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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...

The mechanism of ionic conduction in Ca-doped lanthanum oxychloride (LaOCl) was investigated using first-principles calculations based on density functional theory. The calculations of the point defect formation energies suggest that Cl- ion vacancies and substituted Ca2+ ions at La sites were dominant point defects. Although the migration energy of an O2- ion is 0.95 eV, the migration energy of a Cl- ion was calculated to be 0.44 eV, which is consistent with the reported experimental value. These results imply that the main carrier in Ca-doped LaOCl is Cl- ions and ionic conduction occurs by a Cl- ion vacancy mechanism.

There has been increasing effort to achieve high strength and ductility for Ti-based alloys. However, there are limited reports investigating the effect of multiple solute elements on the diffusion mechanisms at the atomic level. In this study, the diffusion mechanism of interstitial solute elements was investigated in α-Ti using the first-principles calculations. Solute elements B, C, N, O, and F were confirmed to be the most stable at octahedral sites with their calculated formation energies consistent with the reported phase diagrams of Ti and solute elements. The migration energies of C, N, and O are high, approximately 2 eV, while those of B and F were approximately 1 eV. A high correlation was observed between the migration energies and remarkable difference in the charge densities of the solute atoms between stable and transition states. These results indicate that migration energies and diffusion could be determined by considering not only the atomic radius of solute atoms but also their charge transfer with Ti atoms.

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...

The diffusion mechanism of interstitial solute elements in α-Ti was investigated using first-principles calculations. Solute elements, B, C, N, O and F, were confirmed to be the most stable at octahedral sites with their calculated formation energies. The migration energies of C, N and O are high, approximately 2 eV, while those of B and F were approximately 1 eV. A high correlation was observed between the migration energies and difference in the charge densities of the solute atoms between stable and transition states. These results indicate that migration energies and resultant diffusion could not be determined only by the atomic radii of solute atoms, and charge transfer must also be taken into consideration. The charge transfer between matrix and solute atoms affects diffusion mechanism of solute atoms in Ti.

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...

We investigated the effects of oxygen vacancy segregation on electronic structures in the vicinity of hetero-interfaces between noble metals (Au and Pt) and yttria stabilized zirconia (YSZ) by performing first-principles calculations and Bader analysis. The density of states (DOS) of the Au/YSZ interface around the Fermi level is less than that of the Pt/YSZ interface, resulting from the lower density of Au-s and -p orbitals around the Fermi level compared with the higher density of Pt-d orbitals. Metal layers adjacent to the interface become negatively charged when the bonding oxygen layer contains a high concentration of oxygen vacancies. These results indicate that segregation of oxygen vacancies to Au or Pt/YSZ hetero-interfaces increase the electronic conductivity of Au or Pt atom layers at the interfaces.

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.

Single β phase titanium alloys are used for fabricating biomedical devices, and it is desirable to improve the strength of these alloys. In this study, the tensile strength of Ti-35 at.% Ta alloys with a single β phase was improved using oxygen solid solution and the strengthening mechanism was also investigated. The lattice constants of these alloys calculated from X-ray diffraction data increased with an increase in the oxygen content. This result suggests that oxygen atoms formed solutes in the alloys, which was supported by results obtained from first-principles calculations. Microstructural analyses of the alloys indicate that Ti, Ta, and O were homogeneously distributed. Tensile tests were conducted for the alloys, and the 0.2%YS increased with the increase in the O content of the alloy. The increments of 0.2%YS which was determined experimentally correspond well with the theoretically determined trend, and this observation is a result of the effect of the change in grain size and O solid solution. The contribution of the grain refinement was negligibly small compared with that of the O solid solution, and the latter is the major reason for the observed increase in the 0.2%YS of the Ti-35 at.% Ta alloys.

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.

Ti-based materials with high oxygen solute contents were fabricated from mixtures of Ti powder and TiO2 particles by selective laser melting (SLM). Uniformly dissolved oxygen (O) from the TiO2 particles caused c lattice expansion in α-Ti crystals, which effectively increased the strengths of as-built SLM Ti–O materials. The as-built SLM Ti–O material using 1.5 wt% TiO2 showed a yield stress of 962 MPa and 15.3% elongation. The yield stress increases calculated by the Hall–Petch equation and Labusch model were equivalent to those observed experimentally, and O solid-solution strengthening was dominant in increasing yield stress.

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.

Perovskite-structured $\mathrm{AgNb}{\mathrm{O}}_{3}$ is a promising lead-free ferroelectric material that at room temperature exhibits weak ferroelectric behavior with a large polarization under an applied electric field. Here we report first-principles molecular dynamics (FPMD) simulations of monocrystalline $\mathrm{AgNb}{\mathrm{O}}_{3}$ over a range of temperatures to examine the microscopic polarization-switching mechanism. Polarization switching is found to occur at temperatures around 200 K and above; regardless of whether the simulations commence from the antiferroelectric Pbcm structure or ferroelectric $Pmc{2}_{1}$ structure, above 200 K the crystal fluctuates between the two forms. The FPMD are consistent with the coexistence of the two phases at room temperature, which can explain the mixed ferroelectric/antiferroelectric behavior, such as double P-E hysteresis loops, observed experimentally.

