Publications by authors named "Akihide Kuwabara"

29 Publications

  • Page 1 of 1

Dehydration of Electrochemically Protonated Oxide: SrCoO with Square Spin Tubes.

J Am Chem Soc 2021 Oct 14;143(42):17517-17525. Epub 2021 Oct 14.

Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan.

Controlling oxygen deficiencies is essential for the development of novel chemical and physical properties such as high- superconductivity and low-dimensional magnetic phenomena. Among reduction methods, topochemical reactions using metal hydrides (e.g., CaH) are known as the most powerful method to obtain highly reduced oxides including NdSrNiO superconductor, though there are some limitations such as competition with oxyhydrides. Here we demonstrate that electrochemical protonation combined with thermal dehydration can yield highly reduced oxides: SrCoO thin films are converted to SrCoO by dehydration of HSrCoO at 350 °C. SrCoO forms square (or four-legged) spin tubes composed of tetrahedra, in contrast to the conventional infinite-layer structure. Detailed analyses suggest the importance of the destabilization of the SrCoO precursor by electrochemical protonation that can greatly alter reaction energy landscape and its gradual dehydration (HSrCoO) for the SrCoO formation. Given the applicability of electrochemical protonation to a variety of transition metal oxides, this simple process widens possibilities to explore novel functional oxides.
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http://dx.doi.org/10.1021/jacs.1c07043DOI Listing
October 2021

Alkali-Rich Antiperovskite MFCh (M = Li, Na; Ch = S, Se, Te): The Role of Anions in Phase Stability and Ionic Transport.

J Am Chem Soc 2021 Jul 6;143(28):10668-10675. Epub 2021 Jul 6.

Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan.

To improve ionic conductivity, solid-state electrolytes with polarizable anions that weakly interact with mobile ions have received much attention, a recent example being lithium/sodium-rich antiperovskite MHCh (M = Li, Na; Ch = S, Se, Te). Herein, in order to clarify the role of anions in antiperovskites, the MFCh family, in which the polarizable H anion at the octahedral center is replaced by the ionic F anion, is investigated theoretically and experimentally. We unexpectedly found that the stronger attractive interaction between F and M ions does not slow down the M ion diffusion, with the calculated energy barrier being as low as that of MHCh. This fact suggests that the low-frequency rotational phonon modes of the octahedron of cubic MFCh (and MHCh) are intrinsic to facilitate the fast ionic diffusion. A systematic analysis further reveals a correlation between the tolerance factor and the ionic transport: as decreases within the cubic phase, the rotational mode becomes softer, resulting in the reduction of the migration energy. The cubic iodine-doped LiFSe has a room-temperature ionic conductivity of 5 × 10 S/cm with a bulk activation energy of 0.18 eV.
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http://dx.doi.org/10.1021/jacs.1c04260DOI Listing
July 2021

Defect Engineering and Anisotropic Modulation of Ionic Transport in Perovskite Solid Electrolyte LiLaNbO.

Molecules 2021 Jun 10;26(12). Epub 2021 Jun 10.

Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya 456-8587, Japan.

Solid electrolytes, such as perovskite LiLaTiO, LiLaNbO and garnet LiLaZrO ceramic oxides, have attracted extensive attention in lithium-ion battery research due to their good chemical stability and the improvability of their ionic conductivity with great potential in solid electrolyte battery applications. These solid oxides eliminate safety issues and cycling instability, which are common challenges in the current commercial lithium-ion batteries based on organic liquid electrolytes. However, in practical applications, structural disorders such as point defects and grain boundaries play a dominating role in the ionic transport of these solid electrolytes, where defect engineering to tailor or improve the ionic conductive property is still seldom reported. Here, we demonstrate a defect engineering approach to alter the ionic conductive channels in LiLaNbO ( = 0.1~0.13) electrolytes based on the rearrangements of La sites through a quenching process. The changes in the occupancy and interstitial defects of La ions lead to anisotropic modulation of ionic conductivity with the increase in quenching temperatures. Our trial in this work on the defect engineering of quenched electrolytes will offer opportunities to optimize ionic conductivity and benefit the solid electrolyte battery applications.
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http://dx.doi.org/10.3390/molecules26123559DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8230448PMC
June 2021

Anion ordering enables fast H conduction at low temperatures.

