Publications by authors named "Yoji Kobayashi"

45 Publications

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

Pressure-Induced Collapse Transition in BaTiPnO (Pn = As, Sb) with an Unusual Pn-Pn Bond Elongation.

Inorg Chem 2021 Feb 27;60(4):2228-2233. Epub 2021 Jan 27.

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

Making and breaking bonds in a solid-state compound greatly influences physical properties. A well-known playground for such bonding manipulation is the ThCrSi-type structure ATX, allowing a collapse transition where a X-X dimer forms by a chemical substitution or external stimuli. Here, we report a pressure-induced collapse transition in the structurally related BaTiPnO (Pn = As, Sb) at a transition pressure of ∼15 GPa. The Pn-Pn bond formation is related with Pn-p band filling, which is controlled by charge transfer from the Ti-3d band. At , the Sb-Sb distance in BaTiSbO shrinks due to bond formation, but interestingly, the Sb-Sb expands with increasing pressure above . This expansion, which was not reported in ThCrSi-type compounds, may arise from heteroleptic coordination geometry around titanium, where a compression of the Ti-O bond plays a role. Electrical resistivity measurements of BaTiSbO up to 55 GPa revealed an increasing trend of the superconducting transition temperature with pressure. This study presents structure motifs that allow flexible bonding manipulation and property control with heteroleptic coordination geometry.
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http://dx.doi.org/10.1021/acs.inorgchem.0c02989DOI Listing
February 2021

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

Site Selectivity of Hydride in Early-Transition-Metal Ruddlesden-Popper Oxyhydrides.

Inorg Chem 2018 Sep 16;57(17):11058-11067. Epub 2018 Aug 16.

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

Layered perovskite titanium oxyhydrides have been prepared by low-temperature topochemical CaH reduction from Ruddlesden-Popper Sr Ti O phases ( n = 1, 2) and structurally characterized by combined synchrotron X-ray and neutron diffraction data refinements. In the single-layered SrTiOD material, hydride anions are statistically disordered with oxides on the apical site only, as opposed to known transition-metal oxyhydrides exhibiting a preferred occupation of the equatorial site. This unprecedented site selectivity of H has been reproduced by periodic DFT+ U calculations, emphasizing for the hydride defect a difference in formation energy of 0.24 eV between equatorial and apical sites. In terms of electronic structure, the model system SrTiOH is found to be slightly metallic and the released electron remains mostly delocalized over several Ti atoms. On the other hand, hydride anions in the double-layered SrTiOH material show a clear preference for the bridging apical site within the perovskite slabs, as confirmed by DFT calculations on the SrTiOH model system. Finally, the influence of the B-site chemical nature on the hydride site selectivity for early 3d transition metals is theoretically explored in the single-layered system by substituting vanadium for titanium. The V electronic polaron is suggested to play a role in stabilizing H on the equatorial site in SrVOH for x = 0.125.
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http://dx.doi.org/10.1021/acs.inorgchem.8b01645DOI Listing
September 2018

Current Ventilator and Oxygen Management during General Anesthesia: A Multicenter, Cross-sectional Observational Study.

Anesthesiology 2018 07;129(1):67-76

From the Department of Anesthesiology and Resuscitology, Okayama University Hospital, Okayama, Japan (S.S., Y.M., Y.H., S.O., H.M.) Department of Medical Statistics, Osaka City University Graduate School of Medicine, Osaka, Japan (T.I., A.S.). Atago Hospital, Kochi Chikamori Hospital, Kochi Fukuyama City Hospital, Hiroshima Fukuyama Medical Center, Hiroshima Himeji Central Hospital, Hyogo Hiroshima City Hiroshima Citizens Hospital, Hiroshima Iwakuni Medical Center, Yamaguchi Japanese Red Cross Kobe Hospital, Hyogo Japanese Red Cross Okayama Hospital, Okayama Japanese Red Cross Society Himeji Hospital, Hyogo Japanese Red Cross Society Mihara Hospital, Hiroshima Jichi Medical University Hospital, Tochigi Kagawa Prefectural Central Hospital, Kagawa Kagawa Rosai Hospital, Kagawa Kajiki Hospital, Okayama Kameda Medical Center, Chiba Kawasaki Medical School General Medical Center, Okayama Kawasaki Medical School Hospital, Okayama Kobe University Hospital, Hyogo Kochi Health Sciences Center, Kochi Kochi Medical School Hospital, Kochi Kurashiki Medical Center, Okayama Maizuru Kyosai Hospital, Kyoto Matsuda Hospital, Okayama Mitoyo General Hospital, Kagawa Mizushima Kyodo Hospital, Okayama National Cancer Center Hospital, Tokyo Okayama City Hospital, Okayama Okayama Kyokuto Hospital, Okayama Okayama Kyoritsu General Hospital, Okayama Okayama Medical Center, Okayama Okayama Rosai Hospital, Okayama Okayama Saiseikai General Hospital, Okayama Onomichi Municipal Hospital, Hiroshima Saiseikai Imabari Hospital, Ehime Shizuoka Cancer Center, Shizuoka Showa University Northern Yokohama Hospital, Kanagawa Takasago Municipal Hospital, Hyogo Takinomiya General Hospital, Kagawa Tottori Municipal Hospital, Tottori Tsuyama Chuo Hospital, Okayama Yashima General Hospital, Kagawa.

