Publications by authors named "Ikuya Yamada"

34 Publications

High-Pressure Synthesis and Magnetic States of Magnetoplumbite Cobaltates CaCoO and BaCoO.

Inorg Chem 2021 Jun 20;60(11):7680-7686. Epub 2021 May 20.

Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan.

Novel cobalt oxides, CaCoO and BaCoO, have been synthesized under high-pressure and high-temperature conditions of 7 GPa and 1373 K, respectively. Rietveld refinement using synchrotron X-ray diffraction data indicates that the CaCoO and BaCoO crystallize in a magnetoplumbite structure with a hexagonal space group of 6/ (No. 194) as well as SrCoO. The magnetic study demonstrates that itinerant and localized 3d electrons coexist in all CoO ( = Ca, Sr, Ba) and the magnetic ground state transforms from antiferromagnetic ( = Ca) to ferrimagnetic ( = Sr) to antiferromagnetic ( = Ba), which is in stark contrast to the systematic change in the magnetoplumbite-related cobalt oxides of CoO from antiferromagnet ( = Ca) to ferrimagnet ( = Sr) to ferromagnet ( = Ba). The nonmonotonic magnetic evolution with isoelectronic -site substitution in CoO is probably attributed to changes in the interactions between two magnetic sublattices of localized 3d electrons at trigonal-bipyramidal and tetrahedral sites for CoO. This finding proposes the complex magnetic properties in the layered cobalt oxides with multiple magnetic sublattices in the coexistence system of itinerant and localized electrons.
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http://dx.doi.org/10.1021/acs.inorgchem.0c03726DOI Listing
June 2021

Metamagnetic Behavior in a Quadruple Perovskite Oxide.

Inorg Chem 2021 May 27;60(10):7023-7030. Epub 2021 Apr 27.

Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, 1-2 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8570, Japan.

A cubic quadruple perovskite oxide CeMnCrO has been synthesized under high-pressure and high-temperature conditions of 8 GPa and 1273 K. The X-ray absorption spectroscopy reveals that the Ce ions are in a trivalent state, as represented by the ionic model of CeMnCrO. The magnetic study demonstrates three independent antiferromagnetic transitions attributed to Ce (∼10 K), Mn (46 K), and Cr (133 K) ions. Furthermore, a magnetic field-induced antiferromagnetic-to-ferromagnetic (metamagnetic) transition of Ce 4f moments is observed at low temperatures below 20 K, exhibiting a rare example of metamagnetism in the Ce-oxides. This finding represents that the 3d-electron magnetic sublattices play a role in the metamagnetism of 4f-electron magnetic moments, demonstrating a new aspect of the 3d-4f complex electron systems.
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http://dx.doi.org/10.1021/acs.inorgchem.0c03432DOI Listing
May 2021

A Sequential Electron Doping for Quadruple Perovskite Oxides CuCoO ( = Ca, Y, Ce).

Inorg Chem 2020 Jul 12;59(13):8699-8706. Epub 2020 Jun 12.

Department of Physics and Electronics, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan.

A novel quadruple perovskite oxide CeCuCoO has been synthesized in high-pressure and high-temperature conditions of 12 GPa and 1273 K. Rietveld refinement of the synchrotron X-ray powder diffraction pattern reveals that this oxide crystallizes in a cubic quadruple perovskite structure with the 1:3-type ordering of Ce and Cu ions at the -site. X-ray absorption spectroscopy analysis demonstrates the valence-state transitions in the CuCoO series ( = Ca, Y, Ce) from CaCuCoO to YCuCoO to CeCuCoO, where the electrons are doped in the order from -site (Co → Co) to '-site (Cu → Cu). This electron-doping sequence is in stark contrast to the typical -site electron doping for simple O-type perovskite and quadruple perovskites CaCuO ( = V, Cr, Mn), further differing from the monotonical '-site electron doping for NaLaMnTiO and '- and -site electron doping for MnVO ( = Na, Ca, La). The differences in the electron-doping sequences are interpreted by rigid-band models, proposing a wide variety of electronic states for the complex transition-metal oxides containing the multiple valence-variable ions.
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http://dx.doi.org/10.1021/acs.inorgchem.0c00184DOI Listing
July 2020

ϵ-FeOOH: A Novel Negative Electrode Material for Li- and Na-Ion Batteries.

ACS Omega 2020 May 20;5(17):10115-10122. Epub 2020 Apr 20.

Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, 1-2 Gakuen-cho, Sakai, Osaka 599-8570, Japan.

The demand for eco-friendly materials for secondary batteries has stimulated the exploration of a wide variety of Fe oxides, but their potential as electrode materials remains unknown. In this contribution, ϵ-FeOOH was synthesized using a high-pressure/high-temperature method and examined for the first time in nonaqueous Li and Na cells. Under a pressure of 8 GPa, α-FeOOH transformed into ϵ-FeOOH at 400 °C and then decomposed into α-FeO and HO above 500 °C. Here, FeO octahedra form [2 × 1] tunnels in α-FeOOH or [1 × 1] tunnels in ϵ-FeOOH. The ϵ-FeOOH/Li cell exhibited a rechargeable capacity ( ) of ∼700 mA h·g at 0.02-3.0 V, whereas the ϵ-FeOOH/Na cell indicated a of less than 30 mA h·g at 0.02-2.7 V. The discharge and charge profiles of ϵ-FeOOH and α-FeOOH were similar, but the rate capability of ϵ-FeOOH was superior to that of α-FeOOH.
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http://dx.doi.org/10.1021/acsomega.0c00728DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7203964PMC
May 2020

A robust thermal-energy-storage property associated with electronic phase transitions for quadruple perovskite oxides.

Chem Commun (Camb) 2020 May 15;56(41):5500-5503. Epub 2020 Apr 15.

Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, 1-2 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8570, Japan.

The quadruple perovskite oxides RCuFeO (R: rare-earth metals) exhibit large latent-heat capacities (25 J g at maximum) with variable transition temperatures between 254 and 368 K, whereas their transition entropies are almost completely retained. This finding proposes an effective way to design robust thermal-energy-storage materials with various operating temperatures.
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http://dx.doi.org/10.1039/d0cc01715aDOI Listing
May 2020

ZIF-Derived CoNiS Nanoparticles Immobilized on N-Doped Carbons as Efficient Catalysts for High-Performance Zinc-Air Batteries.

ACS Appl Mater Interfaces 2020 Feb 27;12(5):5847-5856. Epub 2020 Jan 27.

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

Bimetallic sulfides have been attracting considerable attention because of their high catalytic activities for oxygen reduction reaction (ORR) and oxygen evolution reaction; thus, they are considered efficient catalysts for important energy conversion devices such as fuel cells and metal-air batteries. Here, the catalytic activity of a novel catalyst composed of CoNiS nanoparticles immobilized on N-doped carbons (CoNiS/NC) is reported. The catalyst is synthesized using a Ni-adsorbed Co-Zn zeolitic imidazolate framework (ZIF) precursor (NiCoZn-ZIF). Because of the porous structure of ZIF and the high intrinsic activity of the bimetallic sulfide nanoparticles, the CoNiS/NC catalyst exhibits high half-wave potential 0.86 V versus reversible hydrogen electrode for ORR and outstanding bifunctional catalytic performance. When CoNiS/NC is applied as a cathode catalyst in zinc-air batteries, considerably higher power density of about 75 mW cm and discharge voltage are achieved compared to those of batteries with commercial Pt/C and other ZIF-derived catalysts. The zinc-air battery with the CoNiS/NC catalyst shows a high cyclability more than 170 cycles for 60 h with almost negligible decline at 10 mA cm. Our work provides a new insight into the design of bimetallic sulfide composites with high catalytic activities.
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http://dx.doi.org/10.1021/acsami.9b19268DOI Listing
February 2020

Structure, Magnetism, and Electrochemistry of LiMgZnVO Spinels with 0 ≤ ≤ 1.

Inorg Chem 2020 Jan 16;59(1):777-789. Epub 2019 Dec 16.

Department of Materials Science, Graduate School of Engineering , Osaka Prefecture University , 1-2 Gakuen-cho , Sakai , Osaka 599-8570 , Japan.

Negative electrode materials with lower operating voltages are urgently required to increase the energy density of lithium-ion batteries. In this study, LiMgVO with a NaCrO-type structure, LiZnVO with a phenacite structure, and their mixture were treated under a high pressure of 12 GPa and a high temperature of 1273 K, and their electrochemical reactivities were examined in a nonaqueous lithium cell. Synchrotron X-ray diffraction (XRD) measurements and Raman spectroscopy revealed that the LiMgZnVO samples with 0 ≤ ≤ 1 are in a single phase of the inverse spinel structure that forms a solid solution compound over the whole range. All of the samples were brown or light black due to the presence of a small amount of V ions with = 1/2 and oxygen deficiencies. Since the majority of the vanadium ions are located at the route of the Li ion conduction pathway, no rechargeable capacity () would be expected. Nevertheless, all LiMgZnVO samples exhibited a value of more than 200 mAh g with an operating voltage of ∼0.8 V. This operating voltage is ∼1.6 V lower than that of LiVO with a normal spinel structure. Furthermore, the = 0.5 sample demonstrated an extremely stable cycle performance over 1 month. Ex situ XRD measurements clarified that the reversible electrochemical reaction can be attributed to the movement of vanadium ions from the tetrahedral 8a to octahedral 16c sites during the initial discharge reaction. Details regarding the crystal structure, magnetism, and electrochemistry of LiMgZnVO are presented.
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http://dx.doi.org/10.1021/acs.inorgchem.9b03058DOI Listing
January 2020

Structural and Electrochemical Analyses on the Transformation of CaFeO-Type LiMnO from Spinel-Type LiMnO.

ACS Omega 2019 Apr 8;4(4):6459-6467. Epub 2019 Apr 8.

Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, 1-2 Gakuen, Sakai, Osaka 599-8570, Japan.

Lithium manganese oxides have received much attention as positive electrode materials for lithium-ion batteries. In this study, a post-spinel material, CaFeO-type LiMnO (CF-LMO), was synthesized at high pressures above 6 GPa, and its crystal structure and electrochemical properties were examined. CF-LMO exhibits a one-dimensional (1D) conduction pathway for Li ions, which is predicted to be superior to the three-dimensional conduction pathway for these ions. The stoichiometric LiMnO spinel (SP-LMO) was decomposed into three phases of LiMnO, MnO, and MnO at 600 °C and then started to transform into the CF-LMO structure above 800 °C. The rechargeable capacity ( ) of the sample synthesized at 1000 °C was limited to ∼40 mA h·g in the voltage range between 1.5 and 5.3 V because of the presence of a small amount of LiMnO phase in the sample (=9.1 wt %). In addition, the Li-rich spinels, Li[Li Mn ]O with = 0.1, 0.2, and 0.333, were also employed for the synthesis of CF-LMO. The sample prepared from = 0.2 exhibited a value exceeding 120 mA h·g with a stable cycling performance, despite the presence of large amounts of the phases LiMnO, MnO, and MnO. Details of the structural transformation from SP-LMO to CF-LMO and the effect of Mn ions on the 1D conduction pathway are discussed.
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http://dx.doi.org/10.1021/acsomega.9b00588DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6648083PMC
April 2019

Synthesis of Rhombohedral LiCoMnO Using a High-Pressure Method.

Inorg Chem 2019 May 8;58(10):6684-6695. Epub 2019 May 8.

Department of Materials Science, Graduate School of Engineering , Osaka Prefecture University , 1-2 Gakuen , Sakai , Osaka 599-8570 , Japan.

Lithium transition metal (M) oxides with a rhombohedral structure, r-LiMO, have attracted a great deal of attention as a positive electrode material for lithium-ion batteries. Despite intensive studies thus far, Mn-rich r-LiMO compounds have remained unattainable, due to a cooperative Jahn-Teller distortion of Mn ions in the MnO octahedra. We employed a high-pressure method for synthesizing r-LiCo MnO ( r-LCMO) with x = 0.5 and examined its electrochemical properties in a nonaqueous lithium cell. The high-pressure method successfully suppressed the Jahn-Teller distortion of Mn ions, and the r-LCMO phase was observed in a wide temperature-pressure region when using a LiOH·HO precursor. The rechargeable capacity of the sample synthesized at 600 °C and 12 GPa reached 126 mAh g, although the r-LCMO phase was contaminated with electrochemically inactive rock-salt LCMO and hexagonal LCMO phases. Compositional and structural analyses clarified that the actual Co/Mn ratio of the r-LCMO phase was 64/36, which deviated slightly from the initial composition (50/50). The high-pressure method was found to be effective for synthesizing Mn-rich r-LiMO compounds, although their electrochemical properties should be improved.
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http://dx.doi.org/10.1021/acs.inorgchem.9b00066DOI Listing
May 2019

Complementary evaluation of structure stability of perovskite oxides using bond-valence and density-functional-theory calculations.

Sci Technol Adv Mater 2018 19;19(1):101-107. Epub 2018 Feb 19.

NanoSquare Research Institute, Research Center for the 21st Century, Organization for Research Promotion, Osaka Prefecture University, Sakai, Japan.

Estimation of structure stability is an essential issue in materials design and synthesis. Global instability index () based on bond-valence method is applied as a simple indication, while density functional theory calculation is adopted for accurate evaluation of formation energy. We compare the and total energy of typical O-type perovskite oxides and rationalize their relationship, proposing that the criteria for empirically unstable structures ( > 0.2 valence unit) correspond to the difference in total energy of 50-200 meV per formula unit.
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http://dx.doi.org/10.1080/14686996.2018.1430449DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5827794PMC
February 2018

Novel catalytic properties of quadruple perovskites.

Authors:
Ikuya Yamada

Sci Technol Adv Mater 2017 27;18(1):541-548. Epub 2017 Jul 27.

Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, Sakai, Japan.

Quadruple perovskite oxides 'O demonstrate a rich variety of structural and electronic properties. A large number of constituent elements for /'/-site cations can be introduced using the ultra-high-pressure synthesis method. Development of novel functional materials consisting of earth-abundant elements plays a crucial role in current materials science. In this paper, functional properties, especially oxygen reaction catalysis, for quadruple perovskite oxides CaCuFeO and MnO ( = Ca, La) composed of earth-abundant elements are reviewed.
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http://dx.doi.org/10.1080/14686996.2017.1350557DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5613907PMC
July 2017

Perovskite-Type InCoO with Low-Spin Co: Effect of In-O Covalency on Structural Stabilization in Comparison with Rare-Earth Series.

Inorg Chem 2017 Sep 7;56(18):11113-11122. Epub 2017 Sep 7.

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

Perovskite rare-earth cobaltites ACoO (A = Sc, Y, La-Lu) have been of enduring interest for decades due to their unusual structural and physical properties associated with the spin-state transitions of low-spin Co ions. Herein, we have synthesized a non-rare-earth perovskite cobaltite, InCoO, at 15 GPa and 1400 °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 ScCoO with an improved cation stoichiometry in comparison to that in a previous study, where synthesis at 6 GPa and 1297 °C yielded a perovskite cobaltite with cation mixing on the A-site, (ScCo)CoO. 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 perovskites (A = In, Sc, Y, Pr-Lu) show that InCoO does not fulfill the general evolution of crystal metrics with A-site cation size, indicating that InCoO 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 InCoO is ascribed to the A-site cation displacements that accompany octahedral tilts; despite the highly tilted CoO network, the In-O covalency makes In 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 InCoO and ScCoO are diamagnetic with a low-spin state of Co below 300 K, in contrast to the case of (ScCo)CoO, where the high-spin Co 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.
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http://dx.doi.org/10.1021/acs.inorgchem.7b01426DOI Listing
September 2017

Covalency Competition in the Quadruple Perovskite CdCuFeO.

Inorg Chem 2017 Aug 19;56(15):9303-9310. Epub 2017 Jul 19.

Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST) , 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan.

Cadmium ions (Cd) are similar to calcium ions (Ca) in size, whereas the Cd ions tend to form covalent bonds with the neighboring anions because of the high electronegativity. The covalent Cd-O bonds affect other metal-oxygen bonds, inducing drastic changes in crystal structures and electronic states. Herein, we demonstrate high-pressure synthesis, crystal structure, and properties of a new quadruple perovskite CdCuFeO. This compound exhibits an electronic phase transition accompanying a charge disproportionation of Fe ions without charge ordering below ∼200 K, unlike charge-disproportionation transition with rock-salt-type charge ordering for CaCuFeO. First-principle calculations and Mössbauer spectroscopy display that covalent Cd-O bonds effectively suppress the Fe-O bond covalency, resulting in an electronic state different from that of CaCuFeO. This finding proposes covalency competition among constituent metal ions dominating electronic states of complex metal oxides.
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http://dx.doi.org/10.1021/acs.inorgchem.7b01405DOI Listing
August 2017

A-Site and B-Site Charge Orderings in an s-d Level Controlled Perovskite Oxide PbCoO.

J Am Chem Soc 2017 03 15;139(12):4574-4581. Epub 2017 Mar 15.

Laboratory for Materials and Structures, Tokyo Institute of Technology , 4259 Nagatsuta, Midori, Yokohama 226-8503, Japan.

Perovskite PbCoO synthesized at 12 GPa was found to have an unusual charge distribution of PbPbCoCoO with charge orderings in both the A and B sites of perovskite ABO. Comprehensive studies using density functional theory (DFT) calculation, electron diffraction (ED), synchrotron X-ray diffraction (SXRD), neutron powder diffraction (NPD), hard X-ray photoemission spectroscopy (HAXPES), soft X-ray absorption spectroscopy (XAS), and measurements of specific heat as well as magnetic and electrical properties provide evidence of lead ion and cobalt ion charge ordering leading to PbPbCoCoO quadruple perovskite structure. It is shown that the average valence distribution of PbCoO between PbCrO and PbNiO can be stabilized by tuning the energy levels of Pb 6s and transition metal 3d orbitals.
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http://dx.doi.org/10.1021/jacs.7b01851DOI Listing
March 2017

Bifunctional Oxygen Reaction Catalysis of Quadruple Manganese Perovskites.

Adv Mater 2017 Jan 25;29(4). Epub 2016 Nov 25.

Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, 143-8505, Japan.

Bifunctional electrocatalysts for oxygen evolution/reduction reaction (OER/ORR) are desirable for the development of energy conversion technologies. It is discovered that the manganese quadruple perovskites CaMn O and LaMn O show bifunctional catalysis in the OER/ORR. A possible origin of the high OER activity is the unique surface structure through corner-shared planar MnO and octahedral MnO units to promote direct OO bond formations.
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http://dx.doi.org/10.1002/adma.201603004DOI Listing
January 2017

Inverse Charge Transfer in the Quadruple Perovskite CaCu3Fe4O12.

Inorg Chem 2016 Feb 27;55(4):1715-9. Epub 2016 Jan 27.

Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University , 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan.

Structural and spectroscopic analyses revealed that the quadruple perovskite CaCu3Fe4O12 undergoes an "inverse" electron charge transfer in which valence electrons move from B-site Fe to A'-site Cu ions (∼3Cu(∼2.4+) + 4Fe(∼3.65+) → ∼3Cu(∼2.2+) + 4Fe(∼3.8+)) simultaneously with a charge disproportionation transition (4Fe(∼3.8+) → ∼2.4Fe(3+) + ∼1.6Fe(5+)), on cooling below 210 K. The direction of the charge transfer for CaCu3Fe4O12 is opposite to those reported for other perovskite oxides such as BiNiO3 and ACu3Fe4O12 (A = Sr(2+) or the large trivalent rare-earth metal ions), in which the electrons move from A/A'-site to B-site ions. This finding sheds a light on a new aspect in intermetallic phenomena for complex transition metal compounds.
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http://dx.doi.org/10.1021/acs.inorgchem.5b02623DOI Listing
February 2016

Covalency-reinforced oxygen evolution reaction catalyst.

Nat Commun 2015 Sep 10;6:8249. Epub 2015 Sep 10.

Department of Materials Science and Engineering, Osaka Prefecture University, Osaka 599-8531, Japan.

The oxygen evolution reaction that occurs during water oxidation is of considerable importance as an essential energy conversion reaction for rechargeable metal-air batteries and direct solar water splitting. Cost-efficient ABO3 perovskites have been studied extensively because of their high activity for the oxygen evolution reaction; however, they lack stability, and an effective solution to this problem has not yet been demonstrated. Here we report that the Fe(4+)-based quadruple perovskite CaCu3Fe4O12 has high activity, which is comparable to or exceeding those of state-of-the-art catalysts such as Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2)O(3-δ) and the gold standard RuO2. The covalent bonding network incorporating multiple Cu(2+) and Fe(4+) transition metal ions significantly enhances the structural stability of CaCu3Fe4O12, which is key to achieving highly active long-life catalysts.
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http://dx.doi.org/10.1038/ncomms9249DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4579779PMC
September 2015

Rattling in the Quadruple Perovskite CuCu3 V4 O12.

Angew Chem Int Ed Engl 2015 Sep 23;54(37):10870-4. Epub 2015 Jul 23.

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

Of particular interest is a peculiar motion of guest atoms or ions confined to nanospace in cage compounds, called rattling. While rattling provides unexplored physical properties through the guest-host interactions, it has only been observed in a very limited class of materials. Herein, we introduce an A-site-ordered quadruple perovskite, CuCu3 V4 O12 , as a new family of cage compounds. This novel AA'3 B4 O12 -type perovskite has been obtained by a high-pressure synthesis technique and structurally characterized to have cubic Im$\bar 3$ symmetry with an ionic model of Cu(2+) Cu(2+) 3 V(4+) 4 O12 . The thermal displacement parameter of the A-site Cu(2+) ion is as large as Uiso ≈0.045 Å(2) at 300 K, indicating its large-amplitude thermal oscillations in the oversized icosahedral cages. Remarkably, the presence of localized phonon modes associated with rattling of the A-site Cu(2+) ion manifests itself in the low-temperature specific heat data. This work sheds new light on the structure-property relations in perovskites.
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http://dx.doi.org/10.1002/anie.201504784DOI Listing
September 2015

Room-temperature polar ferromagnet ScFeO3 transformed from a high-pressure orthorhombic perovskite phase.

J Am Chem Soc 2014 Oct 21;136(43):15291-9. Epub 2014 Oct 21.

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

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 R3c 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 μC/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 Fe(3+) 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.
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http://dx.doi.org/10.1021/ja507958zDOI Listing
October 2014

Charge-order melting in charge-disproportionated perovskite CeCu3Fe4O12.

Inorg Chem 2014 Nov 21;53(21):11794-801. Epub 2014 Oct 21.

Nanoscience and Nanotechnology Research Center, Osaka Prefecture University , 1-2 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8570, Japan.

A novel quadruple perovskite oxide CeCu3Fe4O12 has been synthesized under high-pressure and high-temperature conditions of 15 GPa and 1473 K. (57)Fe Mössbauer spectroscopy displays a charge disproportionation transition of 4Fe(3.5+) → 3Fe(3+) + Fe(5+) below ∼270 K, whereas hard X-ray photoemission and soft X-ray absorption spectroscopy measurements confirm that the Ce and Cu valences are retained at approximately +4 and +2, respectively, over the entire temperature range measured. Electron and X-ray diffraction studies reveal that the body-centered cubic symmetry (space group Im3̅, No. 204) is retained at temperatures as low as 100 K, indicating the absence of any types of charge-ordering in the charge-disproportionated CeCu3Fe4O12 phase. The magnetic susceptibility and neutron powder diffraction data illustrate that the antiferromagnetic ordering of Fe ions is predominant in the charge-disproportionated CeCu3Fe4O12 phase. These findings suggest that CeCu3Fe4O12 undergoes a new type of electronic phase in the ACu3Fe4O12 series and that the melting of the charge-ordering in CeCu3Fe4O12 is caused by the substantial decrease in the Fe valence and the resulting large deviation from the ideal abundance ratio of Fe(3+):Fe(5+) = 1:1 for rock-salt-type charge-ordering.
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http://dx.doi.org/10.1021/ic502138vDOI Listing
November 2014

Valence transitions in negative thermal expansion material SrCu₃Fe₄O₁₂.

Inorg Chem 2014 Oct 11;53(19):10563-9. Epub 2014 Sep 11.

Nanoscience and Nanotechnology Research Center, Osaka Prefecture University , 1-2 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8570, Japan.

The valence states of a negative thermal expansion material, SrCu3Fe4O12, are investigated by X-ray absorption and (57)Fe Mössbauer spectroscopy. Spectroscopic analyses reveal that the appropriate ionic model of this compound at room temperature is Sr(2+)Cu(~2.4+)3Fe(~3.7+)4O12. The valence states continuously transform to Sr(2+)Cu(~2.8+)3Fe(~3.4+)4O12 upon cooling to ~200 K, followed by a charge disproportionation transition into the Sr(2+)Cu(~2.8+)3Fe(3+)(~3.2)Fe(5+)(~0.8)O12 valence state at ~4 K. These observations have established the charge-transfer mechanism in this compound, and the electronic phase transitions in SrCu3Fe4O12 can be distinguished from the first-order charge-transfer phase transitions (3Cu(2+) + 4Fe(3.75+) → 3Cu(3+) + 4Fe(3+)) in Ln(3+)Cu(2+)3Fe(3.75+)4O12 (Ln = trivalent lanthanide ions).
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http://dx.doi.org/10.1021/ic501665cDOI Listing
October 2014

High-pressure synthesis, crystal structure, and unusual valence state of novel perovskite oxide CaCu3Rh4O12.

Inorg Chem 2014 Jul 7;53(14):7089-91. Epub 2014 Jul 7.

Nanoscience and Nanotechnology Research Center, Osaka Prefecture University , 1-2 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8570, Japan.

A novel perovskite oxide, CaCu3Rh4O12, has been synthesized under high-pressure and high-temperature conditions (15 GPa and 1273 K). Rietveld refinement of synchrotron X-ray powder diffraction data indicates that this compound crystallizes in a cubic AA'3B4O12-type perovskite structure. Synchrotron X-ray absorption and photoemission spectroscopy measurements reveal that the Cu and Rh valences are nearly trivalent. The spectroscopic analysis based on calculations suggests that the appropriate ionic model of this compound is Ca(2+)Cu(∼2.8+)3Rh(∼3.4+)4O12, as opposed to the conventional Ca(2+)Cu(2+)3Rh(4+)4O12. The uncommon valence state of this compound is attributed to the relative energy levels of the Cu 3d and Rh 4d orbitals, in which the large crystal-field splitting energy of the Rh 4d orbitals is substantial.
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http://dx.doi.org/10.1021/ic501341xDOI Listing
July 2014

Room-temperature pressure-induced nanostructural CuInTe(2) thermoelectric material with low thermal conductivity.

Inorg Chem 2014 Jul 10;53(13):6844-9. Epub 2014 Jun 10.

Nanoscience and Nanotechnology Research Center, Research Organization for the 21st Century, Osaka Prefecture University , Gakuencho 1-2, Sakai 599-8570, Japan.

A room-temperature high-pressure synthesis method is proposed as an alternative way to induce nanoscale structural disorder in the bulk thermoelectric CuInTe2 matrix. This disorder stems from the coexistence of distinct domains with different degrees and geometries of disorder at Cu/In cation sites. The lattice thermal conductivity of high-pressure-treated CuInTe2 is substantially less than that of hot-pressed CuInTe2. The Debye-Callaway model reveals that the reduced lattice thermal conductivity is mainly attributed to disorder at the Cu/In cation sites and stacking faults, which were probably created during formation of the high-pressure-treated phases. This study demonstrates that room-temperature high-pressure synthesis can produce a radical change in the crystal structure and physical properties of conventional thermoelectric materials.
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http://dx.doi.org/10.1021/ic500688dDOI Listing
July 2014

AgCu3V4O12: a novel perovskite containing mixed-valence silver ions.

Inorg Chem 2013 Dec 5;52(24):13824-6. Epub 2013 Dec 5.

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

A novel silver-containing perovskite, AgCu3V4O12, was synthesized under high-pressure and high-temperature conditions. It crystallizes in an A-site-ordered perovskite structure (space group Im3), in which silver ions occupy the 12-coordinated A sites forming regular icosahedra, and exhibits metallic behavior. Bond-valence-sum 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 Ag(3+), 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.
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http://dx.doi.org/10.1021/ic402579vDOI Listing
December 2013

Control of bond-strain-induced electronic phase transitions in iron perovskites.

Inorg Chem 2013 Dec 13;52(23):13751-61. Epub 2013 Nov 13.

Nanoscience and Nanotechnology Research Center, Osaka Prefecture University , 1-2 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8570, Japan.

Unusual electronic phase transitions in the A-site ordered perovskites LnCu3Fe4O12 (Ln: trivalent lanthanide ion) are investigated. All LnCu3Fe4O12 compounds are in identical valence states of Ln(3+)Cu(2+)3Fe(3.75+)4O12 at high temperature. LnCu3Fe4O12 with larger Ln ions (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb) show an intersite charge transfer transition (3Cu(2+) + 4Fe(3.75+) → 3Cu(3+) + 4Fe(3+)) in which the transition temperature decreases from 360 to 240 K with decreasing Ln ion size. In contrast, LnCu3Fe4O12 with smaller Ln ions (Ln = Dy, Ho, Er, Tm Yb, Lu) transform into a charge-disproportionated (8Fe(3.75+) → 5Fe(3+) + 3Fe(5+)) and charge-ordered phase below ∼250-260 K. The former series exhibits metal-to-insulator, antiferromagnetic, and isostructural volume expansion transitions simultaneously with intersite charge transfer. The latter shows metal-to-semiconductor, ferrimagnetic, and structural phase transitions simultaneously with charge disproportionation. Bond valence calculation reveals that the metal-oxygen bond strains in these compounds are classified into two types: overbonding or compression stress (underbonding or tensile stress) in the Ln-O (Fe-O) bond is dominant in the former series, while the opposite stresses or bond strains are found in the latter. Intersite charge transfer transition temperatures are strongly dependent upon the global instability indices that represent the structural instability calculated from the bond valence sum, whereas the charge disproportionation occurs at almost identical temperatures, regardless of the magnitude of structural instability. These findings provide a new aspect of the structure-property relationship in transition metal oxides and enable precise control of electronic states by bond strains.
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http://dx.doi.org/10.1021/ic402344mDOI Listing
December 2013

A-site-ordered perovskite MnCu3V4O12 with a 12-coordinated manganese(II).

Inorg Chem 2013 Oct 12;52(19):11538-43. Epub 2013 Sep 12.

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

A novel cubic perovskite MnCu3V4O12 has been synthesized at a high pressure and high temperature of 12 GPa and 1373 K. This compound crystallizes in the A-site-ordered perovskite structure (space group Im3) with lattice constant a = 7.26684(10) Å at room temperature. The most notable feature of this compound lies in the fact that the Mn(2+) ion is surrounded by 12 equidistant oxide ions to form a regular icosahedron; the situation of Mn(2+) is unprecedented for the crystal chemistry of an oxide. An anomalously large atomic displacement parameter U(iso)= 0.0222(8) Å(2) is found for Mn(2+) at room temperature, indicating that the thermal oscillation of the small Mn(2+) ion in a large icosahedron is fairly active. Magnetic susceptibility and electric resistivity measurements reveal that 3d electrons of Mn(2+) ions are mainly localized, while 3d electrons in Cu(2+) and V(4+) ions are delocalized and contribute to the metallic conduction.
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http://dx.doi.org/10.1021/ic401855jDOI Listing
October 2013

Suppression of intersite charge transfer in charge-disproportionated perovskite YCu3Fe4O12.

J Am Chem Soc 2013 Apr 15;135(16):6100-6. Epub 2013 Apr 15.

Department of Chemistry, Graduate School of Science and Engineering, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan.

A novel iron perovskite YCu3Fe4O12 was synthesized under high pressure and high temperature of 15 GPa and 1273 K. Synchrotron X-ray and electron diffraction measurements have demonstrated that this compound crystallizes in the cubic AA'3B4O12-type perovskite structure (space group Im3, No. 204) with a lattice constant of a = 7.30764(10) Å at room temperature. YCu3Fe4O12 exhibits a charge disproportionation of 8Fe(3.75+) → 3Fe(5+) + 5Fe(3+), a ferrimagnetic ordering, and a metal-semiconductor-like transition simultaneously at 250 K, unlike the known isoelectronic compound LaCu3Fe4O12 that currently shows an intersite charge transfer of 3Cu(2+) + 4Fe(3.75+) → 3Cu(3+) + 4Fe(3+), an antiferromagnetic ordering, and a metal-insulator transition at 393 K. This finding suggests that intersite charge transfer is not the only way of relieving the instability of the Fe(3.75+) state in the A(3+)Cu(2+)3Fe(3.75+)4O12 perovskites. Crystal structure analysis reveals that bond strain, rather than the charge account of the A-site alone, which is enhanced by large A(3+) ions, play an important role in determining which of intersite charge transfer or charge disproportionation is practical.
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http://dx.doi.org/10.1021/ja312015jDOI Listing
April 2013

B-site deficiencies in A-site-ordered perovskite LaCu3Pt(3.75)O12.

Inorg Chem 2013 Apr 21;52(7):3985-9. Epub 2013 Mar 21.

Department of Chemistry, Graduate School of Science and Engineering, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan.

An A-site-ordered perovskite LaCu3Pt(3.75)O12 was synthesized by replacing Ca(2+) with La(3+) in a cubic quadruple AA'3B4O12-type perovskite CaCu3Pt4O12 under high-pressure and high-temperature of 15 GPa and 1100 °C. In LaCu3Pt(3.75)O12, 1/16 of B-site cations are vacant to achieve charge balance. The B-site deficiencies were evidenced by crystal structure refinement using synchrotron X-ray powder diffraction, hard X-ray photoemission spectroscopy, and soft X-ray absorption spectroscopy, leading to the ionic model La(3+)Cu(2+)3Pt(4+)(3.75)O(2-)12. Magnetic susceptibility data for this compound indicated a spin-glass-like behavior below T(g) = 3.7 K, which is attributed to disturbance of the antiferromagnetic superexchange interaction by the B-site deficiencies.
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http://dx.doi.org/10.1021/ic302809vDOI Listing
April 2013

Pd(2+)-incorporated perovskite CaPd3B4O12 (B = Ti, V).

Inorg Chem 2013 Feb 18;52(3):1604-9. Epub 2013 Jan 18.

Department of Chemistry, Graduate School of Science and Engineering, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan.

Novel A-site ordered perovskites CaPd(3)Ti(4)O(12) and CaPd(3)V(4)O(12) were synthesized under high-pressure and high-temperature of 15 GPa and 1000 °C. These compounds are the first example in which a crystallographic site in a perovskite-type structure is occupied by Pd(2+) ions with a 4d(8) low spin configuration. The ionic models for these compounds were determined to be Ca(2+)Pd(2+)(3)Ti(4+)(4)O(12) and Ca(2+)Pd(2+)(3)V(4+)(4)O(12) by structural refinement using synchrotron X-ray powder diffraction, hard X-ray photoemission, and soft X-ray absorption spectroscopy. Magnetic susceptibility, electrical resistivity, and specific heat measurements demonstrated diamagnetic insulating behavior for CaPd(3)Ti(4)O(12) in contrast to the Pauli-paramagnetic metallic nature of CaPd(3)V(4)O(12).
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http://dx.doi.org/10.1021/ic3025155DOI Listing
February 2013

Giant negative thermal expansion in the iron perovskite SrCu3Fe4O12.

Angew Chem Int Ed Engl 2011 Jul 6;50(29):6579-82. Epub 2011 Jun 6.

Department of Chemistry, Graduate School of Science and Engineering, Ehime University, Matsuyama, Ehime 790-8577, Japan.

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http://dx.doi.org/10.1002/anie.201102228DOI Listing
July 2011
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