Symmetry, magnetic transitions and multiferroic properties of B-site-ordered AMnB'O perovskites (B' = [Co, Ni]).

The presence of magnetic atoms at the A and B sites and the coupling between these two spin subsystems in perovskites gives rise to a variety of exciting effects. In particular this coupling attracts interest from the field of novel multiferroic and magnetoelectric oxides. Moreover, magnetic double perovskites presenting cationic order at the B sites incorporate an additional modulation that can favor symmetry breaking, multiferroic, magnetoelectric and polar phases.

Mechanism Behind the Enhanced Ferromagnetic Contributions Upon Ru Substitution in CaMnO from X-Ray Absorption Spectroscopies.

We have studied the local structure and electronic and magnetic properties of hybrid improper ferroelectric CaMnO upon Ru substitution at the Mn site by a combination of atomic-selective X-ray absorption spectroscopies in the soft and hard X-ray energy regimes. Ru substitution enhances the macroscopic ferromagnetic contributions, whose origin is here elucidated. In particular, soft X-ray magnetic circular dichroism (XMCD) data indicate that the spin moments of Mn and Ru are aligned in opposite directions, with the effective magnetic moments of Ru being about 1 order of magnitude smaller than for Mn.

Identifying the presence of magnetite in an ensemble of iron-oxide nanoparticles: a comparative neutron diffraction study between bulk and nanoscale.

Scientific interest in iron-oxides and in particular magnetite has been renewed due to the broad scope of their fascinating properties, which are finding applications in electronics and biomedicine. Specifically, iron oxide nanoparticles (IONPs) are gathering attraction in biomedicine. Their cores are usually constituted by a mixture of maghemite and magnetite phases.

Relation among Oxygen Stoichiometry, Structure, and Co Valence and Spin State in Single-Layer LaACoO (A = Ca, Sr) Perovskites.

We have investigated the role of oxygen stoichiometry and structural properties in the modulation of Co valence and spin state in single-layer LaACoO (A = Sr, Ca; 0 ≤ ≤ 1) perovskites as well as the interplay between their local structural properties and the magnetic and charge-ordering phenomena. We show the results of high angular resolution powder X-ray diffraction and Co -edge X-ray absorption and emission spectroscopy experiments on polycrystalline and single-crystal samples. The different doping-induced changes in the Co valence and spin state by Ca (or Sr) substitution can be understood in terms of the evolving oxygen stoichiometry.

Structural, Magnetic, and Electronic Properties of CaBaCoM O (M = Fe, Zn).

The effect of substituting iron and zinc for cobalt in CaBaCoO was investigated using neutron diffraction and X-ray absorption spectroscopy techniques. The orthorhombic distortion present in the parent compound CaBaCoO decreases with increasing the content of either Fe or Zn. The samples CaBaCoZnO and CaBaCoFe O with x ≥ 1.

Effects of A-site disorder in the properties of A2CoMnO6 (A = La, Tb).

We have studied the structural and physical properties of the La2-xTbxCoMnO6 series. The crystal and magnetic structures of these compounds were determined by x-ray and neutron diffraction techniques. All samples belong to the family of double perovskites with space group P21/n, but the Co/Mn ordering is not perfect, and antisite defects are formed.

Evolution of magnetoelectric properties of Sc-diluted TbMnO3.

The magnetoelectric properties of the TbMn(1-x)Sc(x)O3 series have been studied at low temperatures by means of heat capacity, magnetic measurements and impedance spectroscopy. TbMnO3 exhibits as expected three transitions upon lowering the temperature corresponding to the magnetic ordering of the two sublattices (Mn and Tb) and the ferroelectric transition. Ferroelectricity disappears with Sc dilution for x > 0.

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.

Incommensurate sinusoidal oxygen modulations in layered manganites La(1-x)Sr(1+x)MnO4 (x≥0.5).

We have studied the incommensurate-ordered phase in overdoped La0.4Sr1.6MnO4 by resonant x-ray diffraction at the Mn K edge.

Structural distortion, charge modulation and local anisotropies in magnetite below the Verwey transition using resonant X-ray scattering.

The pattern of charge modulations and local anisotropies below the Verwey transition has been determined and quantified in high-quality Fe(3)O(4) single crystals and thin films grown on MgO by using resonant X-ray scattering at the Fe K-edge. The energy, polarization and azimuthal angle dependencies of an extensive set of reflections with potential sensitivity to charge or local anisotropy orderings have been analyzed to explore their origins. A charge disproportion on octahedral B sites of 0.

Structural properties of Pb2MnW(1-x)Re(x)O6 double perovskites.

Pb2MnW(1-x)Re(x)O6 samples have been synthesized and their structure determined by powder x-ray diffraction. These samples undergo a first order structural phase transition between 413 and 445 K depending on the composition. Above this temperature, the samples are cubic.

Origin of the pre-peak features in the oxygen K-edge x-ray absorption spectra of LaFeO₃ and LaMnO₃ studied by Ga substitution of the transition metal ion.

We report on experimental oxygen K-edge x-ray absorption near edge structure (XANES) spectra of the LaFe(1 - x)Ga(x)O(3) and LaMn(1 - x)Ga(x)O(3) series. Transition metal substitution by the 3d full shell Ga atom is mainly reflected in a systematic decrease of the pre-edge structures in the XANES spectra of the two series. This result shows that the associated states originate from the hybridization of oxygen 2p and unoccupied Fe (or Mn) 3d states.