Background: Intraoperative oxygen management is poorly understood. It was hypothesized that potentially preventable hyperoxemia and substantial oxygen exposure would be common during general anesthesia.

Methods: A multicenter, cross-sectional study was conducted to describe current ventilator management, particularly oxygen management, during general anesthesia in Japan. All adult patients (16 yr old or older) who received general anesthesia over 5 consecutive days in 2015 at 43 participating hospitals were identified. Ventilator settings and vital signs were collected 1 h after the induction of general anesthesia. We determined the prevalence of potentially preventable hyperoxemia (oxygen saturation measured by pulse oximetry of more than 98%, despite fractional inspired oxygen tension of more than 0.21) and the risk factors for potentially substantial oxygen exposure (fractional inspired oxygen tension of more than 0.5, despite oxygen saturation measured by pulse oximetry of more than 92%).

Results: A total of 1,786 patients were found eligible, and 1,498 completed the study. Fractional inspired oxygen tension was between 0.31 and 0.6 in 1,385 patients (92%), whereas it was less than or equal to 0.3 in very few patients (1%). Most patients (83%) were exposed to potentially preventable hyperoxemia, and 32% had potentially substantial oxygen exposure. In multivariable analysis, old age, emergency surgery, and one-lung ventilation were independently associated with increased potentially substantial oxygen exposure, whereas use of volume control ventilation and high positive end-expiratory pressure levels were associated with decreased potentially substantial oxygen exposure. One-lung ventilation was particularly a strong risk factor for potentially substantial oxygen exposure (adjusted odds ratio, 13.35; 95% CI, 7.24 to 24.60).

Conclusions: Potentially preventable hyperoxemia and substantial oxygen exposure are common during general anesthesia, especially during one-lung ventilation. Future research should explore the safety and feasibility of a more conservative approach for intraoperative oxygen therapy.
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http://dx.doi.org/10.1097/ALN.0000000000002181DOI Listing
July 2018

New chemistry of transition metal oxyhydrides.

Sci Technol Adv Mater 2017 16;18(1):905-918. Epub 2017 Nov 16.

Department of Energy & Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto, Japan.

In this review we describe recent advances in transition metal oxyhydride chemistry obtained by topochemical routes, such as low temperature reduction with metal hydrides, or high-pressure solid-state reactions. Besides the crystal chemistry, magnetic and transport properties of the bulk powder and epitaxial thin film samples, the remarkable lability of the hydride anion is particularly highlighted as a new strategy to discover unprecedented mixed anion materials.
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http://dx.doi.org/10.1080/14686996.2017.1394776DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5784496PMC
November 2017

Titanium-Based Hydrides as Heterogeneous Catalysts for Ammonia Synthesis.

J Am Chem Soc 2017 12 6;139(50):18240-18246. Epub 2017 Dec 6.

