Topochemical Nitridation with Anion Vacancy-Assisted N3–/O2– Exchange

High-Temperature Synthesis of Ferromagnetic Eu3Ta3(O,N)9 with a Triple Perovskite Structure

Europium tantalum perovskite oxynitrides were prepared by a new high-temperature solid-state synthesis under N2 or N2/H2 gas. The nitrogen stoichiometry was tuned from 0.63 to 1.78 atoms per Eu or Ta atom, starting with appropriate N/O ratios in the mixture of the reactants Eu2O3, EuN and Ta3N5, or Eu2O3 and TaON, which was treated at 1200 °C for 3 h. Two phases were isolated with compositions EuTaO2.37N0.63 and Eu3Ta3O3.66N5.34, showing different crystal structures and magnetic properties. Electron diffraction and Rietveld refinement of synchrotron radiation X-ray diffraction indicated that EuTaO2.37N0.63 is a simple perovskite with cubic Pm3̅m structure and cell parameter a = 4.02043(1) Å, whereas the new compound Eu3Ta3O3.66N5.34 is the first example of a triple perovskite oxynitride and shows space group P4/mmm with crystal parameters a = 3.99610(2), c = 11.96238(9) Å. The tripling of the c-axis in this phase is a consequence of the partial ordering of europium atoms with different charges in two A sites of the perovskite structure with relative ratio 2:1, where the formal oxidation states +3 and +2 are respectively dominant. Magnetic data provide evidence of ferromagnetic ordering developing at low temperatures in both oxynitrides, with saturation magnetization of about 6 μB and 3 μB per Eu ion for EuTaO2.37N0.63 and the triple perovskite Eu3Ta3O3.66N5.34 respectively, and corresponding Curie temperatures of about 7 and 3 K, which is in agreement with the lower proportion of Eu2+ in the latter compound.

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.

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.

Fluorine-Assisted Low-Temperature Synthesis of GaN:ZnO-Related Solid Solutions with Visible-Light Photoresponse

Wurtzite-structured Ga_1-xZn_x(N,O,F) was successfully synthesized by nitridation of mixtures of a Ga-containing oxide and ZnF₂. The addition of ZnF₂ lowered the nitridation temperature for the synthesis of Ga_1-xZn_x(N,O,F) to 823 K, even when bulk ZnGa₂O₄ was used as a paired precursor. This lowering of the synthesis temperature was ascribed to the enhancement of nitridation through the addition of fluorine. The low-temperature nitridation achieved by the addition of fluorine suppressed the volatilization of Zn compared with that during the synthesis of a GaN:ZnO solid solution by a conventional high-temperature ammonolysis reaction. The higher concentration of Zn, as well as the higher N concentration in Ga_1-xZn_x(N,O,F) achieved through the fluorine-assisted nitridation, led to a redshift of the absorption edge of Ga_1-xZn_x(N,O,F) to 560 nm compared with that of GaN:ZnO synthesized by the conventional ammonolysis reaction. The visible-light absorption of Ga_1-xZn_x(N,O,F) can be used to drive the photoelectrochemical oxidation of water.

Identification of a metastable uranium metal–organic framework isomer through non-equilibrium synthesis

Since the structure of supramolecular isomers determines their performance, rational synthesis of a specific isomer hinges on understanding the energetic relationships between isomeric possibilities. To this end, we have systematically interrogated a pair of uranium-based metal–organic framework topological isomers both synthetically and through density functional theory (DFT) energetic calculations. Although synthetic and energetic data initially appeared to mismatch, we assigned this phenomenon to the appearance of a metastable isomer, driven by levers defined by Le Châtelier's principle. Identifying the relationship between structure and energetics in this study reveals how non-equilibrium synthetic conditions can be used as a strategy to target metastable MOFs. Additionally, this study demonstrates how defined MOF design rules may enable access to products within the energetic phase space which are more complex than conventional binary (e.g., kinetic vs. thermodynamic) products.

