Monica Ceretti

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.

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

Lattice Dynamics To Trigger Low Temperature Oxygen Mobility in Solid Oxide Ion Conductors

SrFeO(2.5) and SrCoO(2.5) are able to intercalate oxygen in a reversible topotactic redox reaction already at room temperature to form the cubic perovskites Sr(Fe,Co)O(3), while CaFeO(2.5) can only be oxidized under extreme conditions. To explain this significant difference in low temperature oxygen mobility, we investigated the homologous SrFeO(2.5) and CaFeO(2.5) by temperature dependent oxygen isotope exchange as well as by inelastic neutron scattering (INS) studies, combined with ab initio (DFT) molecular dynamical calculations. From (18)O/(16)O isotope exchange experiments we proved free oxygen mobility to be realized in SrFeO(x) already below 600 K. We have also evidence that low temperature oxygen mobility relies on the existence of specific, low energy lattice modes, which trigger and amplify oxygen mobility in solids. We interpret the INS data together with the DFT-based molecular dynamical simulation results on SrFeO(2.5) and CaFeO(2.5) in terms of an enhanced, phonon-assisted, low temperature oxygen diffusion for SrFeO(3-x) as a result of the strongly reduced Fe-O-Fe bond strength of the apical oxygen atoms in the FeO(6) octahedra along the stacking axis. This dynamically triggered phenomenon leads to an easy migration of the oxide ions into the open vacancy channels and vice versa. The decisive impact of lattice dynamics, giving rise to structural instabilities in oxygen deficient perovskites, especially with brownmillerite-type structure, is demonstrated, opening new concepts for the design and tailoring of low temperature oxygen ion conductors.

Growth and characterization of large high quality brownmillerite CaFeO2.5 single crystals

The growth conditions of large and high quality single-crystals of CaFeO2.5 by the floating-zone technique in an image furnace are discussed. Structural characterization of the as grown single crystals have been carried out by neutron and X-ray diffraction as well as by HRTEM revealed the excellent quality in terms of composition homogeneity and crystalline quality. Magnetic measurements have been performed on oriented crystals by SQUID and neutron diffraction in the range of 5-700 K in order to clear up controversial discussions on possible magnetic phase transitions. The magnetic transitions reported elsewhere are discussed in terms of oxygen non-stoichiometries leading to multiphase CaFeO2.5+/-[small delta] related to the crystal growth conditions.

Anisotropy in the Raman scattering of a CaFeO2.5 single crystal and its link with oxygen ordering in Brownmillerite frameworks.

Periodic DFT calculations allow an understanding of the strong orientation-dependent Raman spectra of oriented CaFeO2.5 single crystals. Modes involving the oscillation of the apical oxygen (O(ap)) atoms perturb the induced electric dipoles. These are formed by anisotropy in the charge distribution and are found to be strongly enhanced when the electric field of the linearly polarized laser line is parallel to the b axis. For the CaFeO2.5 ordered system, strong polarizability of these modes corresponds to strong Raman intensities. Conversely, the apical oxygen disorder observed in low-temperature oxygen-conducting SrFeO2.5 destroys the long-range coherence of the respective Raman modes, which consequently show a strongly reduced intensity. This study provides a vibrational tool to discriminate between ordered and disordered isomorporphous ABO2.5 Brownmillerite frameworks. Furthermore, in combination with DFT calculations, we have found that the weakening of the interlayer interactions is responsible for the loss of ordering in Brownmillerite compounds.

Solid-state reactivity explored in situ by synchrotron radiation on single crystals: from SrFeO2.5 to SrFeO3 via electrochemical oxygen intercalation*

In this study we demonstrate the feasibility of following up a chemical reaction by single crystal x-ray (synchrotron) diffraction under operando conditions, carried out in a specially designed electrochemical cell mounted on the BM01A at the European Synchrotron Radiation Facility (ESRF). We investigated in detail the electrochemical oxidation of SrFeO2.5 to SrFeO3 on a spherical single crystal of 70 µm diameter by in situ diffraction at an ambient temperature. Complete data sets were obtained by scanning the whole reciprocal space using a 2M Pilatus detector, resulting in 3600 frames with a resolution of 0.1° per data set, each obtained in 18 min. The crystal was mounted in a specially designed electrochemical cell with 1N KOH used as the electrolyte. During the electrochemical oxidation, the reaction proceeds following the phase sequence SrFeO2.5/SrFeO2.75/SrFeO2.875/SrFeO3, structurally accompanied by establishing a complex series of long-range oxygen vacancy ordering, which gets instantly organized at ambient temperature. The topotactic reaction pathway is discussed in terms of the evolution of the twin domain structure. The formation of SrFeO2.875 is accompanied by the formation of diffuse streaks along the [1 0 0]-direction of the perovskite cell, reaching high d-spacings. The diffuse streaks are discussed and are thought to originate from a modified twin structure induced by the SrFeO2.75 to SrFeO2.875 transition, and the associated changes in the domain structure, developed during the oxygen intercalation. We equally analysed and discussed in detail the twin structure of all the title compounds. We confirm the ground state of SrFeO2.5 is able to adopt the Imma space group symmetry, showing stacking faults of the tetrahedral layers along the stacking axis of the brownmillerite unit cell, indicated by the 1D diffuse rods. We showed that in situ single crystal diffraction has huge potential in the study of non-stoichiometric compounds under operando conditions, in order to obtain structural information i.e. about diffuse scattering, and microstructural information related to domain effects such as twinning—information far beyond that which powder diffraction methods allow us to obtain.