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

The problem of activating N and its subsequent hydrogenation to form NH has been approached from many directions. One of these approaches involves the use of transition metal hydride complexes. Recently, transition metal hydride complexes of Ti and Ta have been shown to activate N, but without catalytic formation of NH. Here, we show that at elevated temperatures (400 °C, 5 MPa), solid-state hydride-containing Ti compounds (TiH and BaTiOH) form a nitride-hydride surface similar to those observed with titanium clusters, but continuously (∼7 days) form NH under H/N flow conditions to achieve a catalytic cycle, with activity (up to 2.8 mmol·g··h) almost comparable to conventional supported Ru catalysts such as Cs-Ru/MgO or Ru/BaTiO that we have tested. As with the homogeneous analogues, the initial presence of hydride within the catalyst is critical. A rare hydrogen-based Mars van Krevelen mechanism may be at play here. Conventional scaling rules of pure metals predict essentially no activity for Ti, making this a previously overlooked element, but our results show that by introducing hydride, the repertoire of heterogeneous catalysts can be expanded to include formerly unexamined compositions without resorting to precious metals.
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http://dx.doi.org/10.1021/jacs.7b08891DOI Listing
December 2017

Promoted Hydride/Oxide Exchange in SrTiO by Introduction of Anion Vacancy via Aliovalent Cation Substitution.

Inorg Chem 2017 Nov;56(21):13035-13040

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

We investigated topochemical anion exchange reactions for a Sc-substituted SrTiO perovskite, Sr(TiSc)O□ (y ≤ 0.1), using CaH. It was found that the initial introduction of a small amount of anion vacancies (y/2) is crucial to enhance the anion (H/O) exchangeability. For example, hydride reduction of Sr(TiSc)O yielded the oxyhydride SrTiScOH in which the hydride concentration is increased by 33% with respect to pristine SrTiO (leading to SrTiOH). This observation highlights the importance of anion vacancies to improve anion (H/O) diffusion, which is a well-known strategy for improving oxide anion conductivity, and suggests that such a vacancy-assisted reaction could be applied to other anion exchange reactions (e.g., F/O and N/O) to extend the solubility range.
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http://dx.doi.org/10.1021/acs.inorgchem.7b01845DOI Listing
November 2017

Pressure-Stabilized Cubic Perovskite Oxyhydride BaScOH.

Inorg Chem 2017 May 11;56(9):4840-4845. Epub 2017 Apr 11.

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

We report a scandium oxyhydride BaScOH prepared by solid state reaction under high pressure. Rietveld refinements against powder synchrotron X-ray and neutron diffraction data revealed that BaScOH adopts the ideal cubic perovskite structure (Pm3̅m), where oxide (O) and hydride (H) anions are disordered. H nuclear magnetic resonance (NMR) spectroscopy provides a positive chemical shift of about +4.4 ppm, which can be understood by the distance to the nearest (and possibly the next nearest) cation from the H nucleus. A further analysis of the NMR data and calculations based on ab initio random structure searches suggest a partial cis preference in ScOH octahedra. The present oxyhydride, if compositionally or structurally tuned, may become a candidate for H conductors.
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http://dx.doi.org/10.1021/acs.inorgchem.6b02834DOI Listing
May 2017

Selective and low temperature transition metal intercalation in layered tellurides.

Nat Commun 2016 12 14;7:13809. Epub 2016 Dec 14.

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

Layered materials embrace rich intercalation reactions to accommodate high concentrations of foreign species within their structures, and find many applications spanning from energy storage, ion exchange to secondary batteries. Light alkali metals are generally most easily intercalated due to their light mass, high charge/volume ratio and in many cases strong reducing properties. An evolving area of materials chemistry, however, is to capture metals selectively, which is of technological and environmental significance but rather unexplored. Here we show that the layered telluride TPTe (T=Ti, Zr) displays exclusive insertion of transition metals (for example, Cd, Zn) as opposed to alkali cations, with tetrahedral coordination preference to tellurium. Interestingly, the intercalation reactions proceed in solid state and at surprisingly low temperatures (for example, 80 °C for cadmium in TiPTe). The current method of controlling selectivity provides opportunities in the search for new materials for various applications that used to be possible only in a liquid.
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http://dx.doi.org/10.1038/ncomms13809DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5171714PMC
December 2016

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

Impact of Lanthanoid Substitution on the Structural and Physical Properties of an Infinite-Layer Iron Oxide.

Inorg Chem 2016 Nov 1;55(22):12093-12099. Epub 2016 Nov 1.

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

The effect of lanthanoid (Ln = Nd, Sm, Ho) substitution on the structural and physical properties of the infinite-layer iron oxide SrFeO was investigated by X-ray diffraction (XRD) at ambient and high pressure, neutron diffraction, and Fe Mössbauer spectroscopy. Ln for Sr substituted samples up to ∼30% were synthesized by topochemical reduction using CaH. While the introduction of the smaller Ln ion reduces the a axis as expected, we found an unusual expansion of the c axis as well as the volume. Rietveld refinements along with pair distribution function analysis revealed the incorporation of oxygen atoms between FeO layers with a charge-compensated composition of (SrLn)FeO, which accounts for the failed electron doping to the FeO layer. The incorporated partial apical oxygen or the pyramidal coordination induces incoherent buckling of the FeO sheet, leading to a significant reduction of the Néel temperature. High-pressure XRD experiments for (SrHo)FeO suggest a possible stabilization of an intermediate spin state in comparison with SrFeO, revealing a certain contribution of the in-plane Fe-O distance to the pressure-induced transition.
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http://dx.doi.org/10.1021/acs.inorgchem.6b02513DOI Listing
November 2016

High-Pressure Synthesis of Manganese Oxyhydride with Partial Anion Order.

Angew Chem Int Ed Engl 2016 08 29;55(33):9667-70. Epub 2016 Jun 29.

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

The high-pressure synthesis of a manganese oxyhydride LaSrMnO3.3 H0.7 is reported. Neutron and X-ray Rietveld analyses showed that this compound adopts the K2 NiF4 structure with hydride ions positioned exclusively at the equatorial site. This result makes a striking contrast to topochemical reductions of LaSrMnO4 that result in only oxygen-deficient phases down to LaSrMnO3.5 . This suggests that high H2 pressure plays a key role in stabilizing the oxyhydride phase, offering an opportunity to synthesize other transition-metal oxyhydrides. Magnetic susceptibility revealed a spin-glass transition at 24 K that is due to competing ferromagnetic (Mn(2+) -Mn(3+) ) and antiferromagnetic (Mn(2+) -Mn(2) , Mn(3+) -Mn(3+) ) interactions.
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http://dx.doi.org/10.1002/anie.201605123DOI Listing
August 2016

Topochemical Nitridation with Anion Vacancy-Assisted N(3-)/O(2-) Exchange.

J Am Chem Soc 2016 Mar 23;138(9):3211-7. Epub 2016 Feb 23.

Center for Neutron Research, National Institute of Standards and Technology , Gaithersburg, MD 20899, United States.

We present how the introduction of anion vacancies in oxyhydrides enables a route to access new oxynitrides, by conducting ammonolysis of perovskite oxyhydride EuTiO3-xHx (x ∼ 0.18). At 400 °C, similar to our studies on BaTiO3-xHx, hydride lability enables a low temperature direct ammonolysis of EuTi(3.82+)O2.82H0.18, leading to the N(3-)/H(-)-exchanged product EuTi(4+)O2.82N0.12□0.06. When the ammonolysis temperature was increased up to 800 °C, we observed a further nitridation involving N(3-)/O(2-) exchange, yielding a fully oxidized Eu(3+)Ti(4+)O2N with the GdFeO3-type distortion (Pnma) as a metastable phase, instead of pyrochlore structure. Interestingly, the same reactions using the oxide EuTiO3 proceeded through a 1:1 exchange of N(3-) with O(2-) only above 600 °C and resulted in incomplete nitridation to EuTiO2.25N0.75, indicating that anion vacancies created during the initial nitridation process of EuTiO2.82H0.18 play a crucial role in promoting anion (N(3-)/O(2-)) exchange at high temperatures. Hence, by using (hydride-induced) anion-deficient precursors, we should be able to expand the accessible anion composition of perovskite oxynitrides.
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http://dx.doi.org/10.1021/jacs.6b00088DOI Listing
March 2016

A labile hydride strategy for the synthesis of heavily nitridized BaTiO3.

Nat Chem 2015 Dec 19;7(12):1017-23. Epub 2015 Oct 19.

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

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 °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 °C), using a perovskite oxyhydride as a host. The lability of H(-) in BaTiO3-xHx (x ≤ 0.6) allows H(-)/N(3-) 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.
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http://dx.doi.org/10.1038/nchem.2370DOI Listing
December 2015

Hydride in BaTiO2.5H0.5: A Labile Ligand in Solid State Chemistry.

J Am Chem Soc 2015 Dec 30;137(48):15315-21. Epub 2015 Nov 30.

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

In synthesizing mixed anion oxides, direct syntheses have often been employed, usually involving high temperature and occasionally high pressure. Compared with these methods, here we show how the use of a titanium perovskite oxyhydride (BaTiO2.5H0.5) as a starting material enables new multistep low temperature topochemical routes to access mixed anion compounds. Similar to labile ligands in inorganic complexes, the lability of H(-) provides the necessary reactivity for syntheses, leading to reactions and products previously difficult to obtain. For example, BaTiO2.5N0.2 can be prepared with the otherwise inert N2 gas at 400-600 °C, in marked contrast with currently available oxynitride synthetic routes. F(-)/H(-) exchange can also be accomplished at 150 °C, yielding the oxyhydride-fluoride BaTi(O, H, F)3. For BaTiO2.4D0.3F0.3, we find evidence that further anionic exchange with OD(-) yields BaTiO2.4(D(-))0.26(OD(-))0.34, which implies stable coexistence of H(+) and H(-) at ambient conditions. Such an arrangement is thermodynamically unstable and would be difficult to realize otherwise. These results show that the labile nature of hydride imparts reactivity to oxide hosts, enabling it to participate in new multistep reactions and form new materials.
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http://dx.doi.org/10.1021/jacs.5b10255DOI Listing
December 2015

A Nearly Ideal One-Dimensional S = 5/2 Antiferromagnet FeF3(4,4'-bpy) (4,4'-bpy =4,4'-bipyridyl) with Strong Intrachain Interactions.

J Am Chem Soc 2015 Aug 28;137(31):9804-7. Epub 2015 Jul 28.

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

An ideal one-dimensional (1D) magnet is expected to show exotic quantum phenomena. For compounds with larger S (S = 3/2, 2, 5/2, ...), however, a small interchain interaction J' tends to drive a conventional long-range ordered (LRO) state. Here, a new layered structure of FeF3(4,4'-bpy) (4,4'-bpy = 4,4'-bipyridyl) with novel S = 5/2 (Fe(3+)) chains has been hydrothermally synthesized by using 4,4'-bpy to separate chains. The temperature-dependent susceptibility exhibits a broad maximum at high as 164 K, suggesting a fairly strong Fe-F-Fe intrachain interaction J. However, no anomaly associated with a LRO is seen in both magnetic susceptibility and specific heat even down to 2 K. This indicates an extremely small J' with J'/J < 3.2 × 10(-5), making this new material a nearly ideal 1D antiferromagnet. Mössbauer spectroscopy at 2.7 K reveals a critical slowing down of the 1D fluctuations toward a possible LRO at lower temperatures.
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http://dx.doi.org/10.1021/jacs.5b06120DOI Listing
August 2015

Effect of calcination conditions on porous reduced titanium oxides and oxynitrides via a preceramic polymer route.

Inorg Chem 2015 Mar 2;54(6):2802-8. Epub 2015 Mar 2.

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

A preceramic polymer route from Ti-based inorganic-organic hybrid networks provides electroconductive N-doped reduced titanium oxides (TinO2n-1) and titanium oxynitrides (TiOxNy) with a monolithic shape as well as well-defined porous structures. This methodology demonstrates an advantageously lower temperature of the crystal phase transition compared to the reduction of TiO2 by carbon or hydrogen. In this study, the effect of calcination conditions on various features of the products has been explored by adopting three different atmospheric conditions and varying the calcination temperature. The detailed crystallographic and elemental analyses disclose the distinguished difference in the phase transition behavior with respect to the calcination atmosphere. The correlation between the crystallization and nitridation behaviors, porous properties, and electric conductivities in the final products is discussed.
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http://dx.doi.org/10.1021/ic502972vDOI Listing
March 2015

An antiferro-to-ferromagnetic transition in EuTiO(3-x)H(x) induced by hydride substitution.

Inorg Chem 2015 Feb 16;54(4):1501-7. Epub 2015 Jan 16.

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

We have prepared the oxyhydride perovskite EuTiO(3-x)H(x) (x ≤ 0.3) by a low temperature CaH2 reduction of pyrochlore Eu2Ti2O7 and perovskite EuTiO3. The reduced EuTiO(3-x)H(x) crystallizes in the ideal cubic perovskite (Pm3̅m), where O/H anions are randomly distributed. As a result of electron doping by the aliovalent anion exchange, the resistivity of EuTiO(3-x)H(x) shows metallic temperature dependence. Moreover, an antiferromagnetic-to-ferromagnetic transition is observed even when a small amount of hydride (x ∼ 0.07) is introduced. The Curie temperature TC of 12 K is higher than those of any other EuTiO3-derived ferromagnets. The ferromagnetism can be explained by the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the Eu(2+) spins mediated by the itinerant Ti 3d electrons. The present study shows that controlling the oxide/hydride ratio is a versatile method to tune magnetic and transport properties.
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http://dx.doi.org/10.1021/ic502486eDOI Listing
February 2015

MnTaO2N: polar LiNbO3-type oxynitride with a helical spin order.

Angew Chem Int Ed Engl 2015 Jan 21;54(2):516-21. Epub 2014 Nov 21.

Graduate School of Engineering, Kyoto University, Kyoto 615-8510 (Japan); The Hakubi Center for Advanced Research, Kyoto University, Kyoto 606-8501 (Japan).

The synthesis, structure, and magnetic properties of a polar and magnetic oxynitride MnTaO2N are reported. High-pressure synthesis at 6 GPa and 1400 °C allows for the stabilization of a high-density structure containing middle-to-late transition metals. Synchrotron X-ray and neutron diffraction studies revealed that MnTaO2N adopts the LiNbO3-type structure, with a random distribution of O(2-) and N(3-) anions. MnTaO2N with an "orbital-inactive" Mn(2+) ion (d(5); S=5/2) exhibits a nontrivial helical spin order at 25 K with a propagation vector of [0,0,δ] (δ≈0.3), which is different from the conventional G-type order observed in other orbital-inactive perovskite oxides and LiNbO3-type oxides. This result suggests the presence of strong frustration because of the heavily tilted MnO4N2 octahedral network combined with the mixed O(2-)/N(3-) species that results in a distribution of (super)-superexchange interactions.
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http://dx.doi.org/10.1002/anie.201408483DOI Listing
January 2015

Direct synthesis of chromium perovskite oxyhydride with a high magnetic-transition temperature.

Angew Chem Int Ed Engl 2014 Sep 12;53(39):10377-80. Epub 2014 Aug 12.

Graduate School of Engineering, Kyoto University, Kyoto 615-8510 (Japan); The Hakubi Center for Advanced Research, Kyoto University, Kyoto (Japan).

We report a novel oxyhydride SrCrO2H directly synthesized by a high-pressure high-temperature method. Powder neutron and synchrotron X-ray diffraction revealed that this compound adopts the ideal cubic perovskite structure (Pm3̄m) with O(2-)/H(-) disorder. Surprisingly, despite the non-bonding nature between Cr 3d t(2g) orbitals and the H 1s orbital, it exhibits G-type spin ordering at T(N)≈380 K, which is higher than that of RCrO3 (R=rare earth) and any chromium oxides. The enhanced T(N) in SrCrO2H with four Cr-O-Cr bonds in comparison with RCr(3+)O3 with six Cr-O-Cr bonds is reasonably explained by the tolerance factor. The present result offers an effective strategy to tune octahedral tilting in perovskites and to improve physical and chemical properties through mixed anion chemistry.
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http://dx.doi.org/10.1002/anie.201405453DOI Listing
September 2014

High energy density rechargeable magnesium battery using earth-abundant and non-toxic elements.

Sci Rep 2014 Jul 11;4:5622. Epub 2014 Jul 11.

Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-nihonmatsu-cho, Sakyo-ku, Kyoto 606-8501, JAPAN.

Rechargeable magnesium batteries are poised to be viable candidates for large-scale energy storage devices in smart grid communities and electric vehicles. However, the energy density of previously proposed rechargeable magnesium batteries is low, limited mainly by the cathode materials. Here, we present new design approaches for the cathode in order to realize a high-energy-density rechargeable magnesium battery system. Ion-exchanged MgFeSiO4 demonstrates a high reversible capacity exceeding 300 Ah · g(-1) at a voltage of approximately 2.4 V vs. Mg. Further, the electronic and crystal structure of ion-exchanged MgFeSiO4 changes during the charging and discharging processes, which demonstrates the (de)insertion of magnesium in the host structure. The combination of ion-exchanged MgFeSiO4 with a magnesium bis(trifluoromethylsulfonyl)imide-triglyme electrolyte system proposed in this work provides a low-cost and practical rechargeable magnesium battery with high energy density, free from corrosion and safety problems.
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http://dx.doi.org/10.1038/srep05622DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4092329PMC
July 2014

CaH2-assisted low temperature synthesis of metallic magnetic nanoparticle-loaded multiwalled carbon nanotubes.

Chem Commun (Camb) 2014 Jul;50(52):6866-8

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

We studied synthesis of Ni or Fe nanoparticle-loaded multiwalled carbon nanotubes (MWCNTs) by pyrolyzing metal organic salts with CaH2, a very strong reductant. The use of CaH2 lowered the formation temperature of MWCNTs down to 400 °C without the use of toxic halogen-containing precursors and assistance of plasma.
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http://dx.doi.org/10.1039/c4cc01592dDOI Listing
July 2014

Sr2FeO3 with stacked infinite chains of FeO4 square planes.

Inorg Chem 2013 May 8;52(10):6096-102. Epub 2013 May 8.

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

The synthesis of Sr2FeO3 through a hydride reduction of the Ruddlesden-Popper layered perovskite Sr2FeO4 is reported. Rietveld refinements using synchrotron and neutron powder diffraction data revealed that the structure contains corner-shared FeO4 square-planar chains running along the [010] axis, being isostructural with Sr2CuO3 (Immm space group). Fairly strong Fe-O-Fe and Fe-Fe interactions along [010] and [100], respectively, make it an S = 2 quasi two-dimensional (2D) rectangular lattice antiferromagnet. This compound represents the end-member (n = 1) of the serial system Sr(n+1)FenO(2n+1), together with previously reported Sr3Fe2O5 (n = 2) and SrFeO2 (n = ∞), thus giving an opportunity to study the 2D-to-3D dimensional crossover. Neutron diffraction and Mössbauer spectroscopy show the occurrence of G-type antiferromagnetic order below 179 K, which is, because of dimensional reduction, significantly lower than those of the other members, 296 K in Sr3Fe2O5 and 468 K in SrFeO2. However, the temperature dependence of magnetic moment shows a universal behavior.
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http://dx.doi.org/10.1021/ic400444uDOI Listing
May 2013

Oxyhydrides of (Ca,Sr,Ba)TiO3 perovskite solid solutions.

Inorg Chem 2012 Nov 19;51(21):11371-6. Epub 2012 Oct 19.

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

The oxyhydride solid solutions (Ca,Sr)TiO(3-x)H(x) and (Sr,Ba)TiO(3-x)H(x) have been prepared by reducing the corresponding ATiO(3) oxides with calcium hydride. Under the reaction conditions examined, a hydride content of x = 0.1-0.3 was obtained for all compositions. Compared to our previous result with BaTiO(3-x)H(x), the larger particle size in this study (20-30 μm vs 170 nm) resulted in a somewhat lower hydride amount despite prolonged reaction times. We examined changes in cell volume, octahedral tilt angle, and site occupancy of different anion sites after conversion to oxyhydrides; it appears that these oxyhydrides fit the geometrical descriptions typical for regular ABO(3) perovskites quite well. The hydrogen release temperature, previously shown to be indicative of the hydride exchange temperature, however, does not scale linearly with the A-site composition, indicating a potential effect of chemical randomness.
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http://dx.doi.org/10.1021/ic300859nDOI Listing
November 2012

Low temperature solventless synthesis and characterization of Ni and Fe magnetic nanoparticles.

Chem Commun (Camb) 2012 Aug 11;48(66):8237-9. Epub 2012 Jul 11.

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

We have successfully implemented a facile, one-pot solventless synthesis procedure starting from acetylacetonate salts and CaH(2) to obtain carbon-coated ferromagnetic metallic Ni and Fe nanoparticles at low temperature. The use of CaH(2) as a reductant drastically reduces reaction temperature down to 140 °C.
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http://dx.doi.org/10.1039/c2cc33830kDOI Listing
August 2012

(Sr(1-x)Ba(x))FeO2 (0.4 ≤ x ≤ 1): a new oxygen-deficient perovskite structure.

J Am Chem Soc 2012 Jul 5;134(28):11444-54. Epub 2012 Jul 5.

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

Topochemical reduction of (layered) perovskite iron oxides with metal hydrides has so far yielded stoichiometric compositions with ordered oxygen defects with iron solely in FeO(4) square planar coordination. Using this method, we have successfully obtained a new oxygen-deficient perovskite, (Sr(1-x)Ba(x))FeO(2) (0.4 ≤ x ≤ 1.0), revealing that square planar coordination can coexist with other 3-6-fold coordination geometries. This BaFeO(2) structure is analogous to the LaNiO(2.5) structure in that one-dimensional octahedral chains are linked by planar units, but differs in that one of the octahedral chains contains a significant amount of oxygen vacancies and that all the iron ions are exclusively divalent in the high-spin state. Mössbauer spectroscopy demonstrates, despite the presence of partial oxygen occupations and structural disorders, that the planar-coordinate Fe(2+) ions are bonded highly covalently, which accounts for the formation of the unique structure. At the same time, a rigid 3D Fe-O-Fe framework contributes to structural stabilization. Powder neutron diffraction measurements revealed a G-type magnetic order with a drastic decrease of the Néel temperature compared to that of SrFeO(2), presumably due to the effect of oxygen disorder/defects. We also performed La substitution at the Ba site and found that the oxygen vacancies act as a flexible sink to accommodate heterovalent doping without changing the Fe oxidation and spin state, demonstrating the robustness of this new structure against cation substitution.
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http://dx.doi.org/10.1021/ja3007403DOI Listing
July 2012

Selective preparation of macroporous monoliths of conductive titanium oxides Ti(n)O(2n-1) (n = 2, 3, 4, 6).

J Am Chem Soc 2012 Jul 22;134(26):10894-8. Epub 2012 Jun 22.

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

Monolithic conductive titanium oxides Ti(n)O(2n-1) (n = 2, 3, 4, 6) with well-defined macropores have been successfully prepared as a single phase, via reduction of a macroporous TiO(2) precursor monolith using zirconium getter. Despite substantial removal of oxide ions, all the reduced monoliths retain the macropore properties of the precursor, i.e., uniform pore size distribution and pore volume. Furthermore, compared to commercial porous Ebonex (shaped conductive Ti(n)O(2n-1)), the bulk densities (1.8 g cm(-3)) are half, and the porosities (60%) are about 3 times higher. The obtained Ti(n)O(2n-1) (n = 2, 3, 4, 6) macroporous monoliths could find applications as electrodes for many electrochemical reactions.
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http://dx.doi.org/10.1021/ja302083nDOI Listing
July 2012

Epitaxial thin films of ATiO(3-x)H(x) (A = Ba, Sr, Ca) with metallic conductivity.

J Am Chem Soc 2012 May 18;134(21):8782-5. Epub 2012 May 18.

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

Epitaxial thin films of titanium perovskite oxyhydride ATiO(3-x)H(x) (A = Ba, Sr, Ca) were prepared by CaH(2) reduction of epitaxial ATiO(3) thin films deposited on a (LaAlO(3))(0.3)(SrAl(0.5)Ta(0.5)O(3))(0.7) substrate. Secondary ion mass spectroscopy detected a substantial amount and uniform distribution of hydride within the film. SrTiO(3)/LSAT thin film hydridized at 530 °C for 1 day had hydride concentration of 4.0 × 10(21) atoms/cm(3) (i.e., SrTiO(2.75)H(0.25)). The electric resistivity of all the ATiO(3-x)H(x) films exhibited metallic (positive) temperature dependence, as opposed to negative as in BaTiO(3-x)H(x) powder, revealing that ATiO(3-x)H(x) are intrinsically metallic, with high conductivity of 10(2)-10(4) S/cm. Treatment with D(2) gas results in hydride/deuteride exchange of the films; these films should be valuable in further studies on hydride diffusion kinetics. Combined with the materials' inherent high electronic conductivity, new mixed electron/hydride ion conductors may also be possible.
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http://dx.doi.org/10.1021/ja302465cDOI Listing
May 2012
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