In this study, we prepared a brookite-type titanium dioxide particle layer on the surface of titanium substrates via hydrothermal treatment in aqueous urea solutions containing sodium chloride (NaCl) and examined its in vitro apatite-forming ability. Increasing the urea concentration suppressed the formation of anatase-type titanium dioxide on the titanium substrate, forming a particle layer composed of pure brookite-type titanium dioxide. The size and packing density of brookite-type titanium dioxide particles formed on the titanium substrate increased with the NaCl concentration in a 7.0 mol·dm⁻³ urea solution. When titanium substrates hydrothermally treated in aqueous solutions of 7.0 mol·dm⁻³ urea and 2.0 mol·dm⁻³ NaCl were soaked in a simulated body fluid for various periods up to 7 d, the substrate surface was densely covered with hemispherical apatite particles (5.3 µm in diameter) within 3 d, indicating that the brookite-type titanium dioxide particle layer had an excellent apatite-forming ability comparable to that of the anatase-type titanium dioxide particle layer.

Keatite, a polymorph of silica rare in nature, was synthesized by hydrothermal treatment of silicon and water at 100 MPa and 600 °C. The crystal structure of keatite at 24 °C was refined by the Rietveld method using synchrotron X-ray diffraction data. The obtained structure is consistent with the results of previous studies in which some constraints were imposed during refinements. The ²⁹Si MAS NMR spectrum of keatite shows two peaks at −113.9 and −114.3 ppm, which can be assigned to Si at the Si1 and Si2 sites, respectively. The Raman spectrum of keatite shows a prominent peak at 473 cm⁻¹, which is attributable to the Si–O–Si bending mode of the 5-membered ring. These spectra, reported for the first time, are expected to be valuable for the identification of keatite in synthetic and natural samples.

Cubic Li_6.25Al_0.25La₃Zr₂O₁₂ exhibits high Li ionic conductivity (>3 × 10⁻⁴ S·cm⁻¹) at room temperature: however, its sintered body disintegrates upon prolonged exposure to air. To prevent this collapse phenomenon, this study sintered Li_6.25Al_0.25La_3−0.67xM_xZr₂O₁₂ (M = Mg, Ca, Sr, Ba, x = 0.1 to 0.5), wherein a fraction of the La in Li_6.25Al_0.25La₃Zr₂O₁₂ is substituted with alkaline earth metal elements, in a 100 % O₂ atmosphere and allowed the material to cool naturally. The supply of 100 % O₂ during natural cooling is particularly important. Li_6.25Al_0.25La_2.8Mg_0.3Zr₂O₁₂ exhibits a high Li ionic conductivity of 2.7 × 10⁻⁴ S·cm⁻¹ at 25 °C. The X-ray diffraction pattern and Li ionic conductivity of the Li_6.25Al_0.25La_2.8Mg_0.3Zr₂O₁₂ sintered body remains unchanged even after exposure to air for 1 year.

When dealing with crystalline inorganic compounds, a great deal of skill and expertise are entailed in constructing an initial input structure for the Rietveld analysis from their powder X-ray diffraction (XRD) profiles. For newly synthesized materials in particular, determining the space group to which the compound belongs, the first step to begin the Rietveld analysis, often relies on experimentalists’ intuition and experience. Occasionally, we need to try several candidate space groups through trial and error, which takes a significant amount of time. To streamline this initial step, this study demonstrates a scheme for determining the valid space group and lattice parameters of an unknown compound using the Le Bail analysis. This scheme uses a deep-learning model, SpgVolNet, developed in our previous study to predict candidate space groups from an experimental XRD profile. Although we had known the structure of the model compound in this study, perovskite-type La_0.6Sr_0.4Mn_0.9Co_0.1O₃, we assumed it to be “unknown.” The SpgVolNet predicted that this compound belongs to R3c with the highest probability. Le Bail analysis under R3c and several other plausible space groups led us to the “correct” conclusion that the compound indeed belongs to R3c with reliable lattice parameters.

Li_xLa_(1−x)/3NbO₃ with an A-site-deficient perovskite structure was investigated with a focus on the relationship between its atomic configuration and Li⁺ diffusion properties. To this end, total scattering (diffraction) measurements were performed, and then reverse Monte Carlo modeling using the data was employed to construct the atomic configuration. The results suggest that the partial occupancy of La in the La-poor layer facilitate Li⁺ diffusion across the layer owing to the volume contraction. Furthermore, topological analyses conducted via persistent homology using the constructed atomic configuration indicate that a large fourfold ring formed by Nb and O is one of the reasons for superior Li⁺ diffusion in Li_xLa_(1−x)/3NbO₃.

Chemical heterogeneity in Ba(Zr_0.1Ti_0.9)O₃ (BZT) ceramics was formed by an incomplete solid-state reaction. XRD and SEM-EDS analyses revealed the calcined powder contained a mixture of BZT particles with varying concentrations of Zr. Such particles inhibited grain growth and resulted in diffuse phase transition in BZT ceramics due to higher grain-boundary density and chemical heterogeneity. As the chemical heterogeneity was reduced by increasing calcination and sintering temperatures, the grain size increased from 1.65 to 38.5 µm, resulting in their dielectric behavior changing from diffuse phase transition to a proper ferroelectric. These results indicate that the incomplete solid-state reaction plays an important role in microstructure evolution and electrical properties in BZT-based ceramics synthesized via solid-state reaction involving ZrO₂.

We investigated the fabrication of three-dimensional molding of porous bodies and dense bodies of alumina ceramics using direct ink writing (DIW). Ceramic pastes were prepared by adding appropriate amounts of binder, plasticizer, and water to the starting raw material of alumina powder, and various shapes were molded. The characteristics of the molded and sintered bodies were evaluated. Improved shape retention was observed with the addition of polyethylene glycol and an increase in the solid content. However, if these become excessive, extrusion difficulties and delamination of the laminated interface occur; therefore, it is necessary to reduce the apparent viscosity of the ceramic paste within a range that does not cause deformation. It was suggested that porous and dense bodies of three-dimensional alumina ceramics with shapes close to the design could be fabricated using DIW by adjusting the CAD models to account for anisotropic sintering shrinkage.

Wollastonite has been widely studied as a ceramic raw material of calcium silicate (CaSiO₃). Recently, Mg-doped wollastonite has attracted increasing attention due to its enhanced mechanical, insulative and bioactive properties. However, synthesizing single phase Mg-doped wollastonite is not easy because wollastonite undergoes crystal phase transition from β-CaSiO₃ to α-CaSiO₃ at around 1200 °C. Therefore, it is important to establish practical synthesis method of Mg-doped wollastonite for future application studies. In this study, β-Ca_0.92Mg_0.08SiO₃ was successfully synthesized at 1300 °C by two-step sintering of Mg-containing xonotlite, which inhibits the transformation from β to α at high temperatures. The first step facilitates complete Mg diffusion into β-CaSiO₃, while the second step promotes Mg diffusion from diopside (CaMgSi₂O₆) to β-CaSiO₃ without occurring β-α phase transformation. Owing to the stabilization of the low-temperature phase at high temperatures through magnesia addition, the material was named “magnesia-stabilized β-wollastonite (MSW)” in reference to yttria-stabilized zirconia (YSZ).

High-durability, high-activity electrocatalysts for both polymer electrolyte fuel cells and water electrolyzers are developed by utilization of electronically conducting ceramic nanoparticles. These electrocatalysts require the function of a gas diffusion electrode, which in turn requires electronically conducting pathways, gas diffusion pathways, high surface area and durability at high electrode potentials. The electronically conducting ceramic nanoparticles should be modified to mitigate the resistivity originating from gas adsorption, quantum size effects, and interfacial contact between nanoparticles. The specific microstructure, interface structure, electronic state and crystallinity of the electronically conducting ceramic nanoparticles should also be controlled toward the specific application of the electrocatalyst. These electronically conducting ceramic nanoparticles have demonstrated promising possibilities toward their application in green hydrogen technology.

A novel environmentally friendly inorganic yellow-orange pigment, LiCeW_2−xMn_xO_8−δ (0 ≤ x ≤ 0.07), was synthesized and its optical and color properties were investigated. By partially substituting Mn, a promising source of orange-red coloration, into the octahedrally coordinated W site of the LiCeW₂O₈ host lattice, the absorption intensity of d–d transitions associated with W and Mn in octahedral coordination was modulated, thereby enabling control over light reflection in the visible spectrum. X-ray photoelectron spectroscopy and Rietveld refinement suggested that the electronic states of the constituent elements may have influenced the d–d transition absorption intensities of W^(6−δ)+ and Mn^(4−δ)+ species, thereby altering the overall light absorption behavior. As a result, the reflectance of LiCeW_1.97Mn_0.03O_8−δ was selectively enhanced in the red region, yielding the most vivid and bright orange pigment.

Ceramic proton conductors have attracted increasing attention due to their potential to improve the efficiency of electrochemical devices, such as fuel cells. BaSc_0.8Mo_0.2O_2.8 exhibits high proton conductivity, however, it is not fully hydrated, resulting in a low proton carrier concentration. Here, we report the full hydration and high proton concentration of a new material, large-sized Lu-substituted BaSc_0.8Mo_0.2O_2.8 (BaSc_0.6Lu_0.2Mo_0.2O_2.8). Due to the larger ionic radius of Lu³⁺ than Sc³⁺, the lattice volume of BaSc_0.6Lu_0.2Mo_0.2O_2.8 was larger than that of BaSc_0.8Mo_0.2O_2.8, which can be responsible for the full hydration and higher water uptake of BaSc_0.6Lu_0.2Mo_0.2O_2.8 compared with BaSc_0.8Mo_0.2O_2.8. The bulk conductivity of BaSc_0.6Lu_0.2Mo_0.2O_2.8 (0.01 S cm⁻¹ at 400 °C) was higher than that of the leading materials such as BaZr_0.8Y_0.2O_2.9 and BaCe_0.9Y_0.1O_2.95 (e.g., 19 times higher than BaCe_0.9Y_0.1O_2.95 at 243 °C). This is attributable to the higher proton concentration 0.40 of BaSc_0.6Lu_0.2Mo_0.2O_2.8 compared with 0.17 of BaZr_0.8Y_0.2O_2.9 and 0.08 of BaCe_0.9Y_0.1O_2.95. The bulk conductivity of BaSc_0.6Lu_0.2Mo_0.2O_2.8 was lower than that of BaSc_0.8Mo_0.2O_2.8 and BaSc_0.8W_0.2O_2.8 due to its lower proton diffusion coefficient D. The D of BaSc_0.6Lu_0.2Mo_0.2O_2.8 exhibited a non-Arrhenius behavior, and its activation energy for D increased with decreasing temperature, which is attributable to proton trapping.

Sulfurized polyacrylonitrile (SPAN) is considered a promising sulfur cathode material to suppress the shuttle effect, as the active sulfur in SPAN is atomically dispersed and covalently bonded to the conductive skeleton, thereby eliminating the presence of elemental sulfur. This structure significantly reduces the dissolution of lithium polysulfides into the electrolyte during cycling. To enhance our understanding of the charge–discharge mechanism of SPAN, this study employed transmission electron microscopy (TEM) to investigate and analyze the microstructural changes in SPAN. Our findings revealed that SPAN particles undergo repeated expansion and contraction, leading to the expansion and closure of internal cracks. Li₂S nanocrystals appear in SPAN with lithiation and remain even when SPAN is fully delithiated. In parts of regions, sulfur migrates and accumulates at crack boundaries in the SPAN particles, forming Li₂S nanocrystals with sizes ranging from 20 to 50 nm. These Li₂S nanocrystals also persist even when SPAN is fully delithiated. The accumulation of sulfur and Li₂S at boundaries hinders lithium-ion transport, preventing lithiation of the internal SPAN regions and causing electrochemically inactive zones. Consequently, this contributes to capacity fading and limits the overall battery performance.

In two-dimensional protonated layered Zr phosphates, H⁺ ion exchange with monovalent alkali metal ions (M⁺) of smaller atomic radii (e.g., Li⁺, Na⁺) in aqueous solution proceeds much more easily than with larger ions (e.g., K⁺, Rb⁺, and Cs⁺). Therefore, this study investigated the preparation of M₂ZrP₂O₈·nH₂O (M: Li, Na) and M₂ZrP₂O₈·nH₂O (M: K, Rb, Cs) powders via ion substitution—from H⁺ to Li⁺ and Na⁺ in γ-Zr(PO₄)(H₂PO₄)-2H₂O in aqueous solution—and solid-state reactions, respectively. The prepared M₂ZrP₂O₈·nH₂O (M: Li, Na) was monoclinic with a thin plate-like crystal morphology, whereas M₂ZrP₂O₈·nH₂O (M: K, Rb, Cs) was tetragonal with a thick plate-like crystal morphology. Further analyses revealed (002) or (001) peaks, indicating an interlayer distance of approximately 5–10°. Moreover, the interlayer distance of M₂ZrP₂O₈·nH₂O (M: Li, Na) exceeded that of M₂ZrP₂O₈·nH₂O (M: K, Rb, Cs). Finally, the ionic conductivity of M₂ZrP₂O₈·nH₂O (M: K, Rb, Cs) at 30–80 °C exceeded that of M₂ZrP₂O₈·nH₂O (M: Li, Na) by approximately 1.5 orders of magnitude, while the apparent activation energy for ionic conduction was lower. The ionic conductivities of M: K, Rb, and Cs at room temperature were found to be 1.2 × 10⁻³, 9.1 × 10⁻⁴, and 1.3 × 10⁻³ S·cm⁻¹, respectively.