Sci Adv 2021 Jun 2;7(23). Epub 2021 Jun 2.

Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan.

The introduction of chemical disorder by substitutional chemistry into ionic conductors is the most commonly used strategy to stabilize high-symmetric phases while maintaining ionic conductivity at lower temperatures. In recent years, hydride materials have received much attention owing to their potential for new energy applications, but there remains room for development in ionic conductivity below 300°C. Here, we show that layered anion-ordered BaH ( = Cl, Br, and I) exhibit a remarkable conductivity, reaching 1 mS cm at 200°C, with low activation barriers allowing H conduction even at room temperature. In contrast to structurally related BaH (i.e., BaH), the layered anion order in BaH , along with Schottky defects, likely suppresses a structural transition, rather than the traditional chemical disorder, while retaining a highly symmetric hexagonal lattice. This discovery could open a new direction in electrochemical use of hydrogen in synthetic processes and energy devices.
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http://dx.doi.org/10.1126/sciadv.abf7883DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8172174PMC
June 2021

Effects of Nitrogen/Fluorine Codoping on Photocatalytic Rutile TiO Crystal Studied by First-Principles Calculations.

Inorg Chem 2021 Feb 26;60(4):2381-2389. Epub 2021 Jan 26.

Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-NE-2 Ookayama, Meguro-ku, Tokyo 152-8550, Japan.

Nitrogen/fluorine codoping of rutile TiO was recently reported to be effective for introducing visible-light absorption, and the resultant TiO:N,F worked efficiently as an O evolution photocatalyst in a Z-scheme water-splitting system. Although an increase in the amount of nitrogen doped into rutile TiO lattice in the presence of fluorine was experimentally demonstrated, the role of fluorine in the system remained unclear. Here, we report a computational study on TiO:N,F through the construction of supercell models with substitutional defects to reveal the atomic arrangement of the material and the electronic band structure. Calculations for all possible structures of nitrogen/fluorine and nitrogen/oxygen-vacancy relative positions revealed that the defect complexes were preferentially located on the (110) plane and that the distance between defects did not have a strong correlation with the formation energy. The present work also showed that although fluorine did not directly contribute to the narrowing of the band gap of TiO:N,F, the fluorine activity of the synthetic atmosphere promotes the formation of substitutional defect complexes of nitrogen/fluorine for anion sites. This eventually increases the amount of nitrogen incorporated into the rutile TiO lattice and also results in reduction of the amount of oxygen vacancy, which is in qualitative agreement with our previous result of transient absorption measurement for rutile TiO:N,F. The role of fluorine in TiO:N,F is thus clarified through our systematic first-principles calculations.
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http://dx.doi.org/10.1021/acs.inorgchem.0c03262DOI Listing
February 2021

Theoretical investigation of tetrahedral distortion of four-coordinate iron(II) centres in FePd(CN).

Dalton Trans 2021 Feb;50(6):1990-1994

Nanostructures Research Laboratory, Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta, Nagoya 456-8587, Japan.

The tetrahedral distortion of iron(ii) centres in the cyanide-bridged framework FePd(CN)4 was recently demonstrated experimentally. Here, we theoretically confirmed the electronically driven tetrahedral distortion of iron(ii) by comparing the density of states and total energies of FePd(CN)4 (d6) and ZnPd(CN)4 (d10). The calculation results suggested that a Jahn-Teller-like effect is caused on the tetrahedral geometry by the electronic effect of unequally occupied non-bonding 3d orbitals in the corresponding structure.
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http://dx.doi.org/10.1039/d0dt04155fDOI Listing
February 2021

Hydride-based antiperovskites with soft anionic sublattices as fast alkali ionic conductors.

Nat Commun 2021 Jan 8;12(1):201. Epub 2021 Jan 8.

Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto, 615-8510, Japan.

