Published papers

A uniform Na distribution in NaₓAlB₁₄ was achieved using high-pressure diffusion control (HPDC), which promotes Na deintercalation through enhanced diffusion under high pressure, combined with post-annealing. NaₓAlB₁₄ with a non-stoichiometric Na composition is thermodynamically metastable, and conventional solid-state reactions with adjusted starting compositions typically result in the formation of stoichiometric NaAlB₁₄ and side products. While HPDC alone typically leads to concentration gradients, intentionally halting the Na removal process before complete extraction, followed by annealing, enabled a uniform composition across the bulk. This allowed structural and electronic properties to be examined over a wide range of Na concentrations. As Na content decreased, electrical conductivity increased, and the optical band gap narrowed. NMR measurements showed an increase in the density of states at the Fermi level, consistent with DFT calculations predicting boron-related in-gap states. Boron vacancies at specific sites were found to generate deep levels near the band gap center, which can explain experimentally observed optical gap reduction. These results demonstrate that diffusion-controlling methods can be effectively applied to synthesize metastable compounds with tunable compositions in covalent frameworks. Furthermore, they provide a foundation for designing functional boride-based materials with adjustable electronic properties by controlling Na extraction and inducing defect formation.

Na+-containing B-type carbonate apatite ([Ca10-xNa2x/3][(PO4)6-x(CO3)x][(OH)2-x/3(H2O)x], BCA) is a hydroxyapatite (Ca10(PO4)6(OH)2, HA)-derived compounds in which PO43-sites of HA are partially replaced with CO32-. When CO32--composition x exceeds 1, the ionic conductivity of BCA increases four orders of magnitude greater than that of HA, achieving values between 10-4-10-3S cm-1at 600-800 °C. Owing to this unique compositional dependence of ionic conductivity, BCA is not only an important biomaterial, but also a promising material for solid electrolytes. Hence in this study, motivated by the elucidation of the ion conduction mechanism of BCA, two types of BCAs with different x values were prepared, and their physical and ionic conductive properties were investigated. Thermogravimetric analysis result of BCAs sintered under CO2(S-BCAs) showed that dissociation of CO32-ions began at approximately 600 °C during heating in air. However, during three cycles of impedance measurements repeated from room temperature to 850 °C, the decrease in the x (up to-60 %) did not significantly affect the conductivity. The impedance measurements also revealed that the slope of the Arrhenius plot for conductivity exhibited a bend between 500 and 800 °C, which appeared repeatedly across measurement cycles. Based on first-principles calculations regarding the stable structure of the BCA, some OH-(or O2-in OH-sites) might get trapped at the oxygen vacancy close to CO32-, and a defect association could form between Na+and CO32-. Thus, the unique conductivity behavior observed in S-BCAs can be attributed to the changes in the carrier density and conduction pathways, which are influenced by the thermally induced change in the O2--trapping state at oxygen vacancies close to CO32-

This study investigated the effect of intergranular connectivity when Na+ was extracted from polycrystalline NaAlB14 to the Y-type zeolite via anisotropic ion diffusion control (ADC) and high-pressure diffusion control (HPDC) methods. Despite the low chemical intergranular connectivity in NaAlB14 without pre-annealing, Na+ was homogeneously extracted via HPDC process. Although the HPDC can form a strong physical compression between particles, the intergranular domain of the sample was oxidized by the backflow of oxide ions from the zeolite. Such oxidization can be solved by pre-annealing for chemical intergranular connectivity. In the case of Na+ extraction via ADC process, Na+ remained at the cathode side even after the treatment for 230 h. From the comparison between ADC and HPDC processes, a strong physical compression between particles by applying pressure can promote the diffusion of Na+. Therefore, pre-annealing and high-pressure applications are essential for ion diffusion control techniques.

A synthetic protocol was developed for obtaining a single phase of polycrystalline NaAlB14 with strongly connected intergrain boundaries. NaAlB14 has a unique crystal structure with a tunnel-like covalent framework of B that traps monovalent Na and trivalent Al ions. Owing to the atmospheric instability and volatility of Na, the synthesis of polycrystalline NaAlB14 and its physical properties have not been reported yet. This study employed a two-step process to achieve single-phase polycrystalline NaAlB14. As a first step, a mixture of Al and B with excess Al was sintered in the Na vapor atmosphere followed by HCl treatment to remove excess Al as a second step. For obtaining bulk samples with strong grain connection, vacuum or high-pressure (HP) annealing was employed. HP annealing promoted bandgap shrinkage due to the crystal strain and defect levels and suppressed intergranular resistance. As a result, the HP-annealed sample achieved superior transport properties (0.1 kω cm at 300 K) to the vacuum-annealed sample (260 kω cm). Furthermore, from the viewpoint of its crystal structure and DFT calculations, the most probable site for the defect was suggested to be the Na site.

