Gloria Subías

The Verwey transition—a new perspective

This review puts in doubt the classical description of the Verwey (metal?insulator) transition in magnetite on the basis of the wide set of experiments carried out over the last 60 years. We re-analyse here the most relevant experiments used to study the Verwey transition from the point of view of their degree of agreement with the proposed Fe2+?Fe3+ charge ordering model. We will consider three groups of experimental studies, according to their capability of detecting different ionic species and/or a charge periodicity: (1)?Experiments which have been interpreted using the charge ordering model as the starting point though they are not able to demonstrate its validity. This is the case for macroscopic properties such as the electrical resistivity, the heat capacity and the magnetic properties. (2) Experiments which can distinguish different types of Fe ions, such as M?ssbauer, nuclear magnetic resonance (NMR) and electronic spectroscopies. However, we show that they are not able to associate them with a specific valence (2+ or 3+ in our case) and, in some cases, they observe more than two different kinds of iron atoms. (3) Diffraction (x-ray, neutron and electron) experiments, which are the most conclusive ones for determining a periodic ordering of different entities. These experiments, instead, point to the lack of ionic charge ordering. We will focus, in particular, on the discussion of the results of some recent x-ray resonant scattering experiments carried out on magnetite that directly prove the lack of ionic charge ordering in such mixed valence oxide. Furthermore, we also reconsider some so-called