Rietveld refinement of X-ray powder data and bond-valence calculations of NdSrNi0.5Cr0.5O4-δ compound

Compound from the solid-solution NdSrNi 1− x Cr x O 4−δ , 0≤ x ≤1, has been prepared using conventional solid-state method and was characterized by X-ray powder diffraction. The NdSrNi 0.5 Cr 0.5 O 4−δ sample shows the adoption of the K 2 NiF 4 -type structure based on the tolerance factor calculation. X-ray diffraction analysis using the Rietveld method was carried out and it was found that NdSrNi 0.5 Cr 0.5 O 4−δ compound crystallizes in tetragonal symmetry with space group I 4/ mmm . The lattice parameters are found to be at room temperature, a =3.8012(3) A and c =12.4812(1) A. For X-ray diffraction data, the reliability factors are R B =0.034, R wp =0.089, , and χ 2 =1.17. Bond-valence sum calculations were performed for nickel and chromium. The changes in unit-cell parameters are discussed in terms of oxygen stoichiometry and transition metal (3 d ) oxidation state from the perspective of the Brown bond-valence sum calculation theory.

Growth of high quality single crystals of strontium doped (Nd,Pr)-nickelates, Nd2−xSrxNiO4+δ and Pr2−xSrxNiO4+δ

Large size and high quality single crystals of Nd2−xSrxNiO4+δ and Pr2−xSrxNiO4+δ, with selected composition (x = 0.0, 0.1 and 0.5), were grown by a floating-zone technique using an image furnace. We found that even small deviations from the ideal cation stoichiometry led to either NiO segregation or formation of Ni-poor Pr4Ni3O10−x, or RExOy intergrowth phases, rendering the title compounds unstable even under ambient conditions. Related to potential applications as membranes in SOFC, we importantly found Nd2−xSrxNiO4+δ and Pr2−xSrxNiO4+δ powders, obtained from ground stoichiometric single crystals, to be stable in air at high temperature (1000 °C), contrary to the partial decomposition observed when synthesized as polycrystalline powders by classical solid state synthesis. The presence of NiO/Pr4Ni3O10−x or RExOy intergrowth phases are discussed as a drawback limiting high temperature stability for Pr2NiO4+δ. Grown single crystals were characterised by neutron and X-ray diffraction as well as by electron microscopy (SEM/EDS), revealing their excellent quality in terms of composition, homogeneity and crystallinity. For each crystal, the oxygen content was determined by thermogravimetric analysis in a reductive atmosphere. Transmission electron microscopy highlighted a complex incommensurate structure for Pr2NiO4.25 and Nd2NiO4.25 with a 2D modulation vector related to oxygen ordering.

(Nd/Pr)2NiO4+δ: Reaction Intermediates and Redox Behavior Explored by in Situ Neutron Powder Diffraction during Electrochemical Oxygen Intercalation

Oxygen intercalation/deintercalation in Pr2NiO4+δ and Nd2NiO4+δ was followed by in situ neutron powder diffraction during electrochemical oxidation/reduction, in a dedicated reaction cell at room temperature. For both systems three phases, all showing the same line width, were identified. The starting phases Pr2NiO4.23 and Nd2NiO4.24, considered with an average orthorhombic Fmmm symmetry, although both show a slight monoclinic distortion, get reduced in a two-phase reaction step to tetragonal intermediate phases with 0.07 ≤ δ ≤ 0.10 and P42/ ncm space group, which on further reduction transform, again in a two-phase reaction step, toward the respective stoichiometric (Pr/Nd)2NiO4.0 phases, with Bmab space group. Electrochemical oxidation does, however, not proceed fully reversibly for both cases: while the reoxidation of Nd2NiO4+δ is limited to the tetragonal intermediate phase with δ = 0.10, the homologous Pr2NiO4+δ can be reoxidized up to δ = 0.17, showing orthorhombic symmetry. For the intermediate tetragonal phase, we were able to establish for Pr2NiO4.09 a complex anharmonic displacement behavior of the apical oxygen atoms, as analyzed by single-crystal neutron diffraction and maximum entropy analysis, in agreement with a low- T diffusion pathway for oxygen ions, activated by lattice dynamics.

Cubic Sr2ScGaO5 Perovskite: Structural Stability, Oxygen Defect Structure, and Ion Conductivity Explored on Single Crystals

Oxygen-deficient Sr2ScGaO5 single crystals with a cubic perovskite structure were grown by the floating-zone technique. The transparent crystals of this pure 3D oxygen electrolyte are metastable at ambient temperature, showing one-sixth of all oxygen positions vacant. While neutron single-crystal diffraction, followed by maximum entropy analysis, revealed a strong anharmonic displacements for the oxygen atoms, a predominant formation of ScO6 octahedra and GaO4 tetrahedra is indicated by Raman spectroscopic studies, resulting in a complex oxygen defect structure with short-range order. Temperature-dependent X-ray powder diffraction (XPD) and neutron powder diffraction (NPD) studies reveal the cubic Sr2ScGaO5 to be thermodynamically stable only above 1400 °C, while the stable modification below this temperature shows the brownmillerite framework with orthorhombic symmetry. Cubic Sr2ScGaO5 remains surprisingly kinetically stable upon heating from ambient temperature to 1300 °C, indicating a huge inertia for the retransformation toward the thermodynamically stable brownmillerite phase. Ionic conductivity investigated by impedance spectroscopy was found to be 10-4 S/cm at 600 °C, while oxygen 18O/16O isotope exchange indicates a free oxygen mobility to set in at around 500 °C.