Naoaki Hayashi

Large-Scale Synthesis of Single-Crystalline Iron Oxide Magnetic Nanorings

We present an innovative approach to the production of single-crystal iron oxide nanorings employing a solution-based route. Single-crystal hematite (alpha-Fe2O3) nanorings were synthesized using a double anion-assisted hydrothermal method (involving phosphate and sulfate ions), which can be divided into two stages: (1) formation of capsule-shaped alpha-Fe2O3 nanoparticles and (2) preferential dissolution along the long dimension of the elongated nanoparticles (the c axis of alpha-Fe2O3) to form nanorings. The shape of the nanorings is mainly regulated by the adsorption of phosphate ions on faces parallel to c axis of alpha-Fe2O3 during the nanocrystal growth, and the hollow structure is given by the preferential dissolution of the alpha-Fe2O3 along the c axis due to the strong coordination of the sulfate ions. By varying the ratios of phosphate and sulfate ions to ferric ions, we were able to control the size, morphology, and surface architecture to produce a variety of three-dimensional hollow nanostructures. These can then be converted to magnetite (Fe3O4) and maghemite (gamma-Fe2O3) by a reduction or reduction-oxidation process while preserving the same morphology. The structures and magnetic properties of these single-crystal alpha-Fe2O3, Fe3O4, and gamma-Fe2O3 nanorings were characterized by various analytical techniques. Employing off-axis electron holography, we observed the classical single-vortex magnetic state in the thin magnetite nanorings, while the thicker rings displayed an intriguing three-dimensional magnetic configuration. This work provides an easily scaled-up method for preparing tailor-made iron oxide nanorings that could meet the demands of a variety of applications ranging from medicine to magnetoelectronics.

Infinite-layer iron oxide with a square-planar coordination

Conventional high-temperature reactions limit the control of coordination polyhedra in transition-metal oxides to those obtainable within the bounds of known coordination geometries for a given transition metal. For example, iron atoms are almost exclusively coordinated by three-dimensional polyhedra such as tetrahedra and octahedra. However, recent works have shown that binary metal hydrides act as reducing agents at low temperatures, allowing access to unprecedented structures. Here we show the reaction of a perovskite SrFeO3 with CaH2 to yield SrFeO2, a new compound bearing a square-planar oxygen coordination around Fe2+. SrFeO2 is isostructural with ‘infinite layer’ cupric oxides, and exhibits a magnetic order far above room temperature in spite of the two-dimensional structure, indicating strong in-layer magnetic interactions due to strong Fe d to O p hybridization. Surprisingly, SrFeO2 remains free from the structural instability that might well be expected at low temperatures owing to twofold orbital degeneracy in the Fe2+ ground state with D4h point symmetry. The reduction and the oxidation between SrFeO2 and SrFeO3 proceed via the brownmillerite-type intermediate SrFeO2.5, and start at the relatively low temperature of ∼400 K, making the material appealing for a variety of applications, including oxygen ion conduction, oxygen gas absorption and catalysis.

Temperature-induced A-B intersite charge transfer in an A-site-ordered LaCu 3 Fe 4 O 12 perovskite

Changes of valence states in transition-metal oxides often cause significant changes in their structural and physical properties. Chemical doping is the conventional way of modulating these valence states. In ABO3 perovskite and/or perovskite-like oxides, chemical doping at the A site can introduce holes or electrons at the B site, giving rise to exotic physical properties like high-transition-temperature superconductivity and colossal magnetoresistance. When valence-variable transition metals at two different atomic sites are involved simultaneously, we expect to be able to induce charge transfer—and, hence, valence changes—by using a small external stimulus rather than by introducing a doping element. Materials showing this type of charge transfer are very rare, however, and such externally induced valence changes have been observed only under extreme conditions like high pressure. Here we report unusual temperature-induced valence changes at the A and B sites in the A-site-ordered double perovskite LaCu3Fe4O12; the underlying intersite charge transfer is accompanied by considerable changes in the material’s structural, magnetic and transport properties. When cooled, the compound shows a first-order, reversible transition at 393 K from LaCu2+3Fe3.75+4O12 with Fe3.75+ ions at the B site to LaCu3+3Fe3+4O12 with rare Cu3+ ions at the A site. Intersite charge transfer between the A-site Cu and B-site Fe ions leads to paramagnetism-to-antiferromagnetism and metal-to-insulator isostructural phase transitions. What is more interesting in relation to technological applications is that this above-room-temperature transition is associated with a large negative thermal expansion.

BaFeO3: A Ferromagnetic Iron Oxide

Magnetic attraction: The cubic perovskite BaFeO(3) (see picture, Ba blue, Fe brown, O white), which is obtained by a low-temperature reaction using ozone as an oxidant, exhibits ferromagnetism with a fairly large moment of 3.5 μ(B) per Fe ion above a small critical field of approximately 0.3 T. This specific ferromagnetism is attributed to the enhancement of O→Fe charge transfer that arises from deepening of the Fe(4+) d levels.

Spin‐Ladder Iron Oxide: Sr3Fe2O5

14 SPIN-LADDER IRON OXIDE: Sr3Fe2O5 H. Kageyama, T. Watanabe, Y. Tsujimoto, A. Kitada, Y. Sumida, K. Kanamori, K. Yoshimura, N. Hayashi, S. Muranaka, M. Takano , M. Ceretti, W. Paulus, C. Ritter, G. Andre Department of Chemistry, Graduate School of Science, Kyoto University, Japan Graduate School of Human and Environmental Studies, Kyoto University, Japan 3 Institute for Chemical Research, Kyoto University, Uji, Japan 4 Institute for Integrated Cell-Materials Sciences and Research Institute for Production Development, Japan 5 University of Rennes1, Sciences Chimiques de Rennes UMR CNRS 6226, Campus de Beaulieu, Rennes 6 Institute Laue Langevin, BP 156, 38042, Grenoble, France 7 Laboratoire Leon Brillouin, CEA-CNRS Saclay, 91191, Gif-sur-Yvette, France

CaFeO2: a new type of layered structure with iron in a distorted square planar coordination.

CaFeO(2), a material exhibiting an unprecedented layered structure containing 3d(6) iron in a high-spin distorted square-planar coordination, is reported. The new phase, obtained through a low-temperature reduction procedure using calcium hydride, has been characterized through powder neutron diffraction, synchrotron X-ray diffraction, Mossbauer spectroscopy, XAS experiments as well as first-principles DFT calculations. The XAS spectra near the Fe-K edge for the whole solid solution (Sr(1-x)Ca(x))FeO(2) supports that iron is in a square-planar coordination for 0 </= x </= 0.8 but clearly suggests a change of coordination for x = 1. The new structure contains infinite FeO(2) layers in which the FeO(4) units unprecedentedly distort from square-planar toward tetrahedra and rotate along the c-axis, in marked contrast to the well-studied and accepted concept that octahedral rotation in perovskite oxides occurs but the octahedral shape is kept almost regular. The new phase exhibits high-spin configuration and G-type antiferromagnetic ordering as in SrFeO(2). However, the distortion of the FeO(2) layers leads to only a slight decrease of the Neel temperature with respect to SrFeO(2). First-principles DFT calculations provide a clear rationalization of the structural and physical observations for CaFeO(2) and highlight how the nature of the cation influences the structural details of the AFeO(2) family of compounds (A = Ca, Sr, Ba). On the basis of these calculations the driving force for the distortion of the FeO(2) layers in CaFeO(2) is discussed.

Fe-site substitution effect on the structural and magnetic properties in SrFeO2.

We investigated the Fe-site substitution effect on the structural and magnetic properties of the infinite layer iron oxide Sr(Fe(1-x)M(x))O(2) (M = Co, Mn) using synchrotron X-ray diffraction, neutron diffraction, and (57)Fe Mössbauer spectroscopy. Both systems have a similar solubility limit of x ≈ 0.3, retaining the ideal infinite layer structure with a space group of P4/mmm. For the Fe-Co system, both in-plane and out-of-plane axes decrease linearly and only slightly with x, reflecting the ionic radius difference between Fe(2+) and Co(2+). For the Fe-Mn system the lattice evolution also follows Vegards law but is anisotropic: the in-plane axis increases, while the out-of-plane decreases prominently. The magnetic properties are little influenced by Co substitution. On the contrary, Mn substitution drastically destabilizes the G-type magnetic order, featured by a significant reduction and a large distribution of the hyperfine field in the Mössbauer spectra, which suggests the presence of magnetic frustration induced presumably by a ferromagnetic out-of-plane Mn-Fe interaction.

Effects of oxygen gas pressure on structural, electrical, and thermoelectric properties of (ZnO)3In2O3 thin films deposited by rf magnetron sputtering

Zinc indium oxide films were deposited by the rf magnetron sputtering method using a (ZnO)3In2O3 target. The films were prepared at 573 K in various Ar/O2 sputtering gases (O2 content: 0%–25%). The effect of the oxygen gas content in the sputtering gas on the structural, optical, electrical, and thermoelectric properties of the films was investigated. The films had a c-axis oriented layer structure. The films deposited at 0%–3% oxygen gas contents exhibited a high electrical conductivity with a high carrier concentration, n≈1020 cm−3, while the conductivity of the films significantly decreased above the 3% oxygen gas content, having a carrier concentration below 1018 cm−3. From the optical transmission measurement, the band gap of the films was estimated to be 3.01 eV. The films deposited at 3%–8% oxygen gas contents showed a high Seebeck coefficient, −300 μV/K, while the maximum power factor, 4.78×10−5 W/m K2, was obtained at the 2% oxygen gas content. The Seebeck coefficient and the power factor were ca...

Bacterial nanometric amorphous Fe-based oxide: A potential lithium-ion battery anode material

Amorphous Fe(3+)-based oxide nanoparticles produced by Leptothrix ochracea, aquatic bacteria living worldwide, show a potential as an Fe(3+)/Fe(0) conversion anode material for lithium-ion batteries. The presence of minor components, Si and P, in the original nanoparticles leads to a specific electrode architecture with Fe-based electrochemical centers embedded in a Si, P-based amorphous matrix.

Suppression of temperature hysteresis in negative thermal expansion compound BiNi1−xFexO3 and zero-thermal expansion composite

Negative thermal expansion (NTE) of BiNi1−xFexO3 is investigated. All x = 0.05, 0.075, 0.10, and 0.15 samples shows large NTE with the coefficient of linear thermal expansion (CTE) αL exceeding −150 ppm K−1 induced by charge transfer between Bi5+ and Ni2+ in the controlled temperature range near room temperature. Compared with Bi1−xLnxNiO3 (Ln: rare-earth elements), the thermal hysteresis that causes a problem for practical application is suppressed because random distribution of Fe in the Ni site changes the first order transition to second order-like transition. The CTE of BiNi0.85Fe0.15O3 reaches −187 ppm K−1 and it is demonstrated that 18 vol. % addition of the present compound compensates for the thermal expansion of epoxy resin.