Identifying the relationship between structure and energetics in a uranium MOF isomer system reveals how non-equilibrium synthetic conditions can be used as a strategy to target metastable MOFs.

Rank and signature classification of topochemical solid-state reactions

Solid-state reactions are classified into 10 categories based on the unit cells of the reactant and product.

Strain-induced creation and switching of anion vacancy layers in perovskite oxynitrides

Perovskite oxides can host various anion-vacancy orders, which greatly change their properties, but the order pattern is still difficult to manipulate. Separately, lattice strain between thin film oxides and a substrate induces improved functions and novel states of matter, while little attention has been paid to changes in chemical composition. Here we combine these two aspects to achieve strain-induced creation and switching of anion-vacancy patterns in perovskite films. Epitaxial SrVO₃ films are topochemically converted to anion-deficient oxynitrides by ammonia treatment, where the direction or periodicity of defect planes is altered depending on the substrate employed, unlike the known change in crystal orientation. First-principles calculations verified its biaxial strain effect. Like oxide heterostructures, the oxynitride has a superlattice of insulating and metallic blocks. Given the abundance of perovskite families, this study provides new opportunities to design superlattices by chemically modifying simple perovskite oxides with tunable anion-vacancy patterns through epitaxial lattice strain.

Band Gap Adjustment in Perovskite-type Eu1−x Ca x TiO3 via Ammonolysis

Perovskite-type oxynitrides AB(O,N)3 are potential candidates for photoelectrode materials in solar water splitting. A drawback of these materials is their low sintering tendency resulting in low electrical conductivities. Typically, they are prepared by ammonia treatment of insulating, wide band gap oxides. In this study, we propose an approach starting from small band gap oxides Eu1−x Ca x TiO3− δ and then widen the band gaps in a controlled way by ammonolysis and partial Ca2+ substitution. Both together induced a distortion of the octahedral network and dilution of the Eu4f and N2p levels in the valence band. The effect is the stronger the more Ca2+ is present. Within the series of samples, Eu0.4Ca0.6Ti(O,N)3 had the most suitable optical band gap (EG ≈ 2.2 eV) for water oxidation. However, its higher Eu content compared to Eu0.1Ca0.9Ti(O,N)3 slowed down the charge carrier dynamics due to enhanced trapping and recombination as expressed by large accumulation (τ on) and decay (τ off) times of the photovoltage of up to 109 s and 486 s, respectively. In contrast, the highly Ca2+-substituted samples (x ≥ 0.7) were more prone to formation of TiN and oxygen vacancies also leading to Ti3+ donor levels below the conduction band. Therefore, a precise control of the ammonolysis temperature is essential, since even small amounts of TiN can suppress the photovoltage generation by fast recombination processes. Water oxidation tests on Eu0.4Ca0.6Ti(O,N)3 revealed a formation of 7.5 μmol O2 from 50 mg powder together with significant photocorrosion of the bare material. Combining crystal structure, chemical composition, and optical and electronical band gap data, a first simplified model of the electronical band structure of Eu1−x Ca x Ti(O,N)3 could be proposed.

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.

Two-Dimensional-Layered Perovskite ALaTa2O7:Bi3+ (A = K and Na) Phosphors with Versatile Structures and Tunable Photoluminescence