To optimize the giant magnetocaloric properties of HoB2, we synthesized and characterized the magnetocaloric properties of Ho1-xGdxB2 (0.1 ≤ x ≤ 0.4) alloys. We found out that Gd enters stoichiometrically and randomly into the Ho site, leading to a Vegard-type structural change. The addition of spherical S7/2 Gd3+ moments prompts an enhancement in Curie temperature from 15 K to 30 K (at x = 0.4), a reduction in peak value of the magnetic entropy change, from 0.35 (40.1) to 0.17 (20.2) J cm−3 K−1 (J kg−1 K−1), and a broadening of the magnetic entropy change curves. The overall influence is a relatively high refrigerant capacity and relative cooling power, peaking at 6.07 (711) and 7.68 (899) J cm−3 (J kg−1) for x = 0.2, and an extension of the thermal working range to higher temperatures. Unlike Ho1−xDyxB2 alloys, the Gd substituted samples show no magnetic hysteresis. Furthermore, scaling analysis of the entropy curves suggests a second-order phase transition for the ferromagnetic transition in these alloys. Thus, Ho1-xGdxB2 alloys are potential candidates for cryogenic refrigeration applications.

A novel synthesis technique, called the high-pressure diffusion control (HPDC) method, was developed in this study. The method combined the high-pressure synthesis using a cubic anvil apparatus and an anisotropic diffusion control technique; the electrical processing in high-pressure and high-temperature environments of up to 4 GPa and over 1000 °C is enabled by simultaneously adjusting the temperature, pressure, and voltage. This nonequilibrium state is effective in creating metastable materials. The developed novel technique was applied to polycrystalline NaAlB14 with a boron covalent framework. Although electronic conduction is dominant in this material and no Na-ion conduction is observed even at high temperatures, the HPDC method successfully extracted Na ions by utilizing the difference in bond strength between Na and B, creating the metastable material AlB14 while maintaining its basic crystal structure. During the decrease in the Na concentration, applying a high pressure compressed the sample according to the volume change and maintained good contact at the intergrain boundary in the polycrystalline sample, promoting Na-ion diffusion. The Na extraction functioned as electron carrier modulation and significantly reduced the electrical resistivity. The developed HPDC method is expected to be applicable to various compounds with a difference in the bond strength between constituent elements and has the potential to open up new avenues in the inorganic synthesis of polycrystalline metastable materials with dense sintered states and modulate their physical properties.

Intercalation reactions are highly dependent on the electronic and structural relationships between host and guest materials. It is difficult for divalent ions, such as Mg, to undergo intercalation reactions compared with monovalent cations. However, further development of synthetic techniques for controlling divalent ions is strongly demanded to advance applied chemistry and fundamental physics. In this study, the cointercalation of Mg and H into the transition-metal chalcogenide TaS2 was utilized to obtain bulk polycrystalline MgxHyTaS2. Introduced H can be extracted via postannealing at approximately 400 °C without altering the crystal structure. This study clarified the relationship between superconducting properties and electronic carrier density from the perspectives of calculations and experiments, along with the advantages of using hydride as a multivalent intercalation reaction.

Intercalation into 1D transition metal trichalcogenides (TMTs) in which fibers are bonded by a weak van der Waals force can be expected to create various intercalation compounds and develop unique physical properties according to the combination of the host materials and guest ions. However, structural changes via intercalation into 1D TMTs are not as simple as those in 2D transition metal dichalcogenides (TMDs) and are still not understood comprehensively. ZrTe3: a typical compound with a 1D trigonal prismatic structure, belongs to TMTs. Herein, through the Ag introduction to ZrTe3 via solid-state intercalation, a novel crystal phase with a 1D octahedral structure and a quasi-amorphous (QA) phase during the structural transition are discovered; the QA phase is a novel state of matter in which long-range order is lost while retaining 1D order. Based on the Ag concentration, the transport properties are flexibly modulated from superconductivity to semiconductivity. Density functional theory calculations indicate the attraction between Ag ions and the pair diffusion due to their attraction. Furthermore, judging the attraction or repulsion between guest ions predicts whether to induce a QA phase or simple lattice expansion like the intercalation into 2D TMDs.

Transition Metal Dichalcogenides (TMDs) are a major family of compounds for exploring functional materials through intercalation. In this study, the intercalation of Ag ions into 2H-MoTe 2 using the high-pressure diffusion...

The thermoelectric transport properties of the van der Waals-type layered rhombohedral SnAs-based compound, EuSn2As2, have been investigated. A densified polycrystalline sample of EuSn2As2 with porosity (φ) of 2.4(9) vol.% exhibited a weak orientation to the c-axis for hexagonal coordination system; the weak orientation is parallel (P ∥) to the pressing direction of hot pressing. Measurements of electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) were conducted perpendicular (P ⊥) to the pressing direction. The experimental values of ρ and S exhibit metallic temperature dependence and p-type carrier polarity. The power factor (P) was 0.51(8) mW m-1 K-2 at 673(4) K. Using the Wiedemann-Franz-Lorenz law, the phonon thermal conductivity (κ ph) was estimated to be 0.4(6) W m-1 K-1 at 673(6) K. The dimensionless figure of merit, ZT, was 0.092(17) at 673(3) K.