A new type of antiferrodistortive structure in a double perovskite with Pb.

We have found a new structural transition in Pb(2)MnReO(6) at 410 K. Above this temperature, Pb(2)MnReO(6) is cubic with disordered and dynamic atomic displacements manifested in the large thermal parameters of Pb and O atoms. Below 410 K, the antiferrodistortive shift of 2/3 of Pb(2+) cations away from the high-symmetry cubic site produces a new type of monoclinic cell.

On the correlation between the X-ray absorption chemical shift and the formal valence state in mixed-valence manganites.

Here the correlation between the chemical shift in X-ray absorption spectroscopy, the geometrical structure and the formal valence state of the Mn atom in mixed-valence manganites are discussed. It is shown that this empirical correlation can be reliably used to determine the formal valence of Mn, using either X-ray absorption spectroscopy or resonant X-ray scattering techniques. The difficulties in obtaining a reliable comparison between experimental XANES spectra and theoretical simulations on an absolute energy scale are revealed.

Modulation of copper site properties by remote residues determines the stability of plastocyanins.

The metal cofactor determines the thermal stability in cupredoxins, but how the redox state of copper modulates their melting points remains unknown. The metal coordination environment is highly conserved in cyanobacterial plastocyanins. However, the oxidised form is more stable than the reduced one in thermophilic Phormidium, but the opposite occurs in mesophilic Synechocystis.

Reexamination of the temperature dependences of resonant reflections in highly stoichiometric magnetite.

We have studied the temperature dependences of the so-called charge and orbital ordering reflections in the neighborhood of the Verwey transition by means of resonant x-ray scattering at the Fe K and L edges on a high purity single crystal of magnetite. Contrary to recently published results [J. E.

Order-disorder nature of the antiferroelectric transition in Pb(2)MnWO(6).

The nature of the para-antiferroelectric phase transition in Pb(2)MnWO(6) was investigated by x-ray absorption spectroscopy. Extended x-ray absorption spectra (EXAFS) and x-ray absorption near-edge structure (XANES) at the Pb L(3)-edge, W L(3)-edge and Mn K-edge were measured at temperatures above and below the transition temperature, T(c)∼445 K. The temperature dependence of the local geometrical and electronic structure around each of the three atoms was quantitatively determined.

The checkerboard pattern of Bi(0.63)Sr(0.37)MnO(3) determined using resonant x-ray scattering at the Mn K edge.

A study using resonant x-ray scattering at the Mn K edge has been carried out on a Bi(0.63)Sr(0.37)MnO(3) single crystal.

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.

Transient binding of plastocyanin to its physiological redox partners modifies the copper site geometry.

The transient complexes of plastocyanin with cytochrome f and photosystem I are herein used as excellent model systems to investigate how the metal sites adapt to the changes in the protein matrix in transient complexes that are involved in redox reactions. Thus, both complexes from the cyanobacterium Nostoc sp. PCC 7119 (former Anabaena sp.

Electronic structure of Ag2Cu2O4. Evidence of oxidized silver and copper and internal charge delocalization.

The electronic structure of the recently isolated silver copper oxide Ag(2)Cu(2)O(4) is analyzed along with its precursor Ag(2)Cu(2)O(3) and similar binary oxides, Ag(2)O, AgO, CuO, and NaCuO(2), using X-ray photoemission (XPS) and X-ray absorption (XAS) measurements. The results for Ag(2)Cu(2)O(4) reveal an electronic distribution in which silver and copper share a delocalized valence scheme with both metals in formal oxidation states larger than the usual Ag(I) and Cu(II). Only one type of crystallographic silver or copper is found, but disorder-strain parameters are considerable and the possibilities of thermal disorder, atomic motion, oxygen contribution, mixed valence, and internal charge delocalization are considered.

Detecting transient protein-protein interactions by X-ray absorption spectroscopy: The cytochrome c6-photosystem I complex.

Reliable analysis of the functionality of metalloproteins demands a highly accurate description of both the redox state and geometry of the metal centre, not only in the isolated metalloprotein but also in the transient complex with its target. Here, we demonstrate that the transient interaction between soluble cytochrome c(6) and membrane-embedded photosystem I involves subtle changes in the heme iron, as inferred by X-ray absorption spectroscopy (XAS). A slight shift to lower energies of the absorption edge of Fe2+ in cytochrome c6 is observed upon interaction with photosystem I.

Magnetite, a model system for mixed-valence oxides, does not show charge ordering.

We have investigated the charge ordering (CO) in magnetite below the Verwey transition. A new set of half-integer and mixed-integer superlattice reflections of the low-temperature phase have been studied by x-ray resonant scattering. None of these reflections show features characteristic of CO.

Cooperative Jahn-Teller phase transition in LaMnO3 studied by X-ray absorption spectroscopy.

The local structure of LaMnO3 across the Jahn-Teller (JT) transition at T(JT)=750 K was studied by means of x-ray absorption near edge structure and extended x-ray absorption fine structure at the Mn K-edge. Our results indicate a similar electronic local structure for Mn atoms above and below T(JT) and a dynamical tetragonal JT distortion of MnO6 octahedra above T(JT). The structural transition is originated by the ordering of tetragonally distorted octahedra.