Topological chemical reaction methods are indispensable for fabricating new materials or optimizing their functional properties, which is particularly important for two-dimensional (2D)-layered compounds with versatile structures. Herein, we demonstrate a low-temperature (∼350 °C) ion exchange approach to prefabricate metastable phosphors ALa_{1- x}Ta₂O₇: xBi³⁺ (A = K and Na) with RbLa_{1- x}Ta₂O₇: xBi³⁺ serving as precursors. The as-prepared ALa_0.98Ta₂O₇:0.02 Bi³⁺ (A = Rb, K, and Na) share the same Dion-Jacobson type 2D-layered perovskite phase, and photoluminescence analyses show that ALa_0.98Ta₂O₇:0.02 Bi³⁺ (A = Rb, K, and Na) phosphors exhibit broad emission bands peaking at 540, 550, and 510 nm, respectively, which are attributed to the nonradiative transition of Bi³⁺ from excited state ³P₁ or ³P₀ to ground state ¹S₀. The various Bi³⁺ local environments at the crystallographic sites enable the different distributions of emission and excitation spectra, and the photoluminescence tuning of ALa_0.98Ta₂O₇:0.02 Bi³⁺ (A = Rb, K, and Na) phosphors are realized through alkali metal ion exchange. Notably, the combination of superior trivalent bismuth emission and low-temperature ion exchange synthesis leads to a novel yellow-emitting K(La_0.98Bi_0.02)Ta₂O₇ phosphor which is successfully applied in a white LED device based on a commercially available 365 nm LED chip. Our realizable cases of this low-temperature ion exchange strategy could promote exploration into metastable phosphors with intriguing properties.

Air-Stable Porous Fe2N Encapsulated in Carbon Microboxes with High Volumetric Lithium Storage Capacity and a Long Cycle Life

The development of inexpensive electrode materials with a high volumetric capacity and long cycle-life is a central issue for large-scale lithium-ion batteries. Here, we report a nanostructured porous Fe₂N anode fully encapsulated in carbon microboxes (Fe₂N@C) prepared through a facile confined anion conversion from polymer coated Fe₂O₃ microcubes. The resulting carbon microboxes could not only protect the air-sensitive Fe₂N from oxidation but also retain thin and stable SEI layer. The appropriate internal voids in the Fe₂N cubes help to release the volume expansion during lithiation/delithiation processes, and Fe₂N is kept inside the carbon microboxes without breaking the shell, resulting in a very low electrode volume expansion (the electrode thickness variation upon lithiation is ∼9%). Therefore, the Fe₂N@C electrodes maintain high volumetric capacity (1030 mA h cm^-3 based on the lithiation-state electrode volume) comparable to silicon anodes, stable cycling performance (a capacity retention of over 91% for 2500 cycles), and excellent rate performance. Kinetic analysis reveals that the Fe₂N@C shows an enhanced contribution of capacitive charge mechanism and displays typical pseudocapacitive behavior. This work provides a new direction on designing and constructing nanostructured electrodes and protective layer for air unstable conversion materials for potential applications as a lithium-ion battery/capacitor electrode.

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.

Circumventing Diffusion in Kinetically Controlled Solid-State Metathesis Reactions

Solid-state diffusion is often the primary limitation in the synthesis of crystalline inorganic materials and prevents the potential discovery and isolation of new materials that may not be the most stable with respect to the reaction conditions. Synthetic approaches that circumvent diffusion in solid-state reactions are rare and often allow the formation of metastable products. To this end, we present an in situ study of the solid-state metathesis reactions MCl2 + Na2S2 → MS2 + 2 NaCl (M = Fe, Co, Ni) using synchrotron powder X-ray diffraction and differential scanning calorimetry. Depending on the preparation method of the reaction, either combining the reactants in an air-free environment or grinding homogeneously in air before annealing, the barrier to product formation, and therefore reaction pathway, can be altered. In the air-free reactions, the product formation appears to be diffusion limited, with a number of intermediate phases observed before formation of the MS2 product. However, grinding the reactants in air allows NaCl to form directly without annealing and displaces the corresponding metal and sulfide ions into an amorphous matrix, as confirmed by pair distribution function analysis. Heating this mixture yields direct nucleation of the MS2 phase and avoids all crystalline binary intermediates. Grinding in air also dissipates a large amount of lattice energy via the formation of NaCl, and the crystallization of the metal sulfide is a much less exothermic process. This approach has the potential to allow formation of a range of binary, ternary, or higher-ordered compounds to be synthesized in the bulk, while avoiding the formation of many binary intermediates that may otherwise form in a diffusion-limited reaction.