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We report on the hydride (H–) conductivity in fluorite-type LnHO oxyhydrides (Ln = lanthanide) using samples prepared under high pressure. It is found that, despite its “stoichiometric” composition...

A growing number of transition metal oxyhydrides have recently been reported, but they are all confined to perovskite-related structures with corner-shared octahedra. Using high pressure synthesis, we have obtained vanadium oxyhydrides BaVO3–xHx (0.3 ≤ x ≤ 0.8) with a 6H-type hexagonal layer structure consisting of face-shared as well as corner-shared octahedra. Synchrotron X-ray and neutron diffraction measurements revealed that, in BaVO2.7H0.3, H– anions are located selectively at the face-shared sites, as supported by DFT calculations, while BaVO2.2H0.8 contains H– anions at both sites though the face-shared preference is partially retained. The selective hydride occupation for BaVO2.7H0.3 appears to suppress electron hopping along the c axis, making this material a quasi-two-dimensional metal characterized by anomalous temperature dependence of the electrical resistivity and strong antiferromagnetic fluctuations. In contrast, the anion disordered BaVO3–xHx in hexagonal (x ≈ 0.8) and cubic (x ≈ 0.9) fo...

Perovskite oxyhydrides may find diverse applications, ranging from catalysis, topochemical synthesis to solid state ionics, but the understanding of their hydride transport behavior has remained limited. Here, gaseous hydrogen exchange and release experiments were analyzed using the Kissinger method to estimate the activation energy (Ea) for H/D exchange and H2 release in BaTiO3–xHx (x = 0.35–0.60) and LaSrCoO3H0.70. It is revealed that, for each BaTiO3–xHx at a given hydride concentration (x), both H/D exchange and H2 release experiments provide similar Ea values. For BaTiO3–xHx with different x, the obtained Ea values significantly decrease with increasing x until around 0.4; beyond 0.4, it becomes nearly constant (200–220 kJ mol–1). This observation suggests that the diffusion process in the low hydride concentration (x < 0.4) includes oxide as well as hydride diffusion, whereas, for 0.4 < x (

Quality oxide-ion conductors are essential for clean-energy applications. Rare-earth-stabilized bismuth sesquioxide, δ-Bi2O3, exhibits a much greater oxide-ion conductivity at high temperatures than commonly used ZrO2- or CeO2-based electrolytes, but it suffers from serious conductivity degradation while annealing at moderate temperatures of ∼773 K, which is the target temperature for many applications. Here, we demonstrate that a novel set of solute elements for δ-Bi2O3 can significantly enhance the long-term stability at 773 K. A pure oxide-ion conductivity of 0.035 S/cm at 773 K remains unchanged during annealing for 100 h, which is five times greater than the best known solid-state oxide materials after long-term annealing. For materials design, we explore a range of chemical spaces using theoretical methods based on first-principles calculations. The order–disorder transition temperature of the anion sublattice, oxygen-ion diffusivity, and solution free energy are used as descriptors. The design conc...

The main approach for exploring metastable materials is via trial‐and‐error synthesis, and there is limited understanding of how metastable materials are kinetically stabilized. In this study, a metastable phase superionic conductor, β‐Li3YCl6, is discovered through in situ X‐ray diffraction after heating a mixture of LiCl and YCl3 powders. While Cl− arrangement is represented as a hexagonal close packed structure in both metastable β‐Li3YCl6 synthesized below 600 K and stable α‐Li3YCl6 above 600 K, the arrangement of Li+ and Y3+ in β‐Li3YCl6 determined by neutron diffraction brought about the cell with a 1/√3 a‐axis and a similar c‐axis of stable α‐Li3YCl6. Higher Li+ ion conductivity and lower activation energy for Li+ transport are observed in comparison with α‐Li3YCl6. The computationally calculated low migration barrier of Li+ supports the low activation energy for Li+ conduction, and the calculated high migration barrier of Y3+ kinetically stabilizes this metastable phase by impeding phase transformation to α‐Li3YCl6. This work shows that the combination of in situ observation of solid‐state reactions and computation of the migration energy can facilitate the comprehension of the solid‐state reactions allowing kinetic stabilization of metastable materials, and can enable the discovery of new metastable materials in a short time., A metastable superionic conductor, β‐Li3YCl6, is synthesized by heating a mixture of LiCl and YCl3 powders. The computationally calculated low migration barrier of Li+ supports the low activation energy for Li+ conduction, and the calculated high migration barrier of Y3+ kinetically stabilizes this metastable phase by impeding phase transformation to thermodynamically stable α‐Li3YCl6.

