Pressure-Stabilized Cubic Perovskite Oxyhydride BaScO2H

The spectroscopy of hydride in single crystals of SrTiO3 perovskite

Under reducing conditions, SrTiO3 perovskite can exchange up to 20% of its O2- ions for H- (hydride), greatly influencing its material properties. This not only presents intriguing possibilities for material design, but also for hydrogen sequestration in the deep earth, where perovskite-structured minerals are abundant. However, uncertainties remain surrounding hydride incorporation in SrTiO3, including details of the hydride structural state, and how hydride interacts with the broader defect chemistry of SrTiO3. Additionally, experimental studies of hydride in SrTiO3 and other perovskites may face analytical limitations. The most common methods for characterizing hydride, namely 1H NMR, may not be suitable in all experimental contexts, including materials with relatively low hydride concentrations and in situ high-pressure, high-temperature experiments. Here, we present an investigation of hydride in single crystals of SrTiO3 focused on detailed spectroscopic measurements. Through a combination of density functional theory (DFT)-assisted Fourier transform infrared (FTIR) spectroscopy and UV-vis spectroscopy, we observe structural hydride and its effects on the electronic transitions in SrTiO3. These results are compared directly against 1H NMR. We find that, although hydride is sometimes difficult to identify via FTIR, infrared spectroscopy is significantly more sensitive to hydride than 1H NMR. We also find that DFT makes accurate predictions about the spectroscopic behavior of hydride in SrTiO3, pointing to the value of ab initio techniques in future studies.

Mechanochemical Synthesis of Perovskite Oxyhydrides: Insights from Shear Modulus

Perovskite oxyhydrides have attracted recent attention due to their intriguing properties such as ionic conductivity and catalysis, but their repertoire is still restricted compared to perovskite oxynitrides and oxyfluorides. Historically, perovskite oxyhydrides have been prepared mostly by topochemical reactions and high-pressure (HP) reactions, while in this study, we employed a mechanochemical (MC) approach, which enables the synthesis of a series of ABO2H-type oxyhydrides, including those with the tolerance factor (t) much smaller than 1 (e.g., SrScO2H with t = 0.936) which cannot be obtained by HP synthesis. The octahedral tilting, often present in perovskite oxides, does not occur, suggesting that the lack of π-symmetry of the H 1s orbital and the large polarization destabilize tilted low-symmetry structures. Interestingly, SrCrO2H (t = 0.997), previously reported with the HP method, was not achieved with the MC method. A comparative analysis revealed a correlation between the feasibility of MC reactions and the (calculated) shear modulus of the starting reagents (binary oxides and hydrides). Notably, this indicator is not exclusive to oxyhydride perovskites but extends to oxide perovskites (SrMO3). This study demonstrates that MC synthesis offers unique opportunities not only to expand the compositional space in oxyhydrides in various structural types but also to provide a guide for the choice of starting materials for the synthesis of other compounds.

Twists and Puckers: Tuning Crystal Chemistry in the La(AuxGe1-x)2 Compositional Series

The physical properties of solid-state materials are closely tied to their crystal structure, yet our understanding of how competing structural arrangements energetically compare is limited. In this work, we explore how small differences in composition affect the structure in the La(AuxGe1-x)2 series of compounds, which comprises four unique structure types between LaGe2 and LaAu2. This family includes the previously unknown AlB2-type compound with stoichiometry La(Au0.375Ge0.625)2 as well as La(Au0.25Ge0.75)2, an intergrowth of the AlB2 and ThSi2 structure types. We then study the chemical forces driving the structure changes and use phonon band structure calculations and DFT-Chemical Pressure to evaluate atomic-size effects. These calculations show that the parent AlB2 structure type is disfavored in Au-rich compounds due to soft atomic motions along the c axis. The instability of AlB2-type LaAuGe is confirmed by the presence of imaginary modes in the phonon band structure that correspond to a "puckering" of the hexagonal AlB2-type lattice, resulting in the experimentally observed LiGaGe structure type. The impact of size effects is less clear for Au-poor compositions; instead, twisting the AlB2 structure type to form the ThSi2 type opens a pseudogap at the Fermi level in the electronic density of states. This investigation demonstrates how crystal structure in solid-state materials can be compositionally tuned based on balancing size and electronics when multiple structure types are in close thermodynamic competition.

