C. Ritter

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.

Large low-field magnetoresistance and TC in polycrystalline (Ba0.8Sr0.2)2−xLaxFeMoO6 double perovskites

Large low-field magnetoresistance (LFMR) together with high Curie temperatures (TC) are requirements for some applications in magnetoelectronics. In order to optimize both parameters, we have investigated double perovskites (Ba0.8Sr0.2)2−xLaxFeMoO6 (0⩽x⩽0.4). High-temperature neutron diffraction measurements indicate a strong increase in TC with La doping (from ≈345 K for x=0 to ≈405 K for x=0.4). The LFMR is very large for x=0 (at 10 KOe≈27% at 10 K and ≈7% at 290 K) and decreases with La doping. This decrease cannot be attributed to a substantial enhancement of Fe/Mo antisite disorder, which is small as tracked by means of x-ray and high-resolution neutron diffraction, but to grain boundaries modifications.

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

Neutron diffraction studies of the magnetic phase transitions in Ce2Fe17 compound under pressure

The influence of hydrostatic pressure (up to 5 kbar) on the magnetic structure of Ce2Fe17 was investigated using neutron diffraction in the temperature range from 2 to 300 K. The existence of a collinear ferromagnetic phase below 95 K with a magnetic moment of Fe, mFe=2.0 μB, was confirmed at ambient pressure. Magnetic peaks present between 95 and 205 K correspond to an incommensurate antiferromagnetic structure with a wave vector changing its value from τ1=0.026 A−1 at 100 K to τ1=0.034 A−1 at 205 K. A helical model is used to describe the magnetic structure. Application of high pressures leads to significant changes of the magnetic structure. The ferromagnetic phase, suppressed in the studied temperature range by pressures higher than 3 kbar, gets substituted by a new incommensurate antiferromagnetic phase. This phase can be described as a superposition of the helical structure with a second antiferromagnetic coupling with propagation vector τ2≈0.078 A−1 at 40 K under pressures above 3 kbar. The correla...

Magnetocaloric effect in Tb5(SixGe1−x)4

The magnetocaloric effect has been investigated in a series of the R5(SixGe1−x)4 pseudobinary alloys with R=Tb and x=0.0, 0.5, and 1.0. In Tb5Si4 and Tb5Ge4, a maximum magnetic entropy change of 9.8 and 3.7 J/kg K, respectively, has been found at their ordering temperatures for a magnetic field change of 50 kOe. In the case of Tb5Ge4, we have observed the existence of a previously unreported magnetic transition below the Neel temperature. Tb5(Si0.5Ge0.5)4 has been successfully synthesized, and a large entropy change of 21.8 J/kg K (ΔH=50 kOe) is achieved at a first-order para- to ferromagnetic transition, TC≅105 K. This study demonstrates that Tb5(SixGe1−x)4 alloys are good candidates for magnetic refrigeration and that very exciting phenomenology in these systems awaits further investigation.

Structural, magnetic and transport properties of Sr2Fe1−xCrxMoO6−y

Abstract The series of Sr 2 Fe 1− x Cr x MoO 6− y has been synthesized and studied by means of a wide set of experimental techniques. These samples belong to the family of A 2 BB′O 6 double perovskites with a ferrimagnetic ground state. The replacement of Fe by Cr leads to samples with a high oxygen deficiency and to increasing cation disorder between B and B′ sites. The magnetic moments of these samples decrease as the iron content does. Electrical properties change as well strongly along the series: while Sr 2 FeMoO 6 is metallic, the rest of the samples show a semiconducting behavior with an activation energy that increases as the content of Cr does. Only Fe-rich samples exhibit large negative magnetoresistance with the low-field response characteristic of a half-metallic system. The Cr-rich samples display instead a small linear negative magnetoresistance.

Oxygen diffusion mechanism in the mixed ion-electron conductor NdBaCo2O5+x

Double perovskite cobaltites were recently presented as promising cathode materials for solid oxide fuel cells. While an atomistic mechanism was proposed for oxygen diffusion in this family of materials, no direct experimental proof has been presented so far. We report here the first study that directly compares experimental and theoretical diffusion pathways of oxygen in an oxide, namely in the double cobaltite compound, NdBaCo2O5+x. Model-free experimental nuclear density maps are obtained from the maximum entropy method combined with Rietveld refinement against high resolution neutron diffraction data collected at 1173 K. They are then compared to theoretical maps resulting from classical molecular dynamics calculations. The analysis of 3D maps of atomic densities allows identifying unambiguously the pathways and the mechanisms involved in the oxide ion diffusion. It is shown that oxygen diffusion occurs along a complex trajectory between Nd- and Co-containing a,b planes. The study also reveals that Ba-containing planes act as a barrier for oxygen diffusion. The diffusion mechanism is also supported through the oxygen sites occupancy analysis that confirms the increase of oxygen vacancies in the cobalt-planes on heating. The use of such combined experimental and theoretical analysis should be considered as a very powerful approach for materials design.

Enhancement of ferromagnetic correlations on multiferroic TbMnO3 by replacing Mn with Co

The structural, electronic and magnetic properties of TbMn(1-x)Co(x)O(3) (0.1 ≤ x ≤ 0.9) compounds are reported. The samples are isostructural to TbMnO(3) adopting the orthorhombic distorted perovskite structure (Pbnm), except for x = 0.4, 0.5 and 0.6, where an ordered double perovskite structure (P2(1)/n) is found. X-ray absorption spectra at the Mn and Co K edges show an incomplete charge transfer between Mn and Co atoms yielding a mixed valence state Mn(3+)/Mn(4+) and Co(3+)/Co(2+) for the whole series. Neutron powder diffraction measurements show the development of a ferromagnetic ground state for the intermediate compositions (0.3 ≤ x ≤ 0.6) indicating that the ferromagnetic superexchange Mn(4+)-O-Co(2+) interaction is the strongest among a wide set of competitive interactions. The ferromagnetic ordering is, however, not fully achieved and coexists with glassy magnetic properties. With increasing concentration of Co (x ≥ 0.7) the long range ordering vanishes and only a glassy magnetic behavior with slow dynamics is found. These properties could be related to the existence of magnetically inhomogeneous small clusters arising from competitive magnetic interactions.

Phase Diagram of NdFeAsO1−xFx: Essential Role of Chemical Composition

In this Article, we provided the complete phase diagram of NdFeAsO(1-x)F(x) solid solution as a function of electron doping thanks to the careful determination of F- and O-content and phase content. We gave direct evidence of the source of F-depletion in the superconducting main phase, that is, the formation of oxyfluoride spurious phase. The approach reported in this work clearly showed that to give reliable results on these complex new superconducting materials, a rigorous control of the chemical composition of the considered phases has to be carried out.

High-field magnetization measurements in Sr2CrReO6 double perovskite: Evidence for orbital contribution to the magnetization

We have synthesized a Sr2CrReO6 double perovskite sample with the expected high Curie temperature ≈ 600 K, which shows negligible impurity phases and ≈ 15% Cr/Re structural antisite disorder as shown by X-ray and neutron diffraction. High-field magnetization measurements up to 47 T show that the saturation magnetization below room temperature is much higher than the one predicted by simple models that do not take into account the Re spin-orbit coupling. The crucial role of the large Re orbital moment in the physics of Sr2CrReO6 and related compound is highlighted.