, solid state chemistry does not currently benefit from a toolbox of reactions that can allow for the synthesis of any arbitrary material. The diversity and complexity of the solid state phase space likely inhibits chemists from ever having such a toolbox. However, a thorough understanding of the various synthetic techniques involved in the synthesis of stable and metastable solids may be realized through an understanding of the reaction kinetics and thermodynamics. In the Account, we review the common synthesis techniques involved in the formation of metastable materials and break down their underlying chemistry to the simplest reaction mechanisms involved. The synthesis reactions of most metastable materials can be understood through these three reaction driving parameters, which include the exploitation of Le Chatelier's principle, thermo-kinetic reaction coupling, and lowering the activation energy of formation of the metastable product, and we identify several materials whose syntheses are described either by one or a combination of these driving parameters. We identify what exists at the frontier of materials discovery by design, including novel applications of supercritical fluids for tuning between "gas" and "solvent"-like environments. While conventional solvation requires changes in either the temperature or composition of the system, supercritical fluid solvation requires only changes in the fluid density, which opens up the possibilities for the synthesis of new materials. Most importantly, however, we look toward the future of materials synthesis by design and see that it must be a collaborative one. At present, chemists design materials using knowledge about chemical structure and reactivity but often target specific materials with very specific properties. In contrast, computational chemists perform calculations on millions of different elemental combinations and find many candidates of possible materials with interesting properties, though most of these are not realizable synthetically due to limitations in reactivity, kinetics, or thermodynamics. Synthetic harmony can be achieved through active collaboration and communication between these two subfields of chemistry, such that new calculations can incorporate complete knowledge about reaction kinetics and thermodynamics, and new syntheses target computationally predicted materials derived from an understanding of mapped reaction landscapes.

Hydride Reduction of BaTiO3 - Oxyhydride Versus O Vacancy Formation

We investigated the hydride reduction of tetragonal BaTiO₃ using the metal hydrides CaH₂, NaH, MgH₂, NaBH₄, and NaAlH₄. The reactions employed molar BaTiO₃/H ratios of up to 1.8 and temperatures near 600 °C. The air-stable reduced products were characterized by powder X-ray diffraction (PXRD), transmission electron microscopy, thermogravimetric analysis (TGA), and ¹H magic angle spinning (MAS) NMR spectroscopy. PXRD showed the formation of cubic products-indicative of the formation of BaTiO_3-x H _x -except for NaH. Lattice parameters were in a range between 4.005 Å (for NaBH₄-reduced samples) and 4.033 Å (for MgH₂-reduced samples). With increasing H/BaTiO₃ ratio, CaH₂-, NaAlH₄-, and MgH₂-reduced samples were afforded as two-phase mixtures. TGA in air flow showed significant weight increases of up to 3.5% for reduced BaTiO₃, suggesting that metal hydride reduction yielded oxyhydrides BaTiO_3-x H _x with x values larger than 0.5. ¹H MAS NMR spectroscopy, however, revealed rather low concentrations of H and thus a simultaneous presence of O vacancies in reduced BaTiO₃. It has to be concluded that hydride reduction of BaTiO₃ yields complex disordered materials BaTiO_3-x H _y □_(x-y) with x up to 0.6 and y in a range 0.04-0.25, rather than homogeneous solid solutions BaTiO_3-x H _x . Resonances of (hydridic) H substituting O in the cubic perovskite structure appear in the -2 to -60 ppm spectral region. The large range of negative chemical shifts and breadth of the signals signifies metallic conductivity and structural disorder in BaTiO_3-x H _y □_(x-y). Sintering of BaTiO_3-x H _y □_(x-y) in a gaseous H₂ atmosphere resulted in more ordered materials, as indicated by considerably sharper ¹H resonances.

Mixed X-Site Formate-Hypophosphite Hybrid Perovskites

Following the recent discovery of a new family of hybrid ABX₃ perovskites where X=(H₂ POO)^- (hypophosphite), this work reports a facile synthesis for mixed X-site formate perovskites of composition [GUA]Mn(HCOO)_3-x (H₂ POO)_x , with two crystallographically distinct, partially ordered intermediate phases with x=0.84 and 1.53, corresponding to ca. 30 and 50 mol % hypophosphite, respectively. These phases are characterised by single-crystal XRD and solid-state NMR spectroscopy, and their magnetic properties are reported.

Transformation of Perovskite BaBiO3 into Layered BaBiO2.5 Crystals Featuring Unusual Chemical Bonding and Luminescence

Engineering oxygen coordination environments of cations in oxides has received intense interest thanks to the opportunities for the discovery of novel oxides with unusual properties. Herein, the synthesis of stoichiometric layered BaBiO_2.5 by a nontopotactic phase transformation of perovskite BaBiO₃ is presented. By analyzing the synchrotron X-ray diffraction data by the maximum-entropy method/Rietveld technique, it was found that Bi is involved in an unusual chemical bonding situation with four oxygen atoms featuring one ionic bond and three covalent bonds, which results in an asymmetric coordination geometry. Photophysical characterization revealed that this peculiar structure shows near-infrared luminescence differing from that of conventional Bi-containing compounds. Experimental and theoretical results led to the proposal of an excitonic nature of the luminescence. This work highlights that synthesizing materials with uncommon Bi-O bonding and Bi coordination geometry provides a pathway to the discovery of systems with new functionalities. This could inspire interest in the exploration of a range of materials containing heavier p-block elements with prospects for finding systems with unusual properties.

LaSr3 NiRuO4 H4 : A 4d Transition-Metal Oxide-Hydride Containing Metal Hydride Sheets

The synthesis of the first 4d transition metal oxide-hydride, LaSr₃ NiRuO₄ H₄ , is prepared via topochemical anion exchange. Neutron diffraction data show that the hydride ions occupy the equatorial anion sites in the host lattice and as a result the Ru and Ni cations are located in a plane containing only hydride ligands, a unique structural feature with obvious parallels to the CuO₂ sheets present in the superconducting cuprates. DFT calculations confirm the presence of S=1/2  Ni⁺ and S=0, Ru²⁺ centers, but neutron diffraction and μSR data show no evidence for long-range magnetic order between the Ni centers down to 1.8 K. The observed weak inter-cation magnetic coupling can be attributed to poor overlap between Ni 3dz2 and H 1s in the super-exchange pathways.

Expanding frontiers in materials chemistry and physics with multiple anions

During the last century, inorganic oxide compounds laid foundations for materials synthesis, characterization, and technology translation by adding new functions into devices previously dominated by main-group element semiconductor compounds. Today, compounds with multiple anions beyond the single-oxide ion, such as oxyhalides and oxyhydrides, offer a new materials platform from which superior functionality may arise. Here we review the recent progress, status, and future prospects and challenges facing the development and deployment of mixed-anion compounds, focusing mainly on oxide-derived materials. We devote attention to the crucial roles that multiple anions play during synthesis, characterization, and in the physical properties of these materials. We discuss the opportunities enabled by recent advances in synthetic approaches for design of both local and overall structure, state-of-the-art characterization techniques to distinguish unique structural and chemical states, and chemical/physical properties emerging from the synergy of multiple anions for catalysis, energy conversion, and electronic materials.