Potassium-rich antiperovskites K3HTe and K3FTe and their structural relation to lithium and sodium counterparts

Unlike perovskite oxides, antiperovskites M₃HCh and M₃FCh (M = Li, Na; Ch = S, Se, Te) mostly retain their ideal cubic structure over a wide range of compositions owing to anionic size flexibility and low-energy phonon modes that promote their ionic conductivity. In this study, we show the synthesis of potassium-based antiperovskites K₃HTe and K₃FTe and discuss the structural features in comparison with lithium and sodium analogues. It is shown experimentally and theoretically that both compounds maintain a cubic symmetry and can be prepared at ambient pressure, in contrast to most of the reported M₃HCh and M₃FCh which require high pressure synthesis. A systematic comparison of a series of cubic M₃HTe and M₃FTe (M = Li, Na, K) revealed that telluride anions contract in the order of K, Na, Li, with a pronounced contraction in the Li system. This result can be understood in terms of the difference in charge density of alkali metal ions as well as the size flexibility of Ch anions, contributing to the stability of the cubic symmetry.

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.

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.

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.

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.

High-Pressure and High-Temperature Synthesis of Anion-Disordered Vanadium Perovskite Oxyhydrides

Crystallographic order-disorder phenomena in solid state compounds are of fundamental interest due to intimate relationship between the structure and properties. Here, by using high-pressure and high-temperature synthesis, we obtained vanadium perovskite oxyhydrides Sr_1-xNa_xVO_3-yH_y (x = 0, 0.05, 0.1, 0.2) with an anion-disordered structure, which is different from anion-ordered SrVO₂H synthesized by topochemical reduction. High-pressure and high-temperature synthesis from nominal composition SrVO₂H yielded the anion-disordered perovskite SrVO_3-yH_y (y ∼ 0.4) with a significant amount of byproducts, while Na substitution resulted in the almost pure anion-disordered perovskite Sr_1-xNa_xVO_3-yH_y with an increased amount of hydride anion (y ∼ 0.7 for x = 0.2). The obtained disordered phases for x = 0.1 and 0.2 are paramagnetic with almost temperature-independent electronic conductivity, whereas anion-ordered SrVO₂H is an antiferromagnetic insulator. Although we obtained the anion-disordered perovskite under high pressure, a first-principles calculation revealed that the application of pressure stabilizes the ordered phase due to a reduced volume in the ordered structure, suggesting that a further increase of the pressure or reduction of the reaction temperature leads to the anion ordering. This study shows that anion ordering in oxyhydrides can be controlled by changing synthetic pressure and temperature.

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.

Designing disorder into crystalline materials

Crystals are a state of matter characterized by periodic order. Yet, crystalline materials can harbour disorder in many guises, such as non-repeating variations in composition, atom displacements, bonding arrangements, molecular orientations, conformations, charge states, orbital occupancies or magnetic structure. Disorder can sometimes be random but, more usually, it is correlated. Frontier research into disordered crystals now seeks to control and exploit the unusual patterns that persist within these correlated disordered states in order to access functional responses inaccessible to conventional crystals. In this Review, we survey the core design principles that guide targeted control over correlated disorder. We show how these principles - often informed by long-studied statistical mechanical models - can be applied across an unexpectedly broad range of materials, including organics, supramolecular assemblies, oxide ceramics and metal-organic frameworks. We conclude with a forward-looking discussion of the exciting link between disorder and function in responsive media, thermoelectrics and topological phases.

Synthesis and H- conductivity of a new oxyhydride Ba2YHO3 with anion-ordered rock-salt layers

A new layered perovskite oxyhydride Ba₂YHO₃ was synthesized via high pressure synthesis. Powder X-ray and neutron diffraction experiments revealed that in Ba₂YHO₃ the ordered H^- and O^2- anions form [Ba₂H₂] rock-salt layers wherein H^- conduction is allowed. The conductivity reached 0.1 mS cm^-1 at 350 °C, which is higher than that of isostructural Ba₂ScHO₃ with anion-disordered [Ba₂HO] layers.

