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Masaya Fujioka

Publications and source records attributed to Masaya Fujioka.

At least 19 recordsLinked to original sources
arXiv2025

Boron Clusters for Metal-Free Water Splitting

Electron-deficient boron clusters are identified as a fundamentally new class of oxygen evolution reaction (OER) catalysts, entirely free of transition metals. Selective sodium extraction from NaAlB14 and Na2B29 via high-pressure diffusion control introduces hole doping into B12 icosahedral frameworks, resulting in OER activity exceeding that of Co3O4 by more than an order of magnitude, and exceptional durability under alkaline conditions. B12 clusters are known for their superchaotropic character, which destabilizes hydrogen bonding in water. In this system, H2O, instead of OH-, preferentially adsorbs on the catalyst surface, suggesting a distinct OER pathway mediated by molecular water. This adsorption behavior contrasts with conventional transition-metal oxides and reflects the unique interfacial properties of the boron clusters. Density functional theory reveals unoccupied p orbitals and unique local electric fields at the cluster surface, both of which could promote the water activation. These findings suggest a paradigm shift in OER catalysis, in which the unique interaction between B12 clusters and water drives the reaction, replacing the conventional role of redox-active metals. Hole-doped boron clusters thus offer a promising platform for designing high-performance and durable water-splitting catalysts, opening new avenues for OER design beyond conventional transition-metal chemistry.

cond-mat.mtrl-sci↗
arXiv2025

Diffusion-Controlled Anion Conversion into Dense Polycrystalline and Single-Crystalline Oxyhydrides

Oxyhydrides represent a new class of functional materials, yet the synthesis of dense polycrystals or single-crystals suitable for transport studies remains a significant challenge due to hydrogen desorption at elevated temperatures. The co-diffusion of oxygen and hydrogen in densely sintered BaTiO3 enables the topochemical formation of millimeter-scale bulk BaTiO3-xHx via high-pressure diffusion control (HPDC). Hydride ions selectively occupy oxygen-deficient sites, as confirmed by neutron diffraction, TPD, TG, and NMR. Systematic tuning of the hydrogen content and precise control of the electronic conductivity were achieved via HPDC. Hydrogen desorption analysis reveals distinct bonding states between near-surface and interior-bulk regions, which significantly affect the oxynitride conversion under N2 flow. Importantly, the diffusion-based nature of HPDC allows direct anion conversion even in single-crystalline oxides, as demonstrated by the synthesis of SrTiO3-xHx single crystals. These results establish HPDC as a general platform for accessing dense, metastable oxyhydrides with tunable anionic composition and transport properties.

cond-mat.mtrl-sci↗
arXiv2025

Compositional Tuning in NaxAlB14 via Diffusion Control

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.

cond-mat.mtrl-sci↗
arXiv2024

Elemental Reactivity Maps for Materials Discovery

When searching for novel inorganic materials, limiting the combination of constituent elements can greatly improve the search efficiency. In this study, we used machine learning to predict elemental combinations with high reactivity for materials discovery. The essential issue for such prediction is the uncertainty of whether the unreported combinations are non-reactive or not just investigated, though the reactive combinations can be easily collected as positive datasets from the materials databases. To construct the negative datasets, we developed a process to select reliable non-reactive combinations by evaluating the similarity between unreported and reactive combinations. The machine learning models were trained by both datasets and the prediction results were visualized by two-dimensional heatmaps: elemental reactivity maps to identify elemental combinations with high reactivity but no reported stable compounds. The maps predicted high reactivity (i.e., synthesizability) for the Co_Al_Ge ternary system, and two novel ternary compounds were synthesized: Co4Ge3.19Al0.81 and Co2Al1.24Ge1.76.

cond-mat.mtrl-sci↗
arXiv2016

Origin of the Higher-$T_\rm{c}$ Phase in the K$_x$Fe$_{2-y}$Se$_2$ System

Single crystals of K$_x$Fe$_{2-y}$Se$_2$ are prepared by quenching at various temperatures. The crystals obtained at higher quenching temperatures have a surface morphology with mesh-like texture. They show a sharp superconducting transition at $T_\rm{c}$ ~32 K with a large shielding volume fraction. On the other hand, the crystals prepared without quenching show an onset superconducting transition at ~44 K and a zero resistivity around ~33 K, and they possess island-like regions on the surface with a larger amount of Fe incorporation. In-situ high-temperature single crystal X-ray diffraction measurements tell us the Fe-vacancy ordered phase is generated at a temperature region around 270 °C via iron diffusion. The creation of this Fe-vacancy ordered phase may become a driving force of the growth of the higher $T_\rm{c}$ phase. The superconductivity at ~44 K is attributed to a metallic phase with no Fe-vacancy.

cond-mat.supr-con↗
arXiv2015

Electrochemical deposition of FeSe on RABiTS tapes

FeSe film is successfully fabricated onto Rolling Assisted Biaxially Textured Substrate (RABiTS) tapes by an electrochemical deposition technique. The deposited FeSe films tend to become high crystallinity with a decrease in the applied voltage to -1.0 V, and the compositional ratio of Fe to Se approaches 1:1. The sample deposited at -1.0 V shows a superconducting transition approximately 8.0 K in the magnetic susceptibility.

cond-mat.supr-con↗
arXiv2015

Pressure-Induced Superconductivity in BiS2-based EuFBiS2

We measured the electrical resistivity of a BiS2-based compound EuFBiS2 under high pressure. Polycrystalline EuFBiS2 shows insulator-metal transition and pressure-induced superconductivity above 0.7 GPa. The superconducting transition temperature increases with increasing applied pressure and shows the maximum value around 8.6 K at 1.8 GPa.

cond-mat.supr-con↗
arXiv2015

Site Selectivity on Chalcogen Atoms in Superconducting La(O,F)BiSSe

Single crystals of La(O,F)BiSSe were successfully grown by a CsCl flux method. Single crystal X-ray structural analysis revealed that the crystal structure is isostructural with BiS$_2$- or BiSe$_2$-based compounds with space group $P4/nmm$ (lattice parameters $a$ = 4.1110(2) Å, $c$ = 13.6010(7) Å). However, the S atoms are selectively occupied at the apical site of the Bi-SSe pyramids in the superconducting layer. The single crystals show a superconducting transition at 4.2 K in the magnetic susceptibility and resistivity measurement. The superconducting anisotropic parameter is determined to be 34 from its upper critical magnetic field. The anisotropy is in the same range with that of other members of the La(O,F)BiCh$_2$ ($Ch$ = S, Se) family under ambient pressure.

cond-mat.supr-con↗
arXiv2014

The effect of simultaneous substitution on the electronic band structure and thermoelectric properties of Se-doped Co3SnInS2 with the Kagome lattice

The thermoelectric properties and electronic band structures for Se-doped Co3SnInS2 were examined. The parent compound of this material (Co3Sn2S2) has two kinds of Sn sites (Sn1 and Sn2 sites). The density functional theory (DFT) calculations show that the indium substitution at the Sn2 site induces a metallic band structure, on the other hand, a semiconducting band structure is obtained from substitution at the Sn1 site. However, according to the previous reports, since the indium atom prefers to replace the tin atom at the Sn1 site rather than the Sn2 site, the resistivity of Co3SnInS2 shows semiconducting-like behavior. In this study we have demonstrated that metallic behavior and a decrease in resistivity for Se-doped Co3SnInS2 occurs without suppression of the Seebeck coefficient. From the DFT calculations, when the selenium content is above 0.5, the total crystallographic energy shows that a higher indium occupancy at Sn2 site is more stable. Therefore, it is suggested that the selenium doping suppress the site preference for indium substitution. This is one of the possible explanations for the metallic conductivity observed in Se-doped Co3SnInS2

cond-mat.mtrl-sci↗
arXiv2014

Pressure-induced phase transition for single crystalline LaO0.5F0.5BiSe2

We have demonstrated a pressure-induced phase transition from a low-Tc phase to a high-Tc phase in a single crystal of the superconductor LaO0.5F0.5BiSe2. The high-Tc phase appears at 2.16 GPa and the maximum superconducting transition temperature (Tc) is observed at 6.7 K under 2.44 GPa. Although the anisotropy (γ) for the low-Tc phase is estimated to be 20, it is reduced by around half (9.3) in the high-Tc phase. This tendency is the same for the BiS2 system. The Tc of LaO0.5F0.5BiSe2 has continued to increase up to the maximum pressure of this study (2.44 GPa). Therefore applied further pressure has the potential to induce a much higher Tc in this system.

cond-mat.supr-con↗
arXiv2014

High-Tc Phase of PrO0.5F0.5BiS2 single crystal induced by uniaxial pressure

We demonstrated a pressure-induced phase transition from a low-Tc phase (3.5 K) to a high-Tc phase (8.7 K) in single crystalline PrO0.5F0.5BiS2. The high-Tc phase is easily observed even at 0.7 GPa by uniaxial pressure obtained from a geometric effect of a single crystal with an extremely-thin thickness. For this phase transition, superconducting anisotropy and the coherence length along the c axis change from γ = 20 to 9.3 and from ξc = 0.56 to 0.71 nm respectively. It is suggested that a shrinkage in the c lattice constant by applied uniaxial pressure enhances the three dimensionality in the superconducting state.