Most solid-state materials are composed of p-block anions, only in recent years the introduction of hydride anions (1s) in oxides (e.g., SrVOH, BaTi(O,H)) has allowed the discovery of various interesting properties. Here we exploit the large polarizability of hydride anions (H) together with chalcogenide (Ch) anions to construct a family of antiperovskites with soft anionic sublattices. The MHCh antiperovskites (M = Li, Na) adopt the ideal cubic structure except orthorhombic NaHS, despite the large variation in sizes of M and Ch. This unconventional robustness of cubic phase mainly originates from the large size-flexibility of the H anion. Theoretical and experimental studies reveal low migration barriers for Li/Na transport and high ionic conductivity, possibly promoted by a soft phonon mode associated with the rotational motion of HM octahedra in their cubic forms. Aliovalent substitution to create vacancies has further enhanced ionic conductivities of this series of antiperovskites, resulting in NaH(SeI) achieving a high conductivity of ~1 × 10 S/cm (100 °C).
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http://dx.doi.org/10.1038/s41467-020-20370-2DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7794446PMC
January 2021

Ionic conduction mechanism in Ca-doped lanthanum oxychloride.

Dalton Trans 2021 Jan 8;50(1):151-156. Epub 2020 Dec 8.

Joint and Welding Research Institute, Osaka University, Ibaraki, Osaka, Japan.

The mechanism of ionic conduction in Ca-doped lanthanum oxychloride (LaOCl) was investigated using first-principles calculations based on density functional theory. The calculations of the point defect formation energies suggest that Cl ion vacancies and substituted Ca ions at La sites were dominant point defects. Although the migration energy of an O ion is 0.95 eV, the migration energy of a Cl ion was calculated to be 0.44 eV, which is consistent with the reported experimental value. These results imply that the main carrier in Ca-doped LaOCl is Cl ions and ionic conduction occurs by a Cl ion vacancy mechanism.
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http://dx.doi.org/10.1039/d0dt02502jDOI Listing
January 2021

Synthesis and H conductivity of a new oxyhydride BaYHO with anion-ordered rock-salt layers.

Chem Commun (Camb) 2020 Sep 7;56(71):10373-10376. Epub 2020 Aug 7.

Department of Materials Molecular Science, Institute for Molecular Science, 38 Nishigonaka, Myodaiji, Okazaki, Aichi 444-8585, Japan. and SOKENDAI (The Graduate University for Advanced Studies), 38 Nishigonaka, Myodaiji, Okazaki, Aichi 444-8585, Japan.

A new layered perovskite oxyhydride BaYHO was synthesized via high pressure synthesis. Powder X-ray and neutron diffraction experiments revealed that in BaYHO the ordered H and O anions form [BaH] rock-salt layers wherein H conduction is allowed. The conductivity reached 0.1 mS cm at 350 °C, which is higher than that of isostructural BaScHO with anion-disordered [BaHO] layers.
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http://dx.doi.org/10.1039/d0cc03638bDOI Listing
September 2020

Responsive Four-Coordinate Iron(II) Nodes in FePd(CN).

Angew Chem Int Ed Engl 2020 Oct 24;59(43):19254-19259. Epub 2020 Aug 24.

Department of Chemistry, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto, 860-8555, Japan.

Metal node design is crucial for obtaining structurally diverse coordination polymers (CPs) and metal-organic frameworks with desirable properties; however, Fe ions are exclusively six-coordinated. Herein, we present a cyanide-bridged three-dimensional (3D) CP, FePd(CN) , bearing four-coordinate Fe ions, which is synthesized by thermal treatment of a two-dimensional (2D) six-coordinate Fe CP, Fe(H O) Pd(CN) ⋅4 H O, to remove water molecules. Atomic-resolution transmission electron microscopy and powder X-ray and neutron diffraction measurements revealed that the FePd(CN) structure is composed of a two-fold interpenetrated PtS topology network, where the Fe center demonstrates an intermediate geometry between tetrahedral and square-planar coordination. This four-coordinate Fe center with the distorted geometry can act as a thermo-responsive flexible node in the PtS network.
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http://dx.doi.org/10.1002/anie.202008187DOI Listing
October 2020

Direct Measurement of Electronic Band Structures at Oxide Grain Boundaries.

Nano Lett 2020 Apr 5;20(4):2530-2536. Epub 2020 Mar 5.

Institute of Engineering Innovation, The University of Tokyo, Tokyo 113-8656, Japan.