A Partial Anion Disorder in SrVO2H Induced by Biaxial Tensile Strain

SrVO2H, obtained by a topochemical reaction of SrVO3 perovskite using CaH2, is an anion-ordered phase with hydride anions exclusively at the apical site. In this study, we conducted a CaH2 reduction of SrVO3 thin films epitaxially grown on KTaO3 (KTO) substrates. When reacted at 530 °C for 12 h, we observed an intermediate phase characterized by a smaller tetragonality of c/a = 0.96 (vs. c/a = 0.93 for SrVO2H), while a longer reaction of 24 h resulted in the known phase of SrVO2H. This fact suggests that the intermediate phase is a metastable state stabilized by applying tensile strain from the KTO substrate (1.4%). In addition, secondary ion mass spectrometry (SIMS) revealed that the intermediate phase has a hydrogen content close to that of SrVO2H, suggesting a partially disordered anion arrangement. Such kinetic trapping of an intermediate state by biaxial epitaxial strain not only helps to acquire a new state of matter but also advances our understanding of topochemical reaction processes in extended solids.

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.

A Picture of Disorder in Hydrous Wadsleyite-Under the Combined Microscope of Solid-State NMR Spectroscopy and Ab Initio Random Structure Searching

The Earth’s transition zone, at depths of 410–660 km, while being composed of nominally anhydrous magnesium silicate minerals, may be subject to significant hydration. Little is known about the mechanism of hydration, despite the vital role this plays in the physical and chemical properties of the mantle, leading to a need for improved structural characterization. Here we present an ab initio random structure searching (AIRSS) investigation of semihydrous (1.65 wt % H₂O) and fully hydrous (3.3 wt % H₂O) wadsleyite. Following the AIRSS process, k-means clustering was used to select sets of structures with duplicates removed, which were then subjected to further geometry optimization with tighter constraints prior to NMR calculations. Semihydrous models identify a ground-state structure (Mg3 vacancies, O1–H hydroxyls) that aligns with a number of previous experimental observations. However, predicted NMR parameters fail to reproduce low-intensity signals observed in solid-state NMR spectra. In contrast, the fully hydrous models produced by AIRSS, which enable both isolated and clustered defects, are able to explain observed NMR signals via just four low-enthalpy structures: (i) a ground state, with isolated Mg3 vacancies and O1–H hydroxyls; (ii/iii) edge-sharing Mg3 vacancies with O1–H and O3–H species; and (iv) edge-sharing Mg1 and Mg3 vacancies with O1–H, O3–H, and O4–H hydroxyls. Thus, the combination of advanced structure searching approaches and solid-state NMR spectroscopy is able to provide new and detailed insight into the structure of this important mantle mineral.

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.

AgFeOF2: A Fluorine-Rich Perovskite Oxyfluoride

We synthesized a silver iron oxyfluoride AgFeOF₂ by using a high-pressure reaction. Synchrotron X-ray and neutron diffraction, X-ray absorption, and ⁵⁷Fe Mössbauer spectroscopy indicate that AgFeOF₂ crystallizes in the ideal perovskite structure with iron in a trivalent state, although electron microscopy revealed weak super-reflections. A possible partial ordering in the FeO₂F₄ octahedron is inferred from Mössbauer spectroscopy. The synthesis of the fluorine-rich sample offers an opportunity to study a composition-property relation in AFe^IIIO_{3- n}F _n ( n = 0, 1, and 2). AgFeOF₂ exhibits a G-type antiferromagnetic ordering below T_N ≈ 480 K, which is much lower than the n = 0 and 1 cases, suggesting a weaker superexchange interaction between Fe moments via F 2p orbitals (vs O 2p orbitals).

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.

New chemistry of transition metal oxyhydrides

In this review we describe recent advances in transition metal oxyhydride chemistry obtained by topochemical routes, such as low temperature reduction with metal hydrides, or high-pressure solid-state reactions. Besides the crystal chemistry, magnetic and transport properties of the bulk powder and epitaxial thin film samples, the remarkable lability of the hydride anion is particularly highlighted as a new strategy to discover unprecedented mixed anion materials.