We propose a machine-learning method for evaluating the potential barrier governing atomic transport based on the preferential selection of dominant points for the atomic transport. The proposed method generates numerous random samples of the entire potential energy surface (PES) from a probabilistic Gaussian process model of the PES, which enables defining the likelihood of the dominant points. The robustness and efficiency of the method are demonstrated on a dozen model cases for proton diffusion in oxides, in comparison with a conventional nudge elastic band method.

By using a high-pressure reaction, we prepared a new oxynitride ZnTaO

Machine learning techniques are applied to make prediction models of the G0W0 band-gaps for 156 AX binary compounds using Kohn-Sham band-gaps and other fundamental information of constituent elements and crystal structure as predictors. Ordinary least square regression (OLSR), least absolute shrinkage and selection operator (LASSO) and non-linear support vector regression (SVR) methods are applied with several levels of predictor sets. When the Kohn-Sham band-gap by GGA (PBE) or modified Becke-Johnson (mBJ) is used as a single predictor, OLSR model predicts the G0W0 band-gap of a randomly selected test data with the root mean square error (RMSE) of 0.54 eV. When Kohn-Sham band gap by PBE and mBJ methods are used together with a set of various forms of predictors representing constituent elements and crystal structures, RMSE decreases significantly. The best model by SVR yields the RMSE of 0.18 eV. A large set of band-gaps estimated in this way should be useful as predictors for materials exploration., 24 pages, 6 figures, submitted to Phys. Rev. B

In this paper, we propose a selective sampling procedure to preferentially evaluate a potential energy surface (PES) in a part of the configuration space governing a physical property of interest. The proposed sampling procedure is based on a machine-learning method called the Gaussian process, which is used to construct a statistical model of the PES for identifying the region of interest in the configuration space. We demonstrate the efficacy of the proposed procedure for atomic diffusion and ionic conduction, specifically, the proton conduction in a well-studied proton-conducting oxide, barium zirconate $({\mathrm{BaZrO}}_{3})$. The results of the demonstration study indicate that our procedure can efficiently identify the low-energy region characterizing the proton conduction in the host crystal lattice and that the descriptors used for the statistical PES model have a great influence on the performance.

We propose an efficient approach to accurately finding the ground-state structures in alloys based on the cluster expansion method. In this approach, a small number of candidate ground-state structures are obtained without any information of the energy. To generate the candidates, we employ the convex hull constructed from the correlation functions of all possible structures by using an efficient algorithm. This approach is applicable to not only simple lattices but also complex lattices. Firstly, we evaluate the convex hulls for binary alloys with four types of simple lattice. Then we discuss the structures on the vertices. To examine the accuracy of this approach, we perform a set of density functional theory calculations and the cluster expansion for Ag-Au alloy and compare the formation energies of the vertex structures with those of all possible structures. As applications, the ground-state structures of the intermetallic compounds CuAu, CuAg, CuPd, AuAg, AuPd, AgPd, MoTa, MoW and TaW are similarly evaluated. Finally, the energy distribution is obtained for different cation arrangements in MgAl$_2$O$_4$ spinel, for which long-range interactions are essential for the accurate description of its energetics., Comment: 8 pages, 7 figures

The local environment of substitutional Zn(2+) in Ca-deficient hydroxyapatite (HAp) was investigated using experimental and theoretical analyses of the x-ray absorption near edge structure (XANES). For Zn-K XANES calculations, two situations of Zn(2+) were considered. One was Zn(2+) substituted for Ca sites in perfect HAp, and the other was a Ca-deficient HAp model of substitutional Zn(2+) associated with a Ca(2+) vacancy charge compensated by two protons. The model of Zn(2+) in perfect HAp did not reproduce the experimental Zn-K XANES spectrum. In contrast, the Ca-deficient HAp model agreed well with the experimental spectrum. This indicates that substitutional Zn(2+) in Ca-deficient HAp is associated with the Ca(2+) vacancy complex in HAp.

Defect formation energies in materials generally depend on chemical potentials determined by a chemical equilibrium condition. In particular, an aqueous solution environment is important for biomaterials such as hydroxyapatite studied here. Therefore, a methodology to obtain ionic chemical potentials under chemical equilibrium between solid and aqueous solution was introduced, and was applied to substitutional divalent cations formed via ion exchange with Ca(2+) in hydroxyapatite. The calculated ranking of the stability of substitutional cations in HAp was in good agreement with the experimentally observed trend. The present theoretical approach would be useful to explore the thermodynamic stability of defects in materials subjected to an aqueous solution environment.

Hydroxyapatite (HAp) samples were synthesized by a solution reaction method followed by heat-treatments at three different temperatures. Special attention was given to optimizing the processing parameters to obtain the chemical composition near to the stoichiometry of HAp. No trace of secondary crystalline phase was found from powder X-ray diffraction in all samples. X-ray fluorescence measurements found that the Ca/P ratio was 1.68 ± 0.02, which is close to the stoichiometry of HAp, i.e., 1.67. Electron microscope observations revealed that the grain size was uniform within a sample, which was dependent on the heat treatment temperature. Dissolution rates in acid solution and cytotoxicity of the samples were measured. Tendency to decrease both the dissolution rates and the cytotoxicity with increasing crystallite size was observed. After heat-treatment at 1000 °C, the cytotoxicity of the sample was found to be minimal, which had uniaxial and faceted grains with a mean diameter of 200 nm.