cond-mat.supr-con↗
arXiv2014

The effect of exceptionally high fluorine doping on the anisotropy of single crystalline SmFeAsO1-xFx

We succeeded in growing single crystalline SmFeAsO1-xFx with exceptionally high fluorine concentration by using a CsCl flux method. Comparing to conventional flux methods, this method can introduce about double the amount of fluorine into the oxygen site. The obtained single crystal shows the highest superconducting transition temperature (Tc = 57.5 K) in single crystalline iron pnictides. In addition, the residual resistivity ratio (RRR) is almost three times as large as that of previously reported single crystals. This suggests that our single crystals are suitable for investigation of the intrinsic superconducting properties since they have few defects and impurities. Using both the Werthamer-Helfand-Hohenberg model and the effective mass model, we demonstrated that a higher fluorine concentration suppresses the anisotropic superconductivity of SmFeAsO1-xFx.

cond-mat.supr-con↗
arXiv2014

First single crystal growth and structural analysis of superconducting layered bismuth oxyselenide; La(O,F)BiSe2

Single crystal of La(O,F)BiSe2 was successfully grown for the first time by the CsCl flux method. Single crystal X-ray structural analysis clearly showed that La(O,F)BiSe2 crystalizes with space group P4/nmm (lattice parameters a = 4.1408(2) Å, c = 14.1096(6) Å). The structure is composed of alternating BiSe2 and LaO layers. Magnetic susceptibility and resistivity measurements showed the superconducting transition at 3.6 K, which is higher than that of as-grown La(O,F)BiS2. The structural analysis implies that La(O,F)BiSe2 is potential superconductor having higher Tc under high pressure.

cond-mat.supr-con↗
arXiv2014

Superconductivity in FeTe0.8S0.2 induced by battery-like reaction

Superconductivity is successfully induced by utilizing a battery-like reaction found in a typical Li-ion battery. Excess Fe in FeTe0.8S0.2 is electrochemically de-intercalated by applying a voltage in a citric acid solution. The superconducting properties improve with an increase in the applied voltage up to 1.5 V. This result suggests that an electrochemical reaction can be used as a novel method to develop new superconducting materials.

cond-mat.supr-con↗
arXiv2014

Synthesis and physical properties of Ca1-xRExFeAs2 with RE = La ~ Gd

Synthesis of a series of layered iron arsenides Ca1-xRExFeAs2 (112) was attempted by heating at 1000 C under a high-pressure of 2 GPa. The 112 phase successfully forms with RE = La, Ce, Nd, Sm, Eu and Gd, while Tb, Dy and Ho substituted and RE free samples does not contain the 112 phase. The Ce, Nd, Sm, Eu and Gd doped Ca1-xRExFeAs2 are new compounds. All of them exhibit superconducting transition except for the Ce doped sample. The behaviour of the critical temperature, with the RE ionic radii have been investigated.

cond-mat.supr-con↗
arXiv2013

Coexistence of bulk superconductivity and ferromagnetism in CeO1-xFxBiS2

We show the observation of the coexistence of bulk superconductivity and ferromagnetism in CeO1-xFxBiS2(x = 0 - 1.0) prepared by annealing under high-pressure. In CeO1-xFxBiS2 system, both superconductivity and two types of ferromagnetism with respective magnetic transition temperatures of 4.5 K and 7.5 K are induced upon systematic F substitution. This fact suggests that carriers generated by the substitution of O by F are supplied to not only the BiS2 superconducting layers but also the CeO blocking layers. Furthermore, the highest superconducting transition temperature is observed when the ferromagnetism is also enhanced, which implies that superconductivity and ferromagnetism are linked to each other in the CeO1-xFxBiS2 system.

cond-mat.supr-con↗
arXiv2013

α-FeAs-free SmFeAsO1-xFx by low temperature sintering with slow cooling

We obtained amorphous-FeAs-free SmFeAsO1-xFx using a low temperature sintering with slow cooling. SmFeAsO1-xFx is sintered at 980 °C for 40 hours and cooled slowly down to 600 °C. The low temperature sintering suppresses the formation of amorphous FeAs, and the slow cooling introduces much fluorine into SmFeAsO1-xFx. The superconductivity of this sample appears at 57.8 K and the superconducting volume fraction reaches 96 %. To study the change of fluorine concentration during the cooling process, samples are quenched by water at 950 °C, 900 °C, 850 °C, 800 °C, 750 °C and 700 °C. It is found that fluorine is substituted not only at the maximum heating temperature but also during the cooling process. The low temperature sintering with slow cooling is very effective to obtain a homogeneous SmFeAsO1-xFx with high fluorine concentration.

cond-mat.supr-con↗