Origin of the multiferroic-like properties of Er2CoMnO6

We report on the magnetoelectric properties of Er2CoMnO6. This compound adopts the structure of a double perovskite with a strong monoclinic distortion. Our specimen exhibits a nearly perfect Co-Mn order. It undergoes a ferromagnetic transition at TC~70 K due to the Co2+-O-Mn4+ ferromagnetic superexchange interaction. Below 30 K, the Er3+ moments start to order antiferromagnetically to the Co/Mn sublattice. Pyroelectric measurements reveal electrical polarization at low temperature but its strong dependence on the heating rate indicates the lack of a spontaneous ferroelectricity. Instead, electric polarization is derived from thermally stimulated depolarization currents

Local structure of Iridium organometallic catalysts covalently bonded to carbon nanotubes

Hybrid catalysts based on Iridium N-heterocyclic carbenes anchored to carbon nanotubes (CNT) have been studied by XAFS spectroscopy. Oxidation of CNT yields a large amount of functional groups, mainly hydroxyl groups at the walls and carboxylic groups at the tips, defects and edges. Different kinds of esterification reactions were performed to functionalize oxidized CNT with imidazolium salts. Then, the resulting products were reacted with an Ir organometallic compound to form hybrid catalysts efficient in hydrogen transfer processes. XANES spectroscopy agree with the presence of Ir(I) in these catalysts and the EXAFS spectra detected differences in the local structure of Ir atoms between the initial Ir organometallic compound and the Ir complexes anchored to the CNT. Our results confirm that the halide atom, present in the Ir precursor, was replaced by oxygen from -OH groups at the CNT wall in the first coordination shell of Ir. The lability of this group accounts for the good recyclability and the good efficiency shown by these hybrid catalysts

Structural origin for the dipole x-ray resonant scattering in the low temperature phase of Nd0.5Sr0.5MnO3 manganite

We have investigated the low temperature phase of a Nd0.5Sr0.5MnO3 single crystal by x-ray resonant scattering at the Mn K-edge of the (3 0 0), (0 3 0) and (0 5/2 0) reflections. Strong resonances were observed for the s-s' channel in the (3 0 0) and (0 3 0) reflections and for the s-p' channel in the (0 5/2 0) reflection. These resonances show a p-periodicity on the azimuthal angle, having the intensity at the minimum position almost zero. The intensity dependence on the photon energy, azimuthal angle and polarisation dependencies has been analysed using a semi-empirical structural model. Contrary to previous claims of charge (Mn3+-Mn4+) and orbital ordering in this compound, our results show that the dipole resonant superlattice reflections can be explained by the presence of two types of Mn sites with different local geometric structure. One of the Mn sites is surrounded by a tetragonal-distorted oxygen octahedron whereas the other site has a symmetric octahedral environment. This model also establishes that no real space charge ordering is needed to explain the experimental data. Intermediate valence states according to a fractional charge-segregation Mn+3.42-Mn+3.58 were deduced.Comment: 40 pages, 12 figures Presented at the ESRF 14th users meeting, February, 2004. Physical Review B (to be published

Comment on "X-ray resonant scattering studies of orbital and charge ordering in Pr1-xCaxMnO3"

In a recent published paper [Phys. Rev. B 64, 195133 (2001)], Zimmermann et al. present a systematic x-ray scattering study of charge and orbital ordering phenomena in the Pr1-xCaxMnO3 series with x= 0.25, 0.4 and 0.5. They propose that for Ca concentrations x=0.4 and 0.5, the appearance of (0, k+1/2, 0) reflections are originated by the orbital ordering of the eg electrons in the a-b plane while the (0, 2k+1, 0) reflections are due to the charge ordering among the Mn3+ and Mn4+ ions. Moreover, for small Ca concentrations (x<0.3), the orbital ordering is only considered and it occurs at (0, k, 0) reflections. A rigorous analysis of all these resonance reflections will show the inadequacy of the charge-orbital model proposed to explain the experimental results. In addition, this charge-orbital model is highly inconsistent with the electronic balance. On the contrary, these reflections can be easily understood as arising from the anisotropy of charge distribution induced by the presence of local distortions, i.e. due to a structural phase transition.Comment: 10 pages, 2 figures.To be published Phys. Rev.

