掲載種別：研究論文（学術雑誌）   出版者・発行元：ACS Applied Energy Materials  

Impacts of an electrode active material with 1 μm scale thickness on the performance of an oxide-based all-solid-state battery (ASSB) have been investigated by metallic Na | Na3Zr2(SiO4)2(PO4) (NZSP) | Na3Ti2(PO4)3 (NTP) cells. Dense crystalline NTP layer is formed on NZSP ceramic electrolyte using spin coating of glass powder suspension and subsequent crystallization (glass-ceramic process). A sample with 0.6 μm thick NTP layer exhibits 0.1 C charge/discharge cycling with very small polarization (<0.03 V), and its initial capacity of ∼60 mA h g-1 is retained >80% after 60 cycles. Fast cathode kinetics also realized the 0.1 C capacity at −20 °C to be ∼80% retention. Furthermore, a facile control over the thickness of cathode films without any conductive additives in the 1 μm range demonstrated in this work enables the assessment of cathode kinetics. Electrochemical impedance spectroscopy revealed that Warburg resistance originates from an ambipolar diffusion of an electron and Na+ ion. The methodology presented here paves a way for rapid examinations of electrode material compatibility and electrode design for next-generation oxide-based ASSBs.

DOI： 10.1021/acsaem.2c03022

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掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Sol-Gel Science and Technology  

Thermal insulating materials are of importance for efficient and effective use of heat energy. Porous organic foams are widely employed for this purpose owing to their low thermal conductivity and mass productivity at low cost. However, the poor thermal stability of organic polymers limits their availability within a low temperature range (typically, < 150 °C). In this paper, we demonstrate the one-pot sol–gel synthesis of porous poly(amic acid) (PAA) xerogels and their conversion to porous polyimide (PI) monoliths by thermal imidization. The PAA networks crosslinked with either aromatic or aliphatic triamine and the bicontinuous porous morphology tailored via spinodal decomposition allow the evaporative drying at ambient pressure to obtain low-density PAA xerogels without significant shrinkage. The aromatic crosslinker yields the porous PI monoliths with higher porosity and flexible feature as compared with those prepared with the aliphatic triamine. On the other hand, the porous PI monolith based on the aliphatic crosslinker possesses slightly higher thermal stability owing to the stiffer mechanical property. The durability test results verify the porous PI thermal insulators are available in air at up to ~450 °C for a short period and up to ~350 °C for a long term. [Figure not available: see fulltext.]

DOI： 10.1007/s10971-022-05843-8

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その他リンク： https://link.springer.com/article/10.1007/s10971-022-05843-8/fulltext.html

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Inorganic Chemistry  

A novel perovskite fluoride, LixCoF3, which has an exceptionally low tolerance factor (0.81), has been synthesized via low-temperature lithium intercalation into a distorted ReO3-type fluoride CoF3using organolithium reagents. Interestingly, this reaction is completed within 15 min at room temperature. Synchrotron X-ray diffractometry and optical second harmonic generation at room temperature have revealed that this compound shows a high-temperature LiNbO3-type structure (space group: R3¯ c) involving Li-Co antisite defects and A-site splitting along the c direction. A-site splitting is consistent with the prediction based on hybrid Hartree-Fock density functional theory calculations. Co-L2,3edge X-ray absorption spectroscopy, as well as bond valence sum analysis, has verified the divalent oxidation state of Co ions in the lithiated phase, suggesting that its composition is close to LiCoF3(x ≈ 1). This compound exhibits a paramagnetic-to-antiferromagnetic transition at 36 K on cooling, accompanied by weak ferromagnetic ordering. The synthetic route based on low-temperature lithiation of metal fluorides host paves the way for obtaining a new LiNbO3-type fluoride family.

DOI： 10.1021/acs.inorgchem.2c01439

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リポジトリ公開URL： https://hdl.handle.net/2324/7174406

記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Chemistry of Materials  

Tailoring size, shape, and connectivity of oxygen coordination octahedra in perovskite-related oxides is known to play a key role in engineering their material properties, and furthermore, the interplay among the different types of oxygen octahedral distortions can open up new strategies for structure-driven control of functionalities. Here, we report that in layered perovskites AgRTiO4(R: rare earth), the biaxial negative thermal expansion (NTE) arises from the interplay between the structural distortions that alter the octahedral connectivity and shape, which is a fundamentally different mechanism from those in the conventional NTE materials. AgRTiO4was previously identified as having an orthorhombic (Pbcm) structure, but our experimental and theoretical study reveals that this compound adopts an acentric tetragonal (P4¯ 21m) structure due to (φ00)(0φ0)-type TiO6octahedral rotations, as in the previously reported Na and K analogs, NaRTiO4and KRTiO4. Thorough structural analysis reveals that the competition of the octahedral rotations with octahedral deformations drives the biaxial NTE; the decrease in the rotation amplitude caused by heating results in octahedral deformations, i.e., an out-of-plane elongation and an in-plane compression of TiO6octahedra, leading to shrinkage of the lattice parameters a and b (a = b). The Ag+-R3+layered ordering produces a built-in electric field compelling Ti4+to off center, which is the source for the otherwise unfavorable coexistence of octahedral rotations and deformations. Despite the competition between the two octahedral distortions being predicted to be active in other members of the ARTiO4series (A = Na, K, and Rb) as well, we do not observe experimentally the biaxial NTE for the Na members. Detailed analysis of calculated electronic structures highlights the essential role played by Ag-O-Ti covalent bonding in enhancing the octahedral deformation of AgRTiO4, which is directly responsible for the biaxial NTE. The present study provides an important example of functional properties that emerge from the coupling among distinct distortions of octahedral frameworks.

DOI： 10.1021/acs.chemmater.2c01245

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Chemistry of Materials  

The ability to reproducibly synthesize highly conductive solid electrolytes (SEs) is a prerequisite for the widespread usage of solid-state batteries. However, reported ionic conductivities of SEs exhibit significant variation even in materials with the same nominal composition. In this study, the thermodynamic origin of such sample-dependent variations is discussed using sodium-ion conducting Na3SbS4 as a model SE. The impact of uncontrolled variations in elemental chemical potentials on the ionic conductivity is investigated with theory and experiments. The elemental chemical potentials are uniquely defined when the system is constrained to have zero thermodynamic degrees of freedom. First, we establish the relationship between the chemical potentials and sodium-ion conductivity in Na3SbS4 by computing the phase diagram and native defect formation energies. From these calculations, we identify two distinct three-phase equilibrium regions (zero degrees of freedom) with the highest ratio of sodium-ion conductivity, which are then experimentally probed. Transport measurements reveal an abrupt change in the bulk ion transport of the phase-pure samples, with a room-temperature ionic conductivity of 0.16-1.2 mS cm-1 with a standard deviation of 50% when the elemental chemical potentials are not controlled, that is, not uniquely defined. In contrast, we show that by controlling the chemical potentials and therefore, the defect formation energies through the experimental concept of phase boundary mapping, the sample-dependent variation is reduced to 15% with a high average ionic conductivity of 0.94 mS cm-1. This study highlights the existence of "hidden"thermodynamic states defined by their chemical potentials and the need to precisely control these states to achieve reproducibly high ionic conductivity.

DOI： 10.1021/acs.chemmater.2c00944

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記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1016/j.ceramint.2022.05.125

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Inorganic Chemistry  

Narrow-gap semiconductors with visible light absorption capability have attracted attention as photofunctional materials. H--doped BaSn0.7Y0.3O3-δcontaining Sn(II) species was recently reported to absorb visible light up to 600 nm, which represents the first demonstration of oxyhydride-based visible-light-absorbers. In the present study, a more detailed investigation was made to obtain information on the synthesis and properties of H--doped perovskite-type stannate with respect to the A-site cation of the material and the preparation conditions. H--doped ASn0.7Y0.3O3-δ(A = Ba, Ba0.5Sr0.5, and Sr) obtained by the reaction of ASn0.7Y0.3O3-δprecursors with CaH2at 773 K under vacuum conditions was shown to have almost the same bandgap (ca. 2.1 eV), regardless of the A-site cation. Physicochemical measurements and theoretical calculations revealed that the identical bandgaps of H--doped ASn0.7Y0.3O3-δare due to the simultaneous shift of the midgap states composed of Sn2+with the conduction band minimum. Experimental results also indicated that the appropriate preparation conditions with respect to Y3+-substitution and the temperature for the synthesis of the ASn0.7Y0.3O3-δprecursors were essential to obtain H--doped products that have a low density of defects.

DOI： 10.1021/acs.inorgchem.2c00398

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記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevlett.127.215701

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1016/j.envpol.2021.117687

記述言語：英語   掲載種別：研究論文（学術雑誌）  

<jats:p>In the cement system, hydrocalumite is a candidate adsorbent for low-level 129I anionic species. However, the stability of hydrocalumite after immobilizing I− is unclear when they are exposed to pedosphere characterized by organic substances derived from living organisms. In the present work, five amino acids were selected as simplified models of natural organic substances under alkaline conditions. L-cysteine (H2Cys) and L-aspartic acid (H2Asp) accelerated the release of I− from I-hydrocalumite through ion-exchange. Ion-exchange of Cys2− with I− in I-hydrocalumite was faster than Asp2−, and the interlayer spacing (d003) of Cys-hydrocalumite was smaller than that of Asp-hydrocalumite. DFT simulations not only supported the above results but also predicted that there was a positive correlation between the formation energies and interlayer spacings of amino acids intercalated hydrocalumite, depending on the configurations. Moreover, in the DFT predictions, the interaction between amino acids and metallic hydroxide layers was responsible for the formation of hydrogen bonds and Ca-O chemical bonds between the -COO− groups and [Ca2Al(OH)6]+. The other three amino acids did not show intercalation through ion-exchange. The stability of I-hydrocalumite is influenced differently by coexisting amino acids, depending on the ionic sizes, charge numbers, and hydrophilicity, which cause the second contamination.</jats:p>

DOI： 10.3390/min11080836

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Visible-light-absorbing, narrow-gap semiconductors have attracted significant interest in recent years owing to their photofunctional applications, including heterogeneous photocatalysis and solar cells. Herein, we report a new Pb-free perovskite material with a wide range of visible-light absorption that was successfully synthesized via hydride (H-)-ion doping into BaSn0.7Y0.3O3-delta. Chemical reduction that accompanied H--ion doping significantly reduced the optical band gap of the host BaSn0.7Y0.3O3-delta from 4.0 to 2.0 eV. Physicochemical measurements revealed that BaSn0.7Y0.3O3-delta underwent the partial reduction of Sn4+ to Sn2+ at B-sites while maintaining the cubic perovskite structure. Density functional theory calculations showed that the electronic states derived from the Sn(2+ )lone pair, which was located above the top of the valence band, contributed to the visible-light absorption of reduced BaSn0.7Y0.3O3-delta, The prior introduction of oxygen defects into BaSnO3 by Y3+ substitution for Sn2+ was also indispensable to realize a significant reduction of the band gap. The reduced BaSn0.7Y0.3O3-delta doped with H- ions was applied as a photoelectrode material, and a clear anodic photoresponse of up to 600 nm was observed, which demonstrates that the material can be used for photofunctional applications. The results of this work suggest that the utilization of stereoactive lone pair of electrons in combination with H- doping may provide another approach to the band gap reduction of Sn(IV)-based oxides, which are nontoxic and less expensive, but generally wide-gap semiconductors.

DOI： 10.1021/acs.chemmater.1c00460

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1016/j.chemosphere.2021.129927

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Crystallography and dielectric properties in Dion-Jacobson layered perovskites, CsNdNb(2)O(7)and RbNdNb2O7, have been examined in dense polycrystalline samples, and polarization hysteresis loops that substantiate ferroelectricity have been observed at room temperature. The theoretical mechanism for the spontaneous polarization, "hybrid improper ferroelectric mechanism," induced by a combination of two types of non-polar octahedral rotations, is confirmed in these two phases. Our samples show remanent polarizations of 2-3 mu C cm(-2), which are much larger than those obtained in polycrystalline samples with the hybrid improper ferroelectricity reported so far. A dielectric constant in CsNdNb(2)O(7)exhibits an anomaly at 625 K, corresponding to the ferroelectric transition, as previously revealed by X-ray and neutron diffractometry. No dielectric anomaly is observed for RbNdNb(2)O(7)throughout the temperature range studied here (<= 773 K), which is consistent with the previous diffractometry showing the persistence of polar I2cmsymmetry up to 790 K.

DOI： 10.35848/1347-4065/abad46

リポジトリ公開URL： https://hdl.handle.net/2324/7177939

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/PhysRevMaterials.4.104419

記述言語：英語   掲載種別：研究論文（学術雑誌）  

A wide-range control over multimodal pore systems in porous monoliths is a key technology for developing functional materials, as the favorable pore structures in different length scales are required to be tailored depending on their application fields. In the alkoxy-derived sol-gel systems of silica and organosilicates, the synthetic methodology of meso- and macroporous monoliths with tunable pore properties has been developed by combining the supramolecular self-assembly of a Pluronic surfactant and polymerization-induced phase-separation techniques. This strategy has been applied to the sol-gel process of phenolic resins, giving rise to the hierarchically porous polymer gels with ordered mesoporosity and the corresponding carbon monoliths after carbonization. However, the controllable size range has been limited so far. This study has explored the relationship between the starting composition and the bimodal pore properties in further detail aiming at a better control of pore properties in phenolic resins. The enlargement of mesopore size has been achieved, yet associated with broadening the mesopore size distribution and coarsening the macropore morphology, resulting in the particle aggregates. The systematical investigation also reveals that the addition of KCl can improve the micelle arrangement in macroframework and provide the narrower mesopore size distribution.

DOI： 10.1007/s10971-020-05236-9

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1021/acsaem.0c00943

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We demonstrate inversion symmetry breaking induced by oxygen octahedral rotations in layered perovskite oxides KA(R)TiO(4) (A(R) = rare earth) using a combined experimental and theoretical approach including synchrotron x-ray diffraction, optical second harmonic generation, and first-principles lattice dynamics calculations. We experimentally find an interesting but counterintuitive phenomenon, i.e., the acentric-to-centric phase transition temperatures for K family are higher than those for previously reported Na family, in contrast to expectations based on the Goldschmidt tolerance factor, where the octahedral rotation instability toward the acentric phases would reduce with an increase in the radius of A-site alkali metal ions. Our detailed analysis of first-principles calculations for A(A)A(R)TiO(4) (A(A) = Na, K, Rb) reveals that the alkali metal and rare-earth ions play quite different roles in driving the octahedral rotations. Since rare-earth ions attract oxide ions more strongly than alkali metal ions due to the higher valence of the former in comparison with the latter (A(R)(3+) vs A(A)(+)), the optimization of coordination environment of rare-earth ions is the primary driving force of the octahedral rotations. Alkali metal ions serve to impose "bond strains" parallel to the layers, playing a secondary role in the octahedral rotations. Incorporation of large alkali metal ions generates a significant in-plane biaxial bond strain in A(R)O and TiO2 layers through the expanded A(A)O layers, and thereby facilitates the octahedral rotations because of the otherwise highly underbonding of rare-earth ions. Thus, the effect of A-site alkali metal size on the octahedral rotation instability can be explained in terms of the interlayer lattice mismatch. This understanding allows us to propose a geometric descriptor governing the structural instability in A(A)A(R)TiO(4) layered perovskites. We believe that control over the interlayer lattice mismatch could be a useful strategy to tune the octahedral rotations in layered compounds.

DOI： 10.1103/PhysRevMaterials.3.065001

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Polar metals exist as a rather unique class of materials as they combine two seemingly mutually exclusive properties (polar order and metallicity) in one system. So far only a few polar metals have been unambiguously identified; the magnetic ones are exceptionally rare. Here we investigate a 5&#37; Fe-doped polar metal Ca3Ru2O7, via electrical transport, magnetization, microstrain, and optical second-harmonic generation measurements. We report the full magnetic phase diagrams (in the field-temperature space) for magnetic field H parallel to a and H parallel to b, which exhibit distinct field-dependent magnetizations behavior. In particular, for H parallel to a we found a ferromagnetic incommensurate spin structure, which is absent in the pure Ca3Ru2O7. We propose a microscopic spin model to understand this behavior, highlighting the role of Fe doping in tipping the delicate balance of the underlying exchange-interaction energy in this system.

DOI： 10.1103/PhysRevB.99.224411

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Over the past few years, several studies have reported the existence of polar phases in n = 2 Ruddlesden-Popper layer perovskites by trilinear coupling of oxygen octahedral rotations (OOR) and polar distortions, a phenomenon termed as hybrid improper ferroelectricity. This phenomenon has opened an avenue to expand the available compositions of ferroelectric and piezoelectric layered oxides. In this study, we report a new polar n = 2 Ruddlesden-Popper layered niobate, Li₂SrNb₂O₇, which undergoes a structural transformation to an antipolar phase when cooled to 90 K. This structural transition results from a change in the phase of rotation of the octahedral layers within the perovskite slabs across the interlayers. First-principles calculations predicted that the antipolar Pnam phase would compete with the polar A2₁am phase and that both would be energetically lower than the previously assigned centrosymmetric Amam phase. This phase transition was experimentally observed by a combination of synchrotron X-ray diffraction, powder neutron diffraction, and electrical and nonlinear optical characterization techniques. The competition between symmetry breaking to yield polar layer perovskites and hybrid improper antiferroelectrics provides new insight into the rational design of antiferroelectric materials that can have applications as electrostatic capacitors for energy storage.

DOI： 10.1021/acs.chemmater.9b00786

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Ceramic sheets of Na3V2(PO4)(3) (NVP) with NASICON-related structure have been prepared by tape-casting method. The addition of 60Na(2)O-10Nb(2)O(5)-30P(2)O(5) glass to NVP is demonstrated to enhance markedly the density of the sintered NVP sheet and its electrical conductivity. The best densified NVP sheet with a thickness of a few tens of mu m is obtained by sintering tape-casted NVP added with 5 wt&#37; glass at 920 degrees C, which exhibits a total conductivity of 2.9 x 10(-5) S cm(-1) at room temperature. A Pt vertical bar NVP sheet vertical bar Pt cell works as a tiny solid-state battery with a cell potential of 1.7 V, in which NVP acts as both cathode and anode active materials via redox reactions of V4+/V3+ and V3+/V2+ as well as a Na+ ion conducting electrolyte between the two electrode layers. (C) 2019 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.electacta.2019.03.057

記述言語：英語   掲載種別：研究論文（学術雑誌）  

A robust ceramic solid electrolyte with high ionic conductivity is a key component for all-solid-state batteries (ASSBs). In terms of the demand for high-energy-density storage, researchers have been tackling various challenges to use metal anodes, where a fundamental understanding on the metal/solid electrolyte interface is of particular importance. The Na+ superionic conductor, so-called NASICON, has high potential for application to ASSBs with a Na anode due to its high Na+ ion conductivity at room temperature, which has, however, faced a daunting issue of the significantly large interfacial resistance between Na and NASICON. In this work, we have successfully reduced the interfacial resistance as low as 14 Omega cm(2) at room temperature by a simple mechanical compression of a Na/NASICON assembly. We also demonstrate a fundamental study of the Na/NASICON interface in comparison with the Na/beta ''-alumina counterpart by means of the electrochemical impedance technique, which elucidates a stark difference between the activation energies for interfacial charge transfer: similar to 0.6 eV for Na/NASICON and similar to 0.3 eV for Na/beta ''-alumina. This result suggests the formation of a Na+-conductive interphase layer in pressing Na metal on the NASICON surface at room temperature.

DOI： 10.1021/acsaem.9b00250

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Due to the strong dependence of electronic properties on the local bonding environment, a full characterization of the structural dynamics in ultrafast experiments is critical. Here, we report the dynamics and structural refinement at nanosecond time scales of a perovskite thin film by combining optical excitation with time-resolved X-ray diffraction. This is achieved by monitoring the temporal response of both integer and half-integer diffraction peaks of LaVO3 in response to an above-band-gap 800 nm pump pulse. We find that the lattice expands by 0.1&#37; out of plane, and the relaxation is characterized by a biexponential decay with 2 and 12 ns time scales. We analyze the relative intensity change in half-integer peaks and show that the distortions to the substructure are small: the oxygen octahedral rotation angles decrease by similar to 0.3 degrees and La displacements decrease by similar to 0.2 pm, which directly corresponds to an similar to 0.8 degrees increase in the V-O-V bond-angles, an in-plane V-O bond length reduction of similar to 0.3 pm, and an unchanged out-of-plane bond length. This demonstration of tracking the atomic positions in a pump-probe experiment provides experimentally accessible values for structural and electronic tunability in this class of materials and will stimulate future experiments. (C) 2019 Author(s).

DOI： 10.1063/1.5045704

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Polar instabilities are well known to be suppressed on scaling materials down to the nanoscale, when the electrostatic energy increase at surfaces exceeds lowering of the bulk polarization energy. Surprisingly, here we report an emergent low symmetry polar phase arising in nanoscale powders of CaTiO3, the original mineral named perovskite discovered in 1839 and considered nominally nonpolar at any finite temperature in the bulk. Using nonlinear optics and spectroscopy, X-ray diffraction, and microscopy studies, we discover a well-defined polar to non-polar transition at a T-C = 350 K in these powders. The same polar phase is also seen as a surface layer in bulk CaTiO3 single crystals, forming striking domains with in-plane polarization orientations. Density functional theory reveals that oxygen octahedral distortions in the surface layer lead to the stabilization of the observed monoclinic polar phase. These results reveal new ways of overcoming the scaling limits to polarization in perovskites. (C) 2019 Author(s).

DOI： 10.1063/1.5078706

記述言語：英語   掲載種別：研究論文（学術雑誌）  

A mixed valence compound, sodium titanium oxide bronze (NaxTiO2-B), combines intriguing properties of high electric conductivity and good chemical stability together with a unique one-dimensional tunnel crystal structure available for cation storage. However, this compound has not been studied for a long period because of the strongly reductive condition at high temperature required for its preparation, which limits the morphological control such as the preparation of nanocrystals. For the first time in this paper, the topotactic synthesis of nano-sized NaxTiO2-B with high specific surface area (>130 m(2) g(-1)) from TiO2(B) nanoparticles has been demonstrated. The reaction of metastable TiO2(B) with NaBH4 allows carrier electrons to be doped simultaneously with incorporation of Na+ ions into the interstitial sites of the host Ti-O lattice at relatively low temperature. An electrochemical investigation of Li+- and Na+-ion storage behaviors suggests that the incorporated Na+ ions are mainly placed in the 6-fold coordination sites of bronze. In addition, optical measurements including time-resolved transient spectroscopy revealed that the doped electrons in the NaxTiO2-B nanoparticles are predominantly in the Ti3+ state and behave as a small polaron. The pelletized NaxTiO2-B nanoparticles shows a good electronic conductivity of 1.4 x 10(-2) S cm(-1) at 30 degrees C with an activation energy of 0.17 eV, which is attributable to the thermal barrier for the polaron hopping.

DOI： 10.1039/c8nr08372j

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Using first-principles calculations, we predict a strong coupling of lattice, charge, and spin degrees of freedom in Ruddlesden-Popper phase La3Ni2O7, which enables a phase control with epitaxial strain. While the bulk ground state is metallic, moderate compressive strain is found to trigger a Peierls transition to an insulating state in concurrence with a breathing distortion of NiO6 octahedra. The Peierls transition is microscopically interpreted as a band-gap opening arising from lifting and lowering of the Ni d-e(g) states due to the octahedral breathing.

DOI： 10.1103/PhysRevMaterials.2.125001

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Stable structures of metallic antiperovskites ACNi(3) (A = Mg, Zn, and Cd) under epitaxial strain are explored using first- principles lattice dynamics calculations. Although the ground states of ACNi(3) are cubic nonpolar phases without structural distortion, transitions to polar phases are predicted to arise in these compounds under compressive strain while maintaining metallic states. In particular, the polar phase of MgCNi3 would be attainable with moderate strain of a few percent. The polar distortions are considered to be generated by the enhancement of the hybridization between C 2p and Ni 3d states, which is a mechanism analogous to that of the previously discovered polar metal CeSiPt3.

DOI： 10.1103/PhysRevMaterials.2.125004

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Hybrid improper ferroelectricity, which utilizes non polar but ubiquitous rotational/tilting distortions to create polarization, offers an attractive route to the discovery of new ferroelectric and multiferroic materials because its activity derives from geometric rather than electronic origins. Design approaches blending group theory and first principles can be utilized to explore the crystal symmetries of ferroelectric ground states, but in general, they do not make accurate predictions for some important parameters of ferroelectrics, such as Curie temperature (T-C). Here, we establish a predictive and quantitative relationship between T-C and the Goldschmidt tolerance factor, t, by employing n = 2 Ruddlesden-Popper (RP) A(3)B(2)O(7) as a prototypical example of hybrid improper ferroelectrics. The focus is placed on an RP system, (Sr1-xCax)(3)Sn2O7 (x = 0, 0.1, and 0.2), which allows for the investigation of the purely geometric (ionic size) effect on ferroelectric transitions, due to the absence of the second-order Jahn-Teller active (d(0) and 6s(2)) cations that often lead to ferroelectric distortions through electronic mechanisms. We observe a ferroelectric-to-paraelectric transition with T-C = 410 K for Sr3Sn2O7. We also find that the T-C increases linearly up to 800 K upon increasing the Ca2+ content, i.e., upon decreasing the value of t. Remarkably, this linear relationship is applicable to the suite of all known A(3)B(2)O(7) hybrid improper ferroelectrics, indicating that the T-C correlates with the simple crystal chemistry descriptor, t, based on the ionic size mismatch. This study provides a predictive guideline for estimating the T-C of a given material, which would complement the convergent group-theoretical and first-principles design approach.

DOI： 10.1021/jacs.8b07998

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Physical systems evolve from one state to another along paths of least energy barrier. Without apriori knowledge of the energy landscape, multidimensional search methods aim to find such minimum energy pathways between the initial and final states of a kinetic process. However, in many cases, the user has to repeatedly provide initial guess paths, thus implying that the reliability of the final result is heavily user-dependent. Recently, the idea of "distortion symmetry groups" as a complete description of the symmetry of a path has been introduced. Through this, a new framework is enabled that provides a powerful means of classifying the infinite collection of possible pathways into a finite number of symmetry equivalent subsets, and then exploring each of these subsets systematically using rigorous group theoretical methods. The method, which we name the distortion symmetry method, is shown to lead to the discovery of previously hidden pathways for the case studies of bulk ferroelectric switching and domain wall motion in proper and improper ferroelectrics, as well as in multiferroic switching. These provide novel physical insights into the nucleation of switching pathways at experimentally observed domain walls in Ca(3)Ti2O(7), as well as how polarization switching can proceed without reversing magnetization in BiFeO3. Furthermore, we demonstrate how symmetry-breaking from a highly symmetric pathway can be used to probe the non-Ising (Bloch and Neel) polarization components integral to transient states involved in switching in PbTiO3. The distortion symmetry method is applicable to a wide variety of physical phenomena ranging from structural, electronic and magnetic distortions, diffusion, and phase transitions in materials.

DOI： 10.1103/PhysRevB.98.085107

記述言語：英語   掲載種別：研究論文（学術雑誌）  

In contrast to polar cation displacements driving oxides into noncentrosymmetric and ferroelectric states, inversion-preserving anion displacements, such as rotations or tilts of oxygen octahedra about cation coordination centers, are exceedingly common. More than one nonpolar rotational mode in layered perovskites can lift inversion symmetry and combine to induce an electric polarization through a hybrid improper ferroelectric (HIF) mechanism. This form of ferroelectricity expands the compositional palette to new ferroelectric oxides because its activity derives from geometric rather than electronic origins. Here, the new Ruddlesden-Popper HIF Sr3Zr2O7, which is the first ternary lead-free zirconate ferroelectric, is reported and room-temperature polarization switching is demonstrated. This compound undergoes a first-order ferroelectric-to-paraelectric transition, involving an unusual change in the sense of octahedral rotation while the octahedral tilt remains unchanged. Our experimental and first-principles study shows that the paraelectric polymorph competes with the polar phase and emerges from a trilinear coupling of rotation and tilt modes interacting with an antipolar mode. This form of hybrid improper antiferroelectricity is recently predicted theoretically but has remained undetected. This work establishes the importance of understanding anharmonic interactions among lattice degrees of freedom, which is important for the discovery of new ferroelectrics and likely to influence the design of next-generation thermoelectrics.

DOI： 10.1002/adfm.201801856

その他リンク： http://doi.wiley.com/10.1002/adfm.201801856

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Polar metals, commonly defined by the coexistence of polar crystal structure and metallicity, are thought to be scarce because the long-range electrostatic fields favoring the polar structure are expected to be fully screened by the conduction electrons of a metal. Moreover, reducing from three to two dimensions, it remains an open question whether a polar metal can exist. Here we report on the realization of a room temperature two-dimensional polar metal of the B-site type in tri-color (tri-layer) superlattices BaTiO3/SrTiO3/LaTiO3. A combination of atomic resolution scanning transmission electron microscopy with electron energy-loss spectroscopy, optical second harmonic generation, electrical transport, and first-principles calculations have revealed the microscopic mechanisms of periodic electric polarization, charge distribution, and orbital symmetry. Our results provide a route to creating all-oxide artificial non-centrosymmetric quasi-two-dimensional metals with exotic quantum states including coexisting ferroelectric, ferromagnetic, and superconducting phases.

DOI： 10.1038/s41467-018-03964-9

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Using time- and spatially resolved hard x-ray diffraction microscopy, the striking structural and electrical dynamics upon optical excitation of a single crystal of BaTiO3 are simultaneously captured on subnanoseconds and nanoscale within individual ferroelectric domains and across walls. A large emergent photoinduced electric field of up to 20 x 10(6) V/m is discovered in a surface layer of the crystal, which then drives polarization and lattice dynamics that are dramatically distinct in a surface layer versus bulk regions. A dynamical phase-field modeling method is developed that reveals the microscopic origin of these dynamics, leading to gigahertz polarization and elastic waves traveling in the crystal with sonic speeds and spatially varying frequencies. The advances in spatiotemporal imaging and dynamical modeling tools open up opportunities for disentangling ultrafast processes in complex mesoscale structures such as ferroelectric domains.

DOI： 10.1103/PhysRevLett.120.096101

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The group VIb dichalcogenides (MX
₂
, M = Mo, W; X= S, Se) have a layered molybdenite structure in which M atoms are coordinated by a trigonal prism of X atoms. Ternary solid solutions of MS
_x
Se
_2-x
were synthesized, microcrystals were grown by chemical vapor transport, and their morphologies and structures were characterized by using synchrotron X-ray diffraction, Rietveld refinement, DIFFaX simulation of structural disorder, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Double aberration corrected scanning transmission electron microscopy was used to determine the anion distributions in single-layer nanosheets exfoliated from the microcrystals. These experiments indicate that the size difference between S and Se atoms does not result in phase separation, consistent with earlier studies of MX
₂
monolayer sheets grown by chemical vapor deposition. However, stacking faults occur in microcrystals along the layering axis, particularly in sulfur-rich compositions of MS
_x
Se
_2-x
solid solutions.

DOI： 10.1039/c8ra01497c

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Perovskite rare-earth cobaltites ACoO(3) (A = Sc, Y, La Lii) have been of enduring interest for decades due to their unusual structural and physical properties associated with the spin-state transitions of low-spin Co3+ ions. Herein, we have synthesized a non rare-earth perovskite cobaltite, InCoO3, at 15 GPa and 1400 degrees C and investigated its crystal structure and magnetic ground state. Under the same high-pressure and high-temperature conditions, we also prepared a perovskite-type ScCoO3 with an improved cation stoichiometry in comparison to that in a previous study, where synthesis at 6 GPa and 1297 degrees C yielded a perovskite cobaltite with cation mixing on the A-site, (Sc0.95Co0.05)CoO3. The two perovskite phases have nearly stoichiometric cation compositions, crystallizing in the orthorhombic Pnma space group. In the present investigation, comprehensive studies on newly developed and well-known Pnma ACoO(3) perovskites (A = In, Sc, Y, Pr-Lu) show that InCoO3 does not fulfill the general evolution of crystal metrics with A-site cation size, indicating that InCoO3 and rare-earth counterparts have different chemistry for stabilizing the Pnma structures. Detailed structural analyses combined with first-principles calculations reveal that the origin of the anomaly for InCoO3 is ascribed to the A-site cation displacements that accompany octahedral tilts; despite the highly tilted CoO6 network, the In-O covalency makes In3+ ions reluctant to move from their ideal cubic-symmetry position, leading to less orthorhombic distortion than would be expected from electrostatic/ionic size mismatch effects. Magnetic studies demonstrate that InCoO3 and ScCoO3 are diamagnetic with a low-spin state of Co3+ below 300 K, in contrast to the case of (Sc0.95Co0.05)CoO3, where the high-spin Co3+ ions on the A-site generate a large paramagnetic moment. The present work extends the accessible composition range of the low-spin orthocobaltite series and thus should help to establish a more comprehensive understanding of the structure property relation.

DOI： 10.1021/acs.inorgchem.7b01426

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The zinc nitride Zn3N2 is composed of inexpensive and earth-abundant Zn and N elements and shows high electron mobility exceeding 100 cm(2) V-1 s(-1). Although various technological applications of Zn3N2 have been suggested so far, the synthesis of high-quality Zn3N2 samples, especially single crystals, is still challenging, and therefore its basic properties are not yet well understood. Indeed, the reported band gaps of as-grown Zn3N2 widely scatter from 0.85 to 3.2 eV. In this study, we investigate the large gap variation of Zn3N2 in terms of the Burstein-Moss (BM) effect and point-defect energetics using first-principles calculations. First, we discuss the relation between electron carrier concentration and optical gaps based on the electronic structure obtained using the Heyd-Scuseria-Ernzerhof hybrid functional. The calculated fundamental band gap is 0.84 eV in a direct-type band structure. Second, thermodynamic stability of Zn3N2 is assessed using the ideal-gas model in conjunction with the rigid-rotor model for gas phases and first-principles phonon calculations for solid phases. Third, carrier generation and compensation by native point defects and unintentionally introduced oxygen and hydrogen impurities are discussed. The results suggest that a significant BM shift occurs mainly due to oxygen substitutions on nitrogen sites and hydrogen interstitials. However, gaps larger than 2.0 eV would not be due to the BM shift because of the Fermi-level pinning caused by acceptorlike zinc vacancies and hydrogen-on-zinc impurities. Furthermore, we discuss details of peculiar defects such as a nitrogen-on-zinc antisite with azidelike atomic and electronic structures.

DOI： 10.1103/PhysRevApplied.8.014015

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We report the observation of noncentrosymmetricity in the family of HRTiO4 (R = Eu, Gd, Dy) layered oxides possessing a Ruddlesden Popper derivative structure, by second harmonic generation and synchrotron X-ray diffraction with the support of density functional theory calculations. These oxides were previously thought to possess inversion symmetry. Here, inversion symmetry is lifted by rotations of the oxygen-coordinated octahedra, a mechanism that is not active in simple perovskites. We observe a competition between rotations of the oxygen octahedra and sliding of a combined unit of perovskite rocksalt perovskite blocks at the proton layers. For the smaller rare earth ions, R = Eu, Gd, and Dy, which favor the octahedral rotations, noncentrosymmetricity is present but the sliding is absent. For the larger rare earth ions, R = Nd and Sm, the octahedral rotations are absent, but the sliding at the proton layers is present to optimize the length and direction of hydrogen.bonding in the crystal structure. The study reveals a new mechanism for inducing noncentrosymmetricity in layered oxides, and chemical structural effects related to rare earth ion size and hydrogen bonding that can turn this mechanism on and off. We construct a phase diagram of temperature versus rare earth ionic radius for the HRTiO4 family.

DOI： 10.1021/acs.chemmater.6b04103

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Great effort has been devoted to developing single-phase magnetoelectric multiferroics, but room-temperature coexistence of large electric polarization and magnetic ordering still remains elusive. Our recent finding shows that such polar magnets can be synthesized in small-tolerance-factor perovskites AFeO(3) with unusually small cations at the A-sites, which are regarded as having a LiNbO3-type structure (space group R3c). Herein, we experimentally reinforce this finding by preparing a novel room-temperature polar magnet, LiNbO3-type InFeO3. This compound is obtained as a metastable quench product from an orthorhombic perovskite phase stabilized at 15 GPa and an elevated temperature. The structure analyses reveal that the polar structure is characterized by displacements of In3+ (d(10)) and Fe3+ (d(5)) ions along the hexagonal c-axis (pseudocubic [111] axis) from their centrosymmetric positions, in contrast to well-known perovskite ferroelectrics (e.g., BaTiO3, PbTiO3, and BiFeO3) where d(0) transition-metal ions and/or 6s(2) lone-pair cations undergo polar displacements through the so-called second-order Jahn-Teller (SOJT) distortions. Using density functional theory calculations, the electric polarization of LiNbO3-type InFeO3 is estimated to be 96 mu C/cm(2) along the c-axis, comparable to that of an isostructural and SOJT-active perovskite ferroelectric, BiFeO3 (90-100 mu C/cm(2)). Magnetic studies demonstrate weak ferromagnetic behavior at room temperature, as a result of the canted G-type antiferromagnetic ordering of Fe3+ moments below T-N similar to 545 K. The present work shows the functional versatility of small-tolerance-factor perovskites and provides a useful guide for the synthesis and design of room-temperature polar magnets.

DOI： 10.1021/acs.chemmater.6b02783

記述言語：英語  

記述言語：英語  

記述言語：英語  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Cationic rearrangement is a compelling strategy for producing desirable physical properties by atomic-scale manipulation. However, activating ionic diffusion typically requires high temperature, and in some cases also high pressure in bulk oxide materials. Herein, we present the cationic rearrangement in bulk Mn2FeMoO6 at unparalleled low temperatures of 150-300 degrees C. The irreversible ionic motion at ambient pressure, as evidenced by real-time powder synchrotron X-ray and neutron diffraction, and second harmonic generation, leads to a transition from a Ni3TeO6-type to an ordered-ilmenite structure, and dramatic changes of the electrical and magnetic properties. This work demonstrates a remarkable cationic rearrangement, with corresponding large changes in the physical properties in a bulk oxide at unprecedented low temperatures.

DOI： 10.1002/anie.201511360

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The crystal structures of europium niobate, EuNbO3, have been examined over a wide temperature range between 20 and 500 K using synchrotron X-ray diffraction. We have observed two successive structural phase transitions at 360 and 460 K. Below 350 K, EuNbO3 adopts an orthorhombic perovskite structure (space group Imma), which is characterized by NbO6 octahedral tilting about the pseudocubic two-fold axis. The result differs from previous reports in which EuNbO3 was assigned to a cubic aristotype (space group Pm (3) over barm) of perovskite at room temperature. At around 360 K, EuNbO3 undergoes a first-order phase transition to a tetragonal symmetry (space group I4/mcm) with the NbO6 octahedral tilting about the pseudocubic four-fold axis. As the temperature is further increased, the I4/mcm tetragonal phase changes into the Pm (3) over barm cubic aristotype at 460 K. The tetragonal-to-cubic transformation is characterized as a continuous phase transition. (C) 2016 Elsevier Inc. All rights reserved.

DOI： 10.1016/j.jssc.2016.04.032

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The loss of centrosymmetry via oxygen octahedral rotations is demonstrated in the n = 2 Dion-Jacobson family of layered oxide perovskites, A'LaB2O7 (A' = Rb, Cs; B = Nb, Ta). Ab initio density functional theory calculations predict that all four materials should adopt polar space groups, in contrast to the results of previous experimental studies that have assigned these materials to the centrosymmetric P4/mmm space group. Optical second harmonic generation experiments confirm the presence of a noncentrosymmetric phase at ambient temperature. Piezoresponse force microscopy experiments also show that this phase is piezoelectric. To elucidate the symmetry-breaking and assign the appropriate space groups, the crystal structure of CsLaNb2O7 is refined as a function of temperature from synchrotron X-ray diffraction data. Above 550 K, CsLaNb2O7 adopts the previously determined centrosymmetric P4/mmm space group. Between 550 and 350 K, the symmetry is lowered to the noncentrosymmetric space group Amm2. Below 350 K, additional symmetry lowering is observed as peak splitting, but the space group cannot be unambiguously identified.

DOI： 10.1002/adfm.201504046

記述言語：英語  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

PbMnTeO6, a new noncentrosymmetric layered magnetic oxide was synthesized and characterized. The crystal structure is hexagonal, with space group P (6) over bar 2m (No. 189), and consists of edge-sharing (Mn4+/Te6+)O-6 trigonal prisms that form honeycomb-like two-dimensional layers with Pb2+ ions between the layers. The structural difference between PbMnTeO6, with disordered/trigonal prisms of Mn4+/Te6+, versus the similar chiral SrGeTeO6 (space group P312), with long-range order of Ge4+ and Te6+ in octahedral coordination, is attributed to a difference in the electronic effects of Ge4+ and Mn4+. Temperature-dependent second harmonic generation by PbMnTeO6 confirmed the noncentrosymmetric character between 12 and 873 K. Magnetic measurements indicated antiferromagnetic order at T-N approximate to 20 K and a frustration parameter (vertical bar theta vertical bar/TN) of similar to 2.16.

DOI： 10.1021/acs.inorgchem.5b02677

記述言語：英語   掲載種別：研究論文（学術雑誌）  

An improper mechanism for breaking inversion symmetry is revealed and thus inducing piezoelectricity in the family of layered perovskites, LiRTiO4 (R = rare earths), which are previously reported as centrosymmetric. Noncentrosymmetry in this family of compounds arises from TiO6 octahedral rotation represented by a(-)b(o)c(o)/b(o)a(-)c(o) in the perovskite blocks between RO rock salt and LiO antifl uorite layers. X-ray diffraction and optical second harmonic generation complemented by density functional theory predictions are crucial in determining the new structures. High transition temperature (T-ac) of up to 1200 K from noncentrosymmetric to centrosymmetric phase is observed. Piezoelectric coefficients (d(36)) of up to -15 pC/N are predicted, and piezoelectric force microscopy experiments confirm a piezoelectric response. The demonstrated improper mechanism in this and other layered oxide families, with a wide range of available topologies and chemistries, could aid in the search for high temperature piezoelectrics.

DOI： 10.1002/aelm.201500196

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Oxynitrides have been explored extensively in the past decade because of their interesting properties, such as visible-light absorption, photocatalytic activity and high dielectric permittivity. Their synthesis typically requires high-temperature NH3 treatment (800-1,300 degrees C) of precursors, such as oxides, but the highly reducing conditions and the low mobility of N(3-)species in the lattice place significant constraints on the composition and structure-and hence the properties-of the resulting oxynitrides. Here we show a topochemical route that enables the preparation of an oxynitride at low temperatures (< 500 degrees C), using a perovskite oxyhydride as a host. The lability of H-in BaTiO3-xHx (x <= 0.6) allows H-/N3- exchange to occur, and yields a room-temperature ferroelectric BaTiO3-xN2x/3. This anion exchange is accompanied by a metal-to-insulator crossover via mixed O-H-N intermediates. These findings suggest that this 'labile hydride' strategy can be used to explore various oxynitrides, and perhaps other mixed anionic compounds.

DOI： 10.1038/NCHEM.2370

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Oxynitrides have been explored extensively in the past decade because of their interesting properties, such as visible-light absorption, photocatalytic activity and high dielectric permittivity. Their synthesis typically requires high-temperature NH3 treatment (800-1,300 degrees C) of precursors, such as oxides, but the highly reducing conditions and the low mobility of N(3-)species in the lattice place significant constraints on the composition and structure-and hence the properties-of the resulting oxynitrides. Here we show a topochemical route that enables the preparation of an oxynitride at low temperatures (< 500 degrees C), using a perovskite oxyhydride as a host. The lability of H-in BaTiO3-xHx (x <= 0.6) allows H-/N3- exchange to occur, and yields a room-temperature ferroelectric BaTiO3-xN2x/3. This anion exchange is accompanied by a metal-to-insulator crossover via mixed O-H-N intermediates. These findings suggest that this 'labile hydride' strategy can be used to explore various oxynitrides, and perhaps other mixed anionic compounds.

DOI： 10.1038/NCHEM.2370

記述言語：英語  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Mn22+Fe2+W6+O6, a new polar magnetic phase, adopts the corundum-derived Ni3TeO6-type structure with large spontaneous polarization (P-S) of 67.8 mu C cm-2, complex antiferromagnetic order below approximate to 75 K, and field-induced first-order transition to a ferrimagnetic phase below approximate to 30 K. First-principles calculations predict a ferrimagnetic (udu) ground state, optimal switching path along the c-axis, and transition to a lower energy udu-udd magnetic double cell.

DOI： 10.1002/adma.201405244

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Multiferroic materials have been the subject of intense study, but it remains a great challenge to synthesize those presenting both magnetic and ferroelectric polarizations at room temperature. In this work, we have successfully obtained LiNbO3-type ScFeO3, a metastable phase converted from the orthorhombic perovskite formed under 15 GPa at elevated temperatures. A combined structure analysis by synchrotron X-ray and neutron powder diffraction and high-angle annular dark-field scanning transmission electron microscopy imaging reveals that this compound adopts the polar R(3)c symmetry with a fully ordered arrangement of trivalent Sc and Fe ions, forming highly distorted ScO6 and FeO6 octahedra. The calculated spontaneous polarization along the hexagonal c-axis is as large as 100 mu C/cm(2). The magnetic studies show that LiNbO3-type ScFeO3 is a weak ferromagnet with TN = 545 K due to a canted G-type antiferromagnetic ordering of Fe3+ spins, representing the first example of LiNbO3-type oxides with magnetic ordering far above room temperature. A comparison of the present compound and rare-earth orthorhombic perovskites RFeO3 (R = La-Lu and Y), all of which possess the corner-shared FeO6 octahedral network, allows us to find a correlation between TN and the Fe-O-Fe bond angle, indicating that the A-site cation-size-dependent octahedral tilting dominates the magnetic transition through the Fe-O-Fe superexchange interaction. This work provides a general and versatile strategy to create materials in which ferroelectricity and ferromagnetism coexist at high temperatures

DOI： 10.1021/ja507958z

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Rotations of oxygen octahedra are ubiquitous, but they cannot break inversion symmetry in simple perovskites. However, in a layered oxide structure, this is possible, as we demonstrate here in A-site ordered Ruddlesden-Popper NaRTiO4 (R denotes rare-earth metal), previously believed to be centric. By revisiting this series via synchrotron x-ray diffraction, optical second-harmonic generation, piezoresponse force microscopy, and first-principles phonon calculations, we find that the low-temperature phase belongs to the acentric space group P(4) over bar 2(1)m, which is piezoelectric and nonpolar. The mechanism underlying this large new family of acentric layered oxides is prevalent, and could lead to many more families of acentric oxides.

DOI： 10.1103/PhysRevLett.112.187602

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We have prepared amorphous xFeO center dot(100 - x)SiO2 (x = 38.2, 54.8, and 67.5 in mol &#37;) thin films by a pulsed laser deposition method and examined their magnetic properties. The temperature dependence of zero-field-cooled magnetization manifests a cusp-like peak, i.e., spin-freezing in a low temperature region. The spin-freezing temperature depends on the frequency of ac magnetic field, and the dependence is explainable in terms of the scaling law. The aging-memory effect characteristic of spin glass is also observed. Furthermore, superparamagnetic behavior is found in the variation of magnetization with magnetic field at high temperatures. These observations indicate that there exist magnetic clusters of Fe2+ ions in the amorphous FeO-SiO2 thin films and that the intercluster interactions bring about the superspin glass transition at low temperatures. (C) 2014 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.53.05FB11

記述言語：英語   掲載種別：研究論文（学術雑誌）  

A novel silver-containing perovskite, Ag-Cu3V4O12, was synthesized under high-pressure and high-temperature conditions. It crystallizes in an A-site-ordered perovskite structure (space group Im (3) over bar), in which silver ions occupy the 12-coordinated A sites forming regular icosahedra, and exhibits metallic behavior. Bond-valencesum calculations and X-ray photoemission spectroscopy reveal that Ag ions are present in the mixed-valence state, most likely attributable to the coexistence of Ag+ and Ag3+, unlike the case of well-known perovskite-type AgNbO3 and AgTaO3 containing only Ag+ ions. We discuss metallic conduction in relation to electronic structure calculations.

DOI： 10.1021/ic402579v

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Magnetic, optical, and magneto-optical properties have been examined for aluminoborosilicate glasses with high concentration of divalent europium ions. An analysis on temperature dependence of magnetic susceptibility yields positive Weiss temperatures, indicating that magnetic moments of 4f electrons of Eu2+ interact ferromagnetically with each other in the glasses. The glasses show large Faraday rotation angles in the visible range; glass with a composition of 58.0EuO center dot 12.0Al(2)O(3)center dot 20.0B(2)O(3)center dot 10.0SiO(2) (mol&#37;) has a Verdet constant of -1.03 min/Oe cm (-300 rad/T m) at 633 nm. Wavelength dependence of the Verdet constant is interpreted in terms of Eu2+ 4f(7) to 4f(6)5d(1) electronic transitions based on the Van Vleck-Hebb theory. (C) 2012 Elsevier B.V. All rights reserved:

DOI： 10.1016/j.optmat.2012.10.029

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Amorphous Eu2TiO4 and EuTiO3 have been studied by a combination of the Faraday effect in the visible region and polarization-dependent x-ray absorption spectroscopy at the Eu M-4,M-5 and L-2,L-3 edges to examine the role of Eu 4f-5d exchange interactions on the ferromagnetic behavior. The bulk-sensitive x-ray absorption spectra (XAS) for Eu L-2,L-3 edges show that most of the europium ions are present as the divalent state in the amorphous Eu2TiO4 and EuTiO3. The Eu M-4,M-5 edge x-ray magnetic circular dichroism (XMCD) signals, measured for the amorphous Eu2TiO4, dramatically increase upon cooling through the Curie temperature (16 K) determined by a superconducting quantum interference device (SQUID) magnetometer. Sum-rule analysis of the XMCD at Eu M-4,M-5 edges measured at 10 K yields a 4f spin magnetic moment of 6.6 mu(B) per Eu2+ ion. These results confirm that the ferromagnetic properties exclusively arise from 4f spins of Eu2+. In addition, for both the amorphous Eu2TiO4 and EuTiO3, the temperature and magnetic-field dependence of Eu L-2,L-3 edge XMCD signals can be scaled with the corresponding magnetization measured by SQUID, indicating that the 5d magnetic polarization of Eu2+ is involved in the process to cause the ferromagnetic interaction between Eu2+ ions. We further discuss the origin of ferromagnetism in the amorphous system on the basis of the energy diagram of Eu 4f and 5d levels deduced from the Faraday effect in the visible region. From the wavelength dependence of Faraday rotation angles of the amorphous EuO-TiO2 system in comparison with those of the divalent Eu chalcogenides as reported previously, it is found that the magnitude of crystal-field splitting of Eu 5d levels in the former is on the same order as that in the latter, which explains an enhanced ferromagnetic exchange interaction between Eu 4f and 5d states.

DOI： 10.1103/PhysRevB.88.024405

記述言語：英語   掲載種別：研究論文（学術雑誌）  

First-principles calculations reveal that in divalent europium perovskites EuMO3 (M = Ti, Zr, and Hf), antiferromagnetic superexchange interactions via nd states of the B-site M cations (n = 3, 4, and 5, respectively) are enhanced by rotations of the MO6 octahedra. The octahedral rotations involved in a structural change from cubic &#36; Pm{ar 3}m &#36; to orthorhombic Pbnm structures not only reduce energy gaps between the Eu 4f and M nd bands but also point the M nd orbitals at the Eu sites, leading to a significant overlap between the M nd and Eu 4f orbitals. These results reveal that the octahedral rotations are indispensable for antiferromagnetic ordering observed for EuZrO3 and EuHfO3, and put these perovskites into a class of materials exhibiting a novel type of strong coupling between their magnetism and octahedral rotations.

DOI： 10.1002/adfm.201202477

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Lattice volume expansion or amorphization renders EuTiO3 ferromagnetic, although the stable phase of crystalline EuTiO3 is an antiferromagnet. The lattice volume expansion is induced into the crystalline EuTiO3 thin film by utilizing the lattice mismatch between the thin film and a substrate. The magnetization at low temperatures monotonically increases with an increase in lattice volume for the crystalline EuTiO3 thin film, coincident with the results of calculations based on the hybrid Hartree-Fock density functional approach. The ferromagnetic interaction between Eu2+ ions is enhanced by the amorphization as well; the amorphous EuTiO3 thin film becomes a ferromagnet, and the Curie temperature is higher for amorphous Eu2TiO4 than for its crystalline counterpart. The phenomenon, that is, the volume expansion-and amophization-induced ferromagnetism, is explained in terms of the competition between ferromagnetic and antiferromagnetic interactions among Eu2+ ions.

DOI： 10.1557/jmr.2013.60

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Transition metal ions such as Mn²⁺, Fe²⁺, or Co²⁺ provide an interesting alternative to rare earth dopants in optically active glasses. In terms of their magneto-optical properties, they are not yet very well exploited. Here, we report on the effect of Mn²⁺ on Faraday rotation in a metaphosphate glass matrix along the join Mn_xSr_1-x(PO₃)₂ with x = 0...1. Mn²⁺ shows small optical extinction in the visible spectral range and, compared to other transition metal ions, a high effective magnetic moment. At high Mn- levels, however, the magneto-optical activity of Mn²⁺ is strongly quenched due to ionic clustering. The magnetic properties of the heavily Mn²⁺-loaded phosphate matrix are dominated by a superexchange interaction in the Mn²⁺-O-Mn²⁺ bridge with antiparallel spin alignment between Mn²⁺ and O^2- species. The apparent paramagnetic potential of Mn²⁺ species can therefore not be exploited at room temperature.

DOI： 10.1364/OME.3.000184

記述言語：英語   掲載種別：研究論文（その他学術会議資料等）  

Bulk EuTiO₃ is known as a compound in which spin and soft phonon mode is strongly coupled. Recent theoretical study suggests that application of stress or formation of strain leads to a drastic change in magnetic and dielectric properties of EuTiO₃ and that so-called multiferroic properties emerge under such a situation. In the present study, effect of strain induced by a substrate, on which EuTiO₃ thin film is deposited, on the magnetic properties of the film has been experimentally examined. By using a pulsed laser deposition method, EuTiO₃ thin film has been deposited on different kinds of substrate, i.e., LaAlO₃, SrTiO₃, and DyScO₃; the lattice parameter of these compounds is smaller than, just the same as, and larger than that of EuTiO₃, respectively. X-ray diffraction analysis confirms that the strain induced in the plane of as-deposited EuTiO₃ thin films on different substrates is coincident with the lattice parameter of the substrate compounds. Also, all the as-deposited EuTiO₃ thin films manifest elongation of lattice in a direction perpendicular to the film surface. Temperature dependence of magnetization indicates that all the thin films exhibit ferromagnetic behavior at low temperatures. The magnetization at 2 K under a magnetic field of 100 Oe is the highest for EuTiO₃ on DyScO₃ and the lowest for EuTiO₃ on LaAlO₃. The experimental result is coincident with the first-principles calculations which predict that ferromagnetic spin configuration becomes more stable as the lattice volume of EuTiO₃ is increased.

DOI： 10.1557/opl.2012.962

記述言語：英語   掲載種別：研究論文（学術雑誌）  

A comparative study of the crystal and electronic structure and magnetism of divalent europium perovskite oxides EuMO3 (M = Ti, Zr, and Hf) has been performed on the basis of both experimental and theoretical approaches playing complementary roles. The compounds were synthesized via solid-state. reactions. EuZrO3 and EuHfO3 have an orthorhombic structure with a space group Pbnm at room temperature contrary to EuTiO3, which is cubic at room temperature. The optical band gaps of EuZrO3 and EuHfO3 are found to be about 2.4 and 2.7 eV, respectively, much larger than that of EuTiO3 (0.8 eV). On the other hand, the present compounds exhibit similar magnetic properties characterized by paramagnetic-antiferromagnetic transitions at around 5 K, spin flop at moderate magnetic fields lower than 1 T, and the antiferromagnetic nearest-neighbor and ferromagnetic next-nearest-neighbor exchange interactions. First-principles calculations based on a hybrid Hartree-Fock density functional approach yield lattice constants, band gaps, and magnetic interactions in good agreement with those obtained experimentally. The band gap excitations are assigned to electronic transitions from the Eu 4f to M nd states for EuMO3 (M = Ti, Zr, and Hf and n = 3, 4, and 5, respectively).

DOI： 10.1021/ic2024567

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We have investigated the local structure of amorphous oxides in EuO-TiO2 system to understand the mechanism that gives rise to curious ferromagnetic properties recently observed in the system. X-ray absorption spectroscopy has been performed on amorphous EuTiO3 and Eu2TiO4 thin films prepared by the pulsed-laser-deposition method. The Ti K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data analyses reveal that the coordination number of Ti4+ in the amorphous thin films is around 4, which is in sharp contrast to their crystalline counterparts, where Ti4+ ions are octahedrally coordinated by oxide ions. It is therefore inferred that Ti4+ acts as a network-forming cation in the random network structure of the amorphous oxides. From the Eu L-3-edge EXAFS data analyses, it is found that the coordination number of Eu2+ is much lower and the nearest Eu-O bond length is shorter in the amorphous thin films than in the crystalline counterparts. The amorphization-induced enhancement of ferromagnetic interactions observed in the EuO-TiO2 system is discussed in terms of the local environment around Eu2+ ions.

DOI： 10.1111/j.1551-2916.2011.04849.x

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We report on the preparation, structure, and magnetic properties of amorphous EuZrO₃, an amorphous ferromagnetic oxide. The samples were prepared in the form of thin film through a pulsed-laser-deposition method and their structure was confirmed to be amorphous by X-ray diffraction and high-resolution transmission electron microscope observation. Amorphous EuZrO₃ thin films exhibit a ferromagnetic transition and reenter into a spin glass phase at a lower temperature, while crystalline EuZrO₃ is an antiferromagnet as reported previously. The difference in magnetic properties between amorphous and crystalline EuZrO₃ is suggested to arise from the difference in their structures.

DOI： 10.1002/pssc.201000703

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Measurements of fundamental magnetic properties including not only dc and ac susceptibilities but also magnetic aging effects have been performed for aluminoborate glasses with high concentrations of iron and rare-earth R3+ ions (R = Sm, Gd, and Tb) in order to give an insight into the magnetic structures and interactions in amorphous oxides containing both 3d transition metal and 4f rare-earth ions, which manifest magnetic interactions that differ from each other. We demonstrate that the antiferromagnetic interactions between iron and rare-earth ions as well as those between iron ions play a significant role for their magnetic properties, while those between rare-earth ions are of little importance. Most of the rare-earth ions remain paramagnetic even below the spin-freezing temperatures under the strong molecular field caused by the spin-glass freezing of the iron ions, as in the case of rare-earth garnet ferrites.

DOI： 10.1103/PhysRevB.84.144408

記述言語：英語   掲載種別：研究論文（学術雑誌）  

A superexchange mechanism between Eu2+ 4f spins via the 3d states of nonmagnetic Ti4+ ions is proposed through first-principles calculations based on a hybrid Hartree-Fock density functional approach to explain G-type antiferromagnetism in EuTiO3. This mechanism is supported by systematic calculations for related Eu2+-based perovskite oxides. In EuTiO3, the competition between the antiferromagnetic superexchange and an indirect ferromagnetic exchange via the Eu 5d states leads to a delicate balance between antiferromagnetic and ferromagnetic phases. The superexchange mechanism involving the Ti 3d states hints at the microscopic origin of the strong spin-lattice coupling in EuTiO3.

DOI： 10.1103/PhysRevB.83.214421

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Recently, we have reported amorphization-induced ferromagnetism in a magnetic oxide
whereas crystalline EuTiO3 is an antiferromagnet, its amorphous counterpart shows a ferromagnetic transition. In this paper, we have investigated the magnetic properties of amorphous EuO-TiO2 system in detail. The present compounds, i.e., amorphous EuTiO3 and Eu 2 TiO4, exhibit a paramagnetic-ferromagnetic transition followed by a spin-glass transition upon cooling down, thereby being characterized as reentrant ferromagnets. Interestingly, the magnetic ordering temperatures of amorphous EuTiO3 and Eu2 TiO4 are comparable to and higher than those of their crystalline counterparts, respectively, indicating the enhancement of ferromagnetic interactions by amorphization. The origin of these features is discussed in terms of both the essential structural difference between amorphous and crystalline oxides and the magnetism unique to the crystalline EuO-TiO2 compounds. We also find a correlation between the magnitude of ferromagnetic interactions and the covalency of Eu-O bonds. © 2010 The American Physical Society.

DOI： 10.1103/PhysRevB.82.224403

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Recently, we have reported amorphization-induced ferromagnetism in a magnetic oxide; whereas crystalline EuTiO3 is an antiferromagnet, its amorphous counterpart shows a ferromagnetic transition. In this paper, we have investigated the magnetic properties of amorphous EuO-TiO2 system in detail. The present compounds, i.e., amorphous EuTiO3 and Eu2TiO4, exhibit a paramagnetic-ferromagnetic transition followed by a spin-glass transition upon cooling down, thereby being characterized as reentrant ferromagnets. Interestingly, the magnetic ordering temperatures of amorphous EuTiO3 and Eu2TiO4 are comparable to and higher than those of their crystalline counterparts, respectively, indicating the enhancement of ferromagnetic interactions by amorphization. The origin of these features is discussed in terms of both the essential structural difference between amorphous and crystalline oxides and the magnetism unique to the crystalline EuO-TiO2 compounds. We also find a correlation between the magnitude of ferromagnetic interactions and the covalency of Eu-O bonds.

DOI： 10.1103/PhysRevB.82.224403

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Amorphous EuTiO3 thin films have been prepared by a pulsed laser deposition method, and their structural and magnetic properties have been investigated. High-resolution transmission electron microscope image and selected-area electron diffraction pattern as well as X-ray diffraction pattern confirm the amorphous nature of the thin films. Eu-151 conversion-electron Mossbauer effect measurements show that almost all of the europium ions are present as Eu2+ in the thin films. The amorphous EuTiO3 thin film exhibits a positive Weiss temperature of 8.7 K, indicating predominant ferromagnetic interactions among Eu2+ ions. The signature of a ferromagnetic-like transition is observed at around 5.5 K. (C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jnoncrysol.2010.05.014

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Polycrystalline EuZrO3 has been synthesized by the solid-state reaction between EuO and ZrO2, and its structural and magnetic properties have been investigated. Rietveld analysis of the X-ray diffraction pattern indicates that EuZrO3 crystallizes in an orthorhombic perovskite structure. Eu-151 Mossbauer effect measurement reveals that almost all the europium ions are present as the divalent state and Occupy distorted sites with non-axial electric field gradients, in agreement with the orthorhombic structure. In contrast to previous reports, an antiferromagnetic transition was observed around 4.1 K. The magnetic structure below the Neel temperature has been discussed. (C) 2009 Elsevier Inc. All rights reserved.

DOI： 10.1016/j.jssc.2009.10.014

記述言語：英語   掲載種別：研究論文（学術雑誌）  

In most amorphous insulating magnets, the magnetic structure is dominated by the random distribution of magnetic moments as well as the predominant antiferromagnetic interaction among them, inevitably leading to a transition from high-temperature paramagnetic to low-temperature spin glass phase. In this paper, we report our discovery of ferromagnetic amorphous oxides with reentrant spin glass behavior. Unlike most oxide glasses, there is a strong tendency for the magnetic interaction of Eu2+ ions to be ferromagnetic in oxide glasses, as obviously indicated by the positive values of Weiss temperature. Comprehensive investigations of low-temperature magnetic properties for the present Eu2+-containing glasses have revealed a typical behavior of reentrant ferromagnets. We discuss the possible mechanisms behind the ferromagnetic interactions, as well as the origin of reentrant spin glass nature, based on the specific electronic structure of Eu2+ compounds.

DOI： 10.1103/PhysRevB.81.014423

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Magnetic properties of mixed-valence iron phosphate glasses, where there coexist Fe2+ and Fe3+ ions, have been investigated. The molar fraction of Fe3+ with respect to the total iron ion, [Fe3+]/[Fe-total], can be controlled by melting the glass at varied temperatures. Experiments of magnetic aging and memory effects as well as dynamic and static scaling analyses of relaxation time and nonlinear magnetic susceptibility have been performed to get insight into the nature of low-temperature magnetic phase of the glass system. The experimental results reveal that the iron phosphate glasses undergo paramagnet-spin-glass transitions at low temperatures. Temperature dependence of magnetic specific heat suggests that as the temperature is lowered, the magnetic moments start to be frozen at a temperature significantly higher than the spin-glass transition temperature accompanied by a deviation in magnetic susceptibility from Curie-Weiss law. The ratio of the absolute value of Weiss temperature to spin-glass transition temperature increases as the ratio [Fe3+]/[Fe-total] becomes larger. This behavior is explainable in terms of the difference in single-ion anisotropy between Fe3+ and Fe2+ ions.

DOI： 10.1103/PhysRevB.80.134408

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Detailed nanostructures have been investigated for hierarchically porous alumina aerogels and xerogels prepared from ionic precursors via sol-gel reaction. Starting from AlCl3 center dot 6H(2)O and poly(ethylene oxide) (PEO) dissolved in a H2O/EtOH mixed solvent, monolithic wet gels were synthesized using propylene oxide (PO) as a gelation initiator. Hierarchically porous alumina xerogels and aerogels were obtained after evaporative drying and supercritical drying, respectively. Macroporous structures are formed as a result of phase separation, while interstices between the secondary particles in the micrometer-sized gel skeletons work as mesoporous structures. Alumina xerogels exhibit considerable shrinkage during the evaporative drying process, resulting in relatively small mesopores (from 5.4 to 6.2 nm) regardless of the starting composition. For shrinkage-free alumina aerogels, on the other hand, the median mesopore size changes from 13.9 to 33.1 nm depending on the starting composition; the increases in PEO content and H2O/EtOH volume ratio both contribute to producing smaller mesopores. Small-angle X-ray scattering (SAXS) analysis reveals that variation of median mesopore size can be ascribed to the change in agglomeration state of primary particles. As PEO content and H2O/EtOH ratio increase, secondary particles become small, which results in relatively small mesopores. The results indicate that the agglomeration state of alumina primary particles is influenced by the presence of weakly interacting phase separation inducers such as PEO. (C) 2009 Elsevier Inc. All rights reserved.

DOI： 10.1016/j.jcis.2009.06.042

記述言語：英語   掲載種別：研究論文（学術雑誌）  

CdFe2O4 thin films have been prepared by a sputtering method, and their structural and magnetic properties have been examined. As-deposited film shows magnetic transition like that of cluster-spin glass or superspin glass, and possesses large magnetization even at room temperature. The magnetic behaviors are partly attributed to crystal structure with a random distribution of cations in CdFe2O4 nanocrystals. Annealing of the as-deposited film at 200 degrees C leads to an increase in magnetization while the magnetization is decreased when the film is annealed at 400 degrees C. The effect of annealing on the magnetic properties has been discussed in terms of the change of microstructure as well as chemical structure of the thin films.

DOI： 10.1109/TMAG.2008.2001508

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Magnetic and structural properties of iron- containing bismuth borate glasses, whose composition is denoted as xFe(2)O(3) center dot( 80.0 - x)Bi2O3 center dot 20.0B(2)O(3), in mol&#37; ( 18.2 <= x <= 40.0), have been explored. The glasses manifest intriguing magnetic behaviors explainable in terms of the coexistence of a spin glass phase and magnetic clusters. The x = 18.2 glass shows a spin glass transition at 3.5 K, while the contribution of magnetic clusters to the magnetic properties becomes more significant as the content of Fe2O3, x, is increased. We have performed detailed experiments on the x = 32.0 glass for which two different magnetic transitions are observed, as demonstrated by the temperature and frequency of the ac magnetic field dependences of the dc and ac susceptibilities. It is revealed from the measurements of magnetic ageing and memory effects that the magnetic clusters are frozen to form a superspin glass- like state with strong inter- cluster interactions at low temperatures. Transmission electron microscopy clarifies that the magnetic clusters are ascribable not to nanocrystals but to some phases possessing amorphous structures. Also, we have found from the observation of exchange bias effects that an interplay of the clusters with the spin glass phase brings about an exchange anisotropy field after cooling in the presence of magnetic field.

DOI： 10.1088/0953-8984/20/23/235216

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We have prepared glasses having xFeO center dot(100-x)P2O5 (mol &#37;) (x=50.0,54.0,57.1) compositions by melting under mild reducing condition and found that these glasses exhibit fairly high transmittance in the visible range and large Faraday effect at the wavelength of about 400 nm. Fe-57 Mossbauer spectra confirm that almost all the iron ions are present as Fe2+ in the glasses. A spin glass transition is observed at low temperatures in the temperature dependence of magnetic susceptibility. Intense optical absorption in the ultraviolet and infrared wavelength ranges occurs by the charge transfer transition from O2- to Fe2+ and the intra-atomic d-d transition, respectively. The analysis on the wavelength dependence of the Faraday rotation angle using the Van Vleck-Hebb theory has revealed that the charge transfer transition contributes more significantly to the Faraday effect owing to the large effective transition probability, which is comparable to those reported for glasses containing 4f rare-earth ions. The magneto-optical figure of merit shows a maximum at around 380 nm. (c) 2008 American Institute of Physics.

DOI： 10.1063/1.2952460

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Magnetic properties of disordered oxides involving oxide glasses have been investigated. Spin glass-like transition is observed for Fe2O3-TeO2 and MnO-TeO2 glasses in temperature dependence of dc susceptibility, although magnetic transition does not take place for MnO-TeO2 glasses with low concentration of manganese ion above 2 K at least. The mechanism of magnetic transition observed for Fe2O3-TeO2 glasses is discussed on the basis of dc susceptibility obtained under conditions of field cooling and zero field cooling, magnetic field dependence of magnetization at low temperatures, and frequency dependence of spin-freezing temperature derived from temperature dependence of ac susceptibility. Magnetic properties of disordered ZnFe2O4 thin film prepared by a radio frequency sputtering method have been also studied. The disordered ZnFe2O4 thin film exhibits ferrimagnetic behavior with high magnetization even at room temperature. At the same time, the thin film shows spin-freezing at around 320 K. Temperature dependence of nonlinear susceptibility leads to a conclusion that this transition is explainable in terms of superparamagnetism with magnetic interaction among clusters. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jnoncrysol.2007.01.107

記述言語：英語   掲載種別：研究論文（その他学術会議資料等）  

Amorphous oxide thin films of Fe₂O₃-R ₂O₃ (R = La, Gd and Tb) systems have been deposited on silica glass substrates by using a radio frequency sputtering method, and magnetic properties of the thin films have been examined. The Fe ₂O₃-La₂O₃ thin films exhibit cusp-like maxima of dc magnetic susceptibility in their temperature dependences as well as magnetic aging and memory effects characteristic of typical spin glasses. For Fe₂O₃-Gd₂O₃ and Fe ₂O₃-Tb₂O₃ systems, magnetic moments of iron ions take part in formation of a spin glass state, as indicated by the magnetic aging effects, while those of rare-earth ions remain to be in a paramagnetic state even at very low temperatures.

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We have investigated magnetic aging and memory effects in the magnetically ordered phase of 32Fe2O(3)(center dot)48Bi(2)O(3)center dot 20B(2)O(3) (mol&#37;) glass, which exhibits a spin glass-like transition at about 15 K (T,,) and a superparamagnetic blocking at about 28 K (T-sp). The present glass manifests magnetic aging effects similar to those observed in spin glasses in the temperature regimes of not only T < T-seg but also T-sg < T < T-sp, indicating that superparamagnetic clusters existing in the glass strongly interact with each other. The interactions among the magnetic clusters lead to cooperative spin dynamics as observed in superspin glasses. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2006.10.649

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We have examined spin dynamics including magnetic aging and memory effects as well as critical slowing down for 20Fe(2)O(3)center dot 80TeO(2) (mol &#37;) glass. Scaling analysis on critical slowing down reveals that the present glass exhibits a critical behavior as observed in a prototype of spin glasses. Aging and memory effects peculiar to spin glasses have been observed in the magnetically ordered phase of the present glass. These experimental results strongly confirm that the 20Fe(2)O(3)center dot 80TeO(2) glass is converted into a spin glass phase at a very low temperature. It is thought that a disordered structure of the oxide glass gives rise to randomness and frustration in the magnetic structure, leading to the spin glass phase transition.

DOI： 10.1103/PhysRevB.74.012411

開催年月日： 2023年5月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：Physikzentrum, Bad Honnef   国名：ドイツ連邦共和国  

開催年月日： 2023年3月

記述言語：日本語   会議種別：口頭発表（一般）  

国名：日本国  

開催年月日： 2023年3月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：神奈川大学 みなとみらいキャンパス   国名：日本国  

開催年月日： 2023年1月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：岡山大学津島キャンパス   国名：日本国  

開催年月日： 2022年10月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：つくば市、国立研究開発法人物質・材料研究機構   国名：日本国  

開催年月日： 2022年10月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：つくば市、国立研究開発法人物質・材料研究機構   国名：日本国  

開催年月日： 2022年10月

記述言語：日本語   会議種別：口頭発表（一般）  

国名：日本国  

開催年月日： 2022年9月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：徳島大学   国名：日本国  

開催年月日： 2022年9月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：オンライン開催   国名：日本国  

開催年月日： 2022年8月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：オンライン開催   国名：日本国  

開催年月日： 2022年6月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：Kobe, hybrid   国名：日本国  

開催年月日： 2022年6月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：山梨大学（甲府キャンパス） オンライン開催   国名：日本国  

開催年月日： 2022年6月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：山梨大学（甲府キャンパス） オンライン開催   国名：日本国  

開催年月日： 2022年6月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：東京工業大学フロンティア材料研究所オンライン開催   国名：日本国  

開催年月日： 2022年6月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：オンライン開催   国名：日本国  

開催年月日： 2022年6月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：オンライン開催   国名：日本国  

開催年月日： 2020年12月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：オンライン   国名：日本国  

Recently, the development of novel ferroelectrics has been accelerated since Benedek and Fennie rationalized a novel design principle of ferroelectric layered perovskite oxides, in which a spontaneous polarization can be induced symmetrically and energetically by a combination of two non-polar rotation modes of oxygen octahedra via trilinear coupling. This is called hybrid improper ferroelectricity. In this work, we focus on Dion-Jacobson (DJ) layered perovskites CsNdNb2O7 and RbNdNb2O7. Recent detailed structural analyses revealed the crystal structures of CsNdNb2O7 and RbNdNb2O7 belong to polar space groups P21am and I2cm, respectively, but their ferroelectricity is not reported yet. Here we report the experimental demonstration of the ferroelectric switching for the DJ phases prepared by a conventional solid-state reaction method. The ceramic pellets of the DJ phases show the remanent polarizations much larger than those observed for polycrystalline samples of other hybrid improper ferroelectrics (Fig. 1). We will also discuss the results of our first-principles calculations on ferroelectric switching pathways in order to get an insight into the low coercive fields of the DJ phases.

開催年月日： 2020年11月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：オンライン   国名：日本国  

開催年月日： 2020年5月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：コープイン京都   国名：日本国  

開催年月日： 2018年5月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：大阪   国名：日本国  

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Kinetic Analysis of Adsorption and Photodecomposition Processes on TiO2 Particles with Different Crystalline Phases

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開催地：福岡西新プラザ   国名：その他  

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国名：その他  

記述言語：その他  

国名：その他  

記述言語：その他  

国名：その他  

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開催地：京都大学吉田キャンパス   国名：その他  

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国名：その他  

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開催地：北九州国際会議場   国名：その他  

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国名：その他  

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開催地：関西セミナーハウス   国名：その他  

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開催地：関西セミナーハウス   国名：その他  

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国名：その他  

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開催地：工学院大学新宿キャンパス   国名：その他  

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開催地：工学院大学新宿キャンパス   国名：その他  

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国名：その他  

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国名：その他  

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開催地：工学院大学新宿キャンパス   国名：その他  

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国名：その他  

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開催地：北海道大学フロンティア応用科学研究棟   国名：その他  

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開催地：北海道大学フロンティア応用科学研究棟   国名：その他  

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開催地：北海道大学フロンティア応用科学研究棟   国名：その他  

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開催地：北海道大学フロンティア応用科学研究棟   国名：その他  

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開催地：Congress & Exhibition Centre Acropolis   国名：その他  

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開催地：東京理科大学 葛飾キャンパス 図書館ホール   国名：その他  

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開催地：沖縄コンベンションセンター   国名：その他  

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開催地：沖縄コンベンションセンター   国名：その他  

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開催地：沖縄コンベンションセンター   国名：その他  

記述言語：その他  

開催地：沖縄コンベンションセンター   国名：その他  

記述言語：その他  

開催地：沖縄コンベンションセンター   国名：その他  

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開催地：沖縄コンベンションセンター   国名：その他  

記述言語：その他  

開催地：沖縄コンベンションセンター   国名：その他  

記述言語：その他  

開催地：沖縄コンベンションセンター   国名：その他  

記述言語：その他  

開催地：沖縄コンベンションセンター   国名：その他  

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開催地：沖縄コンベンションセンター   国名：その他  

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開催地：物質・材料研究機構 並木地区   国名：その他  

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開催地：物質・材料研究機構 並木地区   国名：その他  

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開催地：物質・材料研究機構 並木地区   国名：その他  

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開催地：明治大学 駿河台キャンパス   国名：その他  

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開催地：明治大学 駿河台キャンパス   国名：その他  

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開催地：明治大学 駿河台キャンパス   国名：その他  

記述言語：その他  

開催地：明治大学 駿河台キャンパス   国名：その他  

種別：査読等 

外国語雑誌　査読論文数：4

種別：査読等 

外国語雑誌　査読論文数：4

種別：査読等 

外国語雑誌　査読論文数：3

日本語雑誌　査読論文数：0

国際会議録　査読論文数：0

国内会議録　査読論文数：0