Grain boundaries (GBs) modulate the macroscopic properties in polycrystalline materials because they have different atomic and electronic structures from the bulk. Despite the progress on the understanding of GB atomic structures, knowledge of the localized electronic band structures is still lacking. Here, we experimentally characterized the atomic structures and the band gaps of four typical GBs in α-AlO by scanning transmission electron microscopy and valence electron energy-loss spectroscopy (EELS). It was found that the band gaps of the GBs are narrowed by 0.5-2.1 eV compared with that of 8.8 eV in the bulk. By combing core-loss EELS with first-principles calculations, we elucidated that the band gap reductions directly correlate with the decrease of the coordination numbers of Al and O ions at the GBs. These results provide in-depth understanding between the local atomic and electronic band structures for GBs and demonstrate a novel electronic-structure analysis for crystalline defects.
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http://dx.doi.org/10.1021/acs.nanolett.9b05298DOI Listing
April 2020

Flux Crystal Growth, Crystal Structure, and Optical Properties of New Germanate Garnet CeCaMgGeO.

Front Chem 2020 18;8:91. Epub 2020 Feb 18.

Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC, United States.

A new germanate garnet compound, CeCaMgGeO, was synthesized via flux crystal growth. Truncated spherical, reddish-orange single crystals with a typical size of 0.1-0.3 mm were grown out of a BaCl-CaCl melt. The single crystals were characterized by single-crystal X-ray diffraction analysis, which revealed that it adopted a cubic garnet-type structure with = 12.5487(3) Å in the space group -3. Its composition is best described as O, where Ce/Ca, Mg, and Ge occupied the , and sites, respectively. A UV-vis absorption spectroscopy measurement on the germanate garnet revealed a clear absorption edge corresponding to a band gap of 2.21 eV (λ = 561 nm). First-principle calculations indicated that the valence band maximum was composed of Ce 4 bands, whereas the conduction band minimum mainly consisted of Ce 5 bands. These findings explain the observed absorption edge through the Ce 4 → 5 absorption. Photoluminescence emission spectra exhibited a very broad peak centered at 600 nm, corresponding to transition from the lowest energy level to the 4 levels.
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http://dx.doi.org/10.3389/fchem.2020.00091DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7039934PMC
February 2020

Transition-Metal Distribution in Brownmillerite CaFeCoO.

Inorg Chem 2019 Aug 11;58(15):10209-10216. Epub 2019 Jul 11.

Institute of Engineering Innovation , University of Tokyo , Bunkyo , Tokyo 113-8656 , Japan.

CaFeCoO (0 ≤ ≤ 1) with higher Co content, which crystallizes in a brownmillerite-type structure, is currently one of the best oxygen-evolution-reaction (OER) catalysts. Identifying the Fe/Co occupancies at the octahedral () and tetrahedral () sites in the structure is the foundation for the understanding of the role of cobalt in each site and the exploration of further improvement in the OER activity. Here, we investigate the Fe/Co distribution in CaFeCoO by means of atomic-resolution energy dispersive X-ray spectroscopy in scanning transmission electron microscopy and dynamical image simulations combined with systematic density functional theory calculations. Our careful microscopic study reveals the absence of long-range Fe/Co order within the transition-metal (TM) layers, but cobalt is slightly enriched at the and sites in the as-synthesized (1100 °C) and 800 °C annealed for a month samples, respectively. The observed Co site preferences are interpretable from the viewpoints of TM ionic size effect and ligand field effect, which are competitive around a crossover point at a certain temperature between 800 and 1100 °C. We also elucidate that the as-synthesized sample with Co enrichment at the site shows the better OER activity, and the optimum annealing temperature for more OER active CaFeCoO should be higher than the crossover temperature.
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http://dx.doi.org/10.1021/acs.inorgchem.9b01356DOI Listing
August 2019

BaScHO: H Conductive Layered Oxyhydride with H Site Selectivity.

Inorg Chem 2019 Apr 20;58(7):4431-4436. Epub 2019 Feb 20.

Department of Materials Molecular Science , Institute for Molecular Science , 38 Nishigonaka, Myodaiji , Okazaki , Aichi 444-8585 , Japan.

Hydride (H) conduction is a new frontier related to hydrogen transport in solids. Here, a new H conductive oxyhydride BaScHO was successfully synthesized using a high-pressure technique. Powder X-ray and neutron diffraction experiments investigated the fact that BaScHO adopts a KNiF-type structure with H ions preferentially occupying the apical sites, as supported by theoretical calculations. Electrochemical impedance spectra showed that BaScHO exhibited H conduction and a conductivity of 5.2 × 10 S cm at 300 °C. This value is much higher than that of BaScOH, which has an ideal perovskite structure, suggesting the advantage of layered structures for H conduction. Tuning site selectivity of H ions in layered oxyhydrides might be a promising strategy for designing fast H conductors applicable for novel electrochemical devices.
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http://dx.doi.org/10.1021/acs.inorgchem.8b03593DOI Listing
April 2019

Systematic analysis of electron energy-loss near-edge structures in Li-ion battery materials.

Phys Chem Chem Phys 2018 Oct;20(38):25052-25061

Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya 456-8587, Japan.

Electrical conductivity, state of charge and chemical stability of Li-ion battery materials all depend on the electronic states of their component atoms, and tools for measuring these reliably are needed for advanced materials analysis and design. Here we report a systematic investigation of electron energy-loss near-edge structures (ELNES) of Li-K and O-K edges for ten representative Li-ion battery electrodes and solid-state electrolytes obtained by performing transmission electron microscopy with a Wien-filter monochromator-equipped microscope. While the peaks of Li-K edges are positioned at about 62 eV for most of the materials examined, the peak positions of O-K edges vary within a range of about 530 to 540 eV, and the peaks can be categorised into three groups based on their characteristic edge shapes: (i) double peaks, (ii) single sharp peaks, and (iii) single broad peaks. The double peaks of group (i) are attributable to the d0 electronic configuration of their transition metal ions bonded to O atoms. The origin of the different peak shapes of groups (ii) and (iii) is more subtle but insights are gained using density functional theory methods to simulate O-K ELNES edges of group (ii) material LiCoO2 and group (iii) material LiFePO4. Comparison of their densities of states reveals that in LiCoO2 the Co-O hybrid orbitals are separated from Li-O hybrid orbitals, resulting in a sharp peak in the O-K edge, while Fe-O, Li-O and P-O hybrid orbitals in LiFePO4 partially overlap each other and produce a broad peak.
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http://dx.doi.org/10.1039/c8cp03390kDOI Listing
October 2018

Chemical Pressure-Induced Anion Order-Disorder Transition in LnHO Enabled by Hydride Size Flexibility.

J Am Chem Soc 2018 09 28;140(36):11170-11173. Epub 2018 Aug 28.

Graduate School of Engineering , Kyoto University , Kyoto 615-8510 , Japan.

While cation order-disorder transitions have been achieved in a wide range of materials and provide crucial effects in various physical and chemical properties, anion analogues are scarce. Here we have expanded the number of known lanthanide oxyhydrides, LnHO (Ln = La, Ce, Pr, Nd), to include Ln = Sm, Gd, Tb, Dy, Ho, and Er, which has allowed the observation of an anion order-disorder transition from the anion-ordered fluorite structure ( P4/ nmm) for larger Ln ions (La-Nd) to a disordered arrangement ( Fm3̅ m) for smaller Ln (Sm-Er). Structural analysis reveals that with the increase of Ln radius (application of negative chemical pressure), the oxide anion in the disordered phase becomes too under-bonded, which drives a change to an anion-ordered structure, with smaller OLn and larger HLn tetrahedra, demonstrating that the size flexibility of hydride anions drives this transition. Such anion ordering control is crucial regarding applications that involve hydride diffusion such as catalysis and electrochemical solid devices.
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http://dx.doi.org/10.1021/jacs.8b06187DOI Listing
September 2018

Microscopic mechanism of biphasic interface relaxation in lithium iron phosphate after delithiation.

Nat Commun 2018 07 20;9(1):2863. Epub 2018 Jul 20.

Nanostructures Research Laboratory, Japan Fine Ceramics Center, Atsuta, Nagoya, 456-8587, Japan.

Charge/discharge of lithium-ion battery cathode material LiFePO is mediated by the structure and properties of the interface between delithiated and lithiated phases. Direct observations of the interface in a partially delithiated single crystal as a function of time using scanning transmission electron microscopy and electron energy-loss spectroscopy help clarify these complex phenomena. At the nano-scale, the interface comprises a thin multiphase layer whose composition varies monotonically between those of the two end-member phases. After partial delithiation, the interface does not remain static, but changes gradually in terms of orientation, morphology and position, as Li ions from the crystal bulk diffuse back into the delithiated regions. First-principles calculations of a monoclinic crystal of composition LiFePO suggest that the interface exhibits higher electronic conductivity than either of the end-member phases. These observations highlight the importance of the interface in enabling LiFePO particles to retain structural integrity during high-rate charging and discharging.
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http://dx.doi.org/10.1038/s41467-018-05241-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6054635PMC
July 2018

Density functional study of the phase stability and Raman spectra of YbO, YbSiO and YbSiO under pressure.

Phys Chem Chem Phys 2018 Jun;20(24):16518-16527

Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya 456-8587, Japan.

The phase stability and Raman spectra of Yb2O3, Yb2SiO5 and Yb2Si2O7 under hydrostatic pressure are investigated using density functional theory calculations. The calculated energies of polymorphs of each compound show that the stable phases at zero pressure, viz., C-type Yb2O3, X2-Yb2SiO5 and β-Yb2Si2O7, exhibit a pressure-induced phase transition as compressive pressure increases, which is consistent with available experimental data. The theoretical Raman spectra at zero pressure are in good agreement with experimental results for the stable phases and can be used to identify each polymorph. Although the calculated pressure dependence of Raman peak positions of C-type Yb2O3 is overestimated compared to available experimental data, piezospectroscopic coefficients extracted from Raman peaks of X2-Yb2SiO5 and β-Yb2Si2O7 suggest that Raman spectroscopy can be used to measure stresses and strains in Yb silicates. Normal mode analyses reveal that characteristic Raman peaks of Yb silicates at frequencies above 600 cm-1 are strongly associated with vibrations of Si-O bonds in SixOy tetrahedral units.
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http://dx.doi.org/10.1039/c8cp02497aDOI Listing
June 2018

Quantitative analysis of Li distributions in battery material Li1-xFePO4 using Fe M2,3-edge and valence electron energy loss spectra.

Microscopy (Oxf) 2017 Aug;66(4):254-260

Nanostructures Research Laboratory, Japan Fine Ceramics Center, Atsuta, Nagoya 456-8587, Japan.

The spatial distribution of Li ions in a lithium iron phosphate (Li1-xFePO4) single crystal after chemical delithiation is quantitatively investigated using Fe M2,3-edge and valence electron energy loss (EEL) spectroscopy techniques. Li contents between those of end-member compositions LiFePO4 and FePO4 are found to correspond to reproducible changes in Fe M2,3-edge and valence EEL spectra across an interface between LiFePO4 and FePO4 regions. Quantitative analysis of these changes is used to estimate the local valence states of Fe ions, from which the Li concentration in the intermediate phase can be deduced. The faster recording time for valence EEL spectra than Fe M2,3-edge spectra makes measurement of the former a more efficient and reproducible means of estimating Li distributions.
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http://dx.doi.org/10.1093/jmicro/dfx012DOI Listing
August 2017

ZnTaON: Stabilized High-Temperature LiNbO-type Structure.

J Am Chem Soc 2016 12 2;138(49):15950-15955. Epub 2016 Dec 2.

Graduate School of Engineering, Kyoto University , Nishikyo-ku, Kyoto 615-8510, Japan.

By using a high-pressure reaction, we prepared a new oxynitride ZnTaON that crystallizes in a centrosymmetric (R3̅c) high-temperature LiNbO-type structure (HTLN-type). The stabilization of the HTLN-type structure down to low temperatures (at least 20 K) makes it possible to investigate not only the stability of this phase, but also the phase transition to a noncentrosymmetric (R3c) LiNbO-type structure (LN-type) which is yet to be clarified. Synchrotron and neutron diffraction studies in combination with transmission electron microscopy show that Zn is located at a disordered 12c site instead of 6a, implying an order-disorder mechanism of the phase transition. It is found that the closed d-shell of Zn, as well as the high-valent Ta ion, is responsible for the stabilization of the HTLN-type structure, affording a novel quasitriangular ZnON coordination. Interestingly, only 3% Zn substitution for MnTaON induces a phase transition from LN- to HTLN-type structure, implying the proximity in energy between the two structural types, which is supported by the first-principles calculations.
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http://dx.doi.org/10.1021/jacs.6b08635DOI Listing
December 2016

Crystalline Grain Interior Configuration Affects Lithium Migration Kinetics in Li-Rich Layered Oxide.

Nano Lett 2016 05 25;16(5):2907-15. Epub 2016 Apr 25.

Institute of Engineering Innovation, School of Engineering, The University of Tokyo , Tokyo 113-8656, Japan.

The electrode kinetics of Li-ion batteries, which are important for battery utilization in electric vehicles, are affected by the grain size, crystal orientation, and surface structure of electrode materials. However, the kinetic influences of the grain interior structure and element segregation are poorly understood, especially for Li-rich layered oxides with complex crystalline structures and unclear electrochemical phenomena. In this work, cross-sectional thin transmission electron microscopy specimens are "anatomized" from pristine Li1.2Mn0.567Ni0.167Co0.067O2 powders using a new argon ion slicer technique. Utilizing advanced microscopy techniques, the interior configuration of a single grain, multiple monocrystal-like domains, and nickel-segregated domain boundaries are clearly revealed; furthermore, a randomly distributed atomic-resolution Li2MnO3-like with an intergrown LiTMO2 (TM = transitional metals) "twin domain" is demonstrated to exist in each domain. Further theoretical calculations based on the Li2MnO3-like crystal domain boundary model reveal that Li(+) migration in the Li2MnO3-like structure with domain boundaries is sluggish, especially when the nickel is segregated in domain boundaries. Our work uncovers the complex configuration of the crystalline grain interior and provides a conceptual advance in our understanding of the electrochemical performance of several compounds for Li-ion batteries.
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http://dx.doi.org/10.1021/acs.nanolett.5b03933DOI Listing
May 2016

Surface design of alloy protection against CO-poisoning from first principles.

J Phys Condens Matter 2014 Sep 31;26(35):355006. Epub 2014 Jul 31.

Department of Materials Science and Engineering, Kyoto University, Sakyo, Kyoto 606-8501, Japan.

Pt-based alloy catalysts are of significant importance in fuel cells due to enhanced electrode reactivity and selectivity. Designing alloy surfaces suitable for catalyst via first-principles predictions has long played a central role in identifying promising candidates. We propose surface design for polymer electrolyte fuel cell (PEFC) based on the use of thermodynamically stable alloy surfaces. Using first-principles calculation, we have explored stable Pt alloy surfaces that possess superior catalytic properties for CO-despoisoning in hydrogen-related reactions of fuel cells. The stable Pt(25)M(75) (M = Rh, Cu) alloy surfaces both exhibited weaker CO and stronger H binding compared to the pure Pt surface, which yielded a significant increase in H coverage by around one order. These modifications of molecular adsorption are attributed to the deeper band centre of surface d electrons. Understanding the adsorption properties of the stable atomic structure at surfaces will help us to design suitable alloy surfaces with high-catalytic activity and prolonged actuation in fuel cells.
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http://dx.doi.org/10.1088/0953-8984/26/35/355006DOI Listing
September 2014

Impact of local strain on Ti-L₂,₃ electron energy-loss near-edge structures of BaTiO₃: a first-principles multiplet study.

Microscopy (Oxf) 2014 Jun 15;63(3):249-54. Epub 2014 Apr 15.

Institute of Industrial Science, The University of Tokyo 4-6-1, Komaba, Meguro, Tokyo 153-8505, Japan

Identification of local strains is crucial because the local strains largely influence the ferroelectric property of BaTiO₃. The effects of local strains induced by external pressures on the Ti-L₂,₃ electron energy-loss near-edge structure (ELNES) of BaTiO₃ were theoretically investigated using first-principles multiplet calculations. We revealed that the effects appear in the position of the spectral threshold, namely the spectrum shifts to lower and higher energy sides by the tensile and compressive pressures, respectively. We concluded that conventional ELNES observations can identify only large strains induced by -10 GPa, and 0.1 eV energy resolution is required to identify ±2% of strains.
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http://dx.doi.org/10.1093/jmicro/dfu011DOI Listing
June 2014

Ferroelectricity driven by twisting of silicate tetrahedral chains.

Angew Chem Int Ed Engl 2013 Jul 14;52(31):8088-92. Epub 2013 Jun 14.

Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan.

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http://dx.doi.org/10.1002/anie.201302188DOI Listing
July 2013

First-principles calculations of lithium-ion migration at a coherent grain boundary in a cathode material, LiCoO(2).

Adv Mater 2013 Jan 2;25(4):618-22. Epub 2012 Nov 2.

Nanostructures Research Laboratory, Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya, 456-8587, Japan.

Results of theoretical calculations are reported, examining the effect of a coherent twin boundary on the electrical properties of LiCoO(2) . This study suggests that internal interfaces in LiCoO(2) strongly affect the battery voltage, battery capacity, and power density of this material, which is of particular concern if it is used in all-solid-state Li-ion batteries.
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http://dx.doi.org/10.1002/adma.201202805DOI Listing
January 2013

On the structural origin of the catalytic properties of inherently strained ultrasmall decahedral gold nanoparticles.

Nano Lett 2012 Apr 9;12(4):2027-31. Epub 2012 Mar 9.

Departments of Physics, University of York, The York JEOL Nanocentre, York, YO10 5DD, United Kingdom.

A new mechanism for reactivity of multiply twinned gold nanoparticles resulting from their inherently strained structure provides a further explanation of the surprising catalytic activity of small gold nanoparticles. Atomic defect structural studies of surface strains and quantitative analysis of atomic column displacements in the decahedral structure observed by aberration corrected transmission electron microscopy reveal an average expansion of surface nearest neighbor distances of 5.6%, with many strained by more than 10%. Density functional theory calculations of the resulting modified gold d-band states predict significantly enhanced activity for carbon monoxide oxidation. The new insights have important implications for the applications of nanoparticles in chemical process technology, including for heterogeneous catalysis.
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http://dx.doi.org/10.1021/nl300067qDOI Listing
April 2012

Oxygen-vacancy ordering at surfaces of lithium manganese(III,IV) oxide spinel nanoparticles.

Angew Chem Int Ed Engl 2011 Mar 23;50(13):3053-7. Epub 2011 Feb 23.

Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya, Japan.

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http://dx.doi.org/10.1002/anie.201004638DOI Listing
March 2011

Local condensation around oxygen vacancies in t-LaNbO4 from first principles calculations.

Phys Chem Chem Phys 2009 Jul;11(27):5550-3

Nanostructures Research Laboratory, Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan.

First principles calculations reveal that formation of a fully ionized oxide vacancy leads to local condensation of coordination polyhedra forming Nb3O11(7-) in t-LaNbO4.
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http://dx.doi.org/10.1039/b813881hDOI Listing
July 2009

First-principles calculations of migration energy of lithium ions in halides and chalcogenides.

J Phys Chem B 2006 Apr;110(16):8258-62

Department of Materials Science and Engineering, Kyoto University, Sakyo, Kyoto 606-8501, Japan.

Migration of Li+ ions via the vacancy mechanism in LiX (X = F, Cl, Br, and I) with the rocksalt and hypothetical zinc blende structures and Li2X (X = O, S, Se, and Te) with the antifluorite structure has been investigated using first-principles projector augmented wave calculations with the generalized gradient approximation. The migration paths and energies, determined by the nudged-elastic-band method, are discussed on the basis of two idealized models: the rigid-sphere and charged-sphere models. The trajectories and energy profiles of the migration in these lithium compounds vary between these two models, depending on the anion species and crystal structure. The migration energies in LiX with both the rocksalt and hypothetical zinc blende structures show a tendency to decrease with increasing periodic number of the anion species in the periodic table. This is consistent with the widely accepted view that anion species with large ionic radii and high polarizabilities are favorable for good ionic conduction. In contrast, Li2O exhibits the lowest migration energy among Li2X compounds, although O is the smallest among the chalcogens, indicating that electrostatic attractive interactions play the dominant role in the inter-ion interactions in Li2O and, therefore, in the ion migration.
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http://dx.doi.org/10.1021/jp0559229DOI Listing
April 2006
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