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This review presents computational and machine learning methodologies developed during a 5-year research project on proton-conducting oxides. The main goal was to develop methodologies that could assist in materials discovery or provide new insights into complex proton-conducting oxides. Through these methodologies, three new proton-conducting oxides, including both perovskite and non-perovskites, have been discovered. In terms of gaining insights, octahedral tilt/distortions and oxygen affinity are found to play a critical role in determining proton diffusivities and conductivities in doped barium zirconates. Replica exchange Monte Carlo approach has enabled to reveal realistic defect configurations, hydration behavior, and their temperature dependence in oxides. Our approach ‘Materials discovery through interpretation’, which integrates new insights or tendencies obtained from computations and experiments to sequential explorations of materials, has also identified perovskites that exhibit proton conductivity exceeding 0.01 S/cm and high chemical stability at 300 [Formula: see text]C.

Abstract 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.

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.

The precipitation and distribution behaviors of in situ-formed titanium boride whiskers (TiB) in TiBw-reinforced Ti-6%Al-4%V (Ti64) composites fabricated from an elemental mixture of Ti64 alloy powder and TiB2 particles by selective laser melting were investigated. The primary precipitation of TiB whiskers strongly depends on B content. For a B content of less than 2 mass%, when the liquid → β-phase transformation occurred and B atoms were discharged, the B-enriched area formed around the β-phase resulted in the generation of TiB whiskers and their agglomeration at the prior β-grain boundaries. When the B content was over 2 mass%, TiB whiskers directly precipitated from the liquid phase and moved to the molten pool boundary via Marangoni convection. As a result, the TiB whiskers were located along the boundary. Furthermore, B-enrichment caused a decrease in the liquidus temperature and thus obstructed β-grain coarsening, and as a result, fine equiaxed α’-grains formed during the phase transformation.

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.

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) forms exhibits a semiconducting behavior. This study offers a wide opportunity to develop transition metal oxyhydrides having various polyhedral linkages, along with site preference of H/O anions, aimed at finding interesting phenomena. [ABSTRACT FROM AUTHOR]

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 concept is verified experimentally. [ABSTRACT FROM AUTHOR]

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 regarding 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. First, 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 the 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 the MgAl2O4 spinel, for which long-range interactions are essential for the accurate description of its energetics. [ABSTRACT FROM AUTHOR]

The article presents the anion ordering enables fast H- conduction at low temperatures. Topics include the solid-state materials with fast ionic conductivity find diverse applications such as batteries, and fuel cells, the some materials drastically lose their ionic conductivity by undergoing a structural phase transition to lower-symmetry structure upon cooling and the best-known strategy to circumvent this problem is the introduction of chemical disorder to stabilize a high-temperature phase.

Machine learning techniques are applied to make prediction models of the G0W0 band gaps for 270 inorganic compounds using Kohn-Sham (KS) band gaps, cohesive energy, crystalline volume per atom, and other fundamental information of constituent elements as predictors. Ordinary least squares regression (OLSR), least absolute shrinkage and selection operator, and nonlinear support vector regression (SVR) methods are applied with two levels of predictor sets. When the KS band gap by generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE) or modified Becke-Johnson (mBJ) is used as a single predictor, the OLSR model predicts the G0W0 band gap of randomly selected test data with the root-mean-square error (RMSE) of 0.59 eV. When KS band gap by PBE and mBJ methods are used together with a set of predictors representing constituent elements and compounds, the RMSE decreases significantly. The best model by SVR yields the RMSE of 0.24 eV. Band gaps estimated in this way should be useful as predictors for virtual screening of a large set of materials. [ABSTRACT FROM AUTHOR]

n 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 (BaZrO3). 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. [ABSTRACT FROM AUTHOR]