Commercial pure Ti alloys are favorable for biomedical applications; however, they need to be strengthened by the addition of alloying elements. Herein, Ti-Fe alloys modified with Zr were explored to produce alloys that exhibit high strength and ductility and are biocompatible. Ti-Fe extruded materials with Zr solutes were prepared by combining solid sintering and hot extrusion. X-ray diffraction results revealed that the lattice constant of α-Ti and β-Ti increased proportionally with the Zr content, indicating the Zr solid solution behavior. In situ and ex situ observations of the microstructure formation revealed the element Zr, a weak β-stabilizer, and a decreased phase transformation temperature. However, during the cooling process after hot extrusion, the grain growth stagnated at the same temperature, irrespective of the Zr content, resulting in both α-Ti grain refinement and increased β-phase area fraction. The tensile test results demonstrated that 1010 MPa 0.2% yield strength of the Ti-4Fe-13Zr extruded material increased to 156%, compared with that of the base Ti-4Fe material. Furthermore, Labusch model and the Hall-Petch equation were used to quantitatively evaluate the strengthening mechanism derived from the Zr solid solution and the changes in the grain size.

Titanium is well-known for its high strength, good corrosion resistance, and biocompatibility. For further improvement of its mechanical properties, it is alloyed with other elements, such as zirconium. In this study, pre-mixed pure Ti powder and ZrH2 particles were employed to prepare extruded Ti–Zr alloys with a low hydrogen content via dehydrogenation after sintering. The Zr solutes were uniformly dissolved in the α-Ti matrix by applying the optimal conditions for homogenization and water-quenching heat treatment. The synthesized Ti–Zr alloys consisted of the equiaxed fine α-Ti grains via dynamic recrystallization during extrusion. When the ZrH2 particle content was less than 10 wt%, the extruded Ti–Zr alloys showed a large tensile elongation of more than 25% at room temperature with a yield stress of approximately 850 MPa. In the case of Ti–10% ZrH2 powder, the mean α-Ti grain size was 2.7 μm, which was smaller than that of extruded pure Ti (3.5 μm). By applying the Hall–Petch equation for grain refinement and Labusch model for Zr solid solution to quantitatively estimate the increase in the yield stress, it was clarified that both factors had an equal effect for Ti–5 and –10 wt% ZrH2 powder even with the dominance of solid-solution strengthening by Zr solutes for alloys with small Zr content.

By using a high-pressure reaction, we prepared a new oxynitride ZnTaO2N that crystallizes in a centrosymmetric (R (3) over barc) high-temperature LiNbO3-type structure (HTLN-type). The stabilization of the HTLN-type structure down to low temperatures (at least 20 K) makes it possible to investigate not only the stability of this phase, but also the phase transition to a noncentrosymmetric (R3c) LiNbO3-type structure (LN-type) which is yet to be clarified. Synchrotron and neutron diffraction studies in combination with transmission electron microscopy show that Zn is located at a disordered 12c site instead of 6a, implying an order disorder mechanism of the phase transition. It is found that the dosed d-shell of Zn2+, as well as the high-valent Ta5+ ion, is responsible for the stabilization of the HTLN-type structure, affording a novel quasitriangular ZnO2N coordination. Interestingly, only 3% Zn substitution for MnTaO2N induces a phase transition from LN- to HTLN-type structure, implying the proximity in energy between the two structural types, which is supported by the first-principles calculations.

Ion-conducting solid electrolytes are widely used for a variety of purposes. Therefore, there is a high demand for the design of highly ion-conductive materials. Theoretical simulations have become effective tools for investigating the performance of ion-conductive materials because of advancements of computers and computational codes, respectively. However, it can be significantly expensive to conduct an extensive search using theoretical computations. Further, dynamic conductivity and static stability must be simultaneously satisfied for practical applications. Therefore, in this study, we propose a computational framework that simultaneously optimizes dynamic conductivity and static stability; this is achieved by combining theoretical calculations and the Pareto hyper-volume criterion-based Bayesian multi-objective optimization. In our framework, we iteratively select the candidate material that maximizes the expected increase in the Pareto hyper-volume criterion; this is a standard optimality criterion of multi-objective optimization. We show that ion-conductive materials with high dynamic conductivity and static stability can be efficiently identified by our framework via two case studies on diffusion of oxygen and lithium.

While cation order–disorder transitions have been achieved in a wide range of materials and provide crucial effects in various physical and chemical properties, anion analogues are scarce. Here we have expanded the number of known lanthanide oxyhydrides, LnHO (Ln = La, Ce, Pr, Nd), to include Ln = Sm, Gd, Tb, Dy, Ho, and Er, which has allowed the observation of an anion order–disorder transition from the anion-ordered fluorite structure (P4/nmm) for larger Ln3+ ions (La–Nd) to a disordered arrangement (Fm3̅m) for smaller Ln3+ (Sm–Er). Structural analysis reveals that with the increase of Ln3+ radius (application of negative chemical pressure), the oxide anion in the disordered phase becomes too under-bonded, which drives a change to an anion-ordered structure, with smaller OLn4 and larger HLn4 tetrahedra, demonstrating that the size flexibility of hydride anions drives this transition. Such anion ordering control is crucial regarding applications that involve hydride diffusion such as catalysis and electrochemical solid devices.