The atypical iron-coordination geometry of cytochrome f remains unchanged upon binding to plastocyanin, as inferred by XAS

The transient complex between cytochrome f and plastocyanin from the cyanobacterium Nostoc sp. PCC 7119 has been analysed by X-ray Absorption Spectroscopy in solution, using both proteins in their oxidized and reduced states. Fe K-edge data mainly shows that the atypical metal coordination geometry of cytochrome f, in which the N-terminal amino acid acts as an axial ligand of the heme group, remains unaltered upon binding to its redox partner, plastocyanin. This fact suggests that cytochrome f provides a stable binding site for plastocyanin and minimizes the reorganization energy required in the transient complex formation, which could facilitate the electron transfer between the two redox partners

Determination of the charge-ordered phases in LuFe

High-resolution synchrotron powder diffraction and Fe K-edge x-ray resonant scattering (RXS) have been combined to determine both the magnitude and sequence of the Fe charge segregation in LuFe2O4. Two phases with different charge disproportionation were found below the so-called charge ordering transition temperature $T_{\mathrm{CO}}\approx 320\ \text{K}$ . The crystal structure between 320 K and 200 K shows a $C2/m$ symmetry where four non-equivalent iron sites with different valences were determined ($\text{Fe}^{2.77+}$ , $\text{Fe}^{2.63+}$ , $\text{Fe}^{2.36+}$ and $\text{Fe}^{2.26+}$ ). Below 200 K the structure further changes to the $P\bar{{1}}$ symmetry and six crystallographic sites for the iron atoms were found. The simplest valence distribution corresponds to a tri-modal one, where the six Fe atoms are grouped in couples with three valences $\text{Fe}^{2.8+}$ , $\text{Fe}^{2.5+}$ and $\text{Fe}^{2.2+}$ . Both polar and anti-polar charge sequences have been discarded

Structural and magnetic properties of Ca3Mn2-xRux O7 (0<x≤0.9)

We here report on the study of the crystallographic and magnetic properties of layered perovskites Ca3Mn2-xRuxO7 (x≤0.9). We observe a solid solution between Mn and Ru atoms in the whole series and all samples present the same orthorhombic structure independently of the Ru content. Different magnetic structures, depending on the Ru content in the sample, have been determined using neutron powder diffraction. For low Ru doping (x≤0.1), there is a dominant G-type antiferromagnetic ordering in the perovskite bilayers but, differently from undoped Ca3Mn2O7, the magnetic moments are located on the ab plane. For higher Ru concentration (x≥0.3), the predominant G-type ordering is preserved along the y axis while an A-type component is developed along the x axis and its intensity increases as Ru content does. This component is characterized by a ferromagnetic ordering in the a direction of one of the Mn(Ru)O6 layers, coupled antiferromagnetically with the neighbor Mn(Ru)O6 layer within the same bilayer. The study of the macroscopic magnetic properties shows that ferromagneticlike correlations are enhanced with increasing Ru content as deduced from the shift to higher temperature of the onset of the magnetic transition temperature. The magnetic transitions take place in two steps. At higher temperatures (140-200 K), short-range magnetic correlations are established. Tiny spontaneous magnetization is observed in the hysteresis loops with small coercive field. At TN≈115-125K, long-range antiferromagnetic ordering is developed. The ferromagnetic component remains with a strong increase of coercivity. We discuss in the paper the possible origins of this ferromagnetic contribution.The authors would like to acknowledge the use of Servicio General de Apoyo a la Investigacion from Universidad de Zaragoza. For financial support, we thank the Spanish Ministerio de Economia y Competitividad [Projects No. RTI2018098537-B-C22 and No. RTI2018-098537-B-C21, cofunded by ERDF from EU, the "Severo Ochoa" Programme (FUNFUTURE (CEX2019-000917) and Diputacion General de Aragon (Project No. E12-17R, RASMIA). We thank the ILL for the beamtime allocation under Experiment No. 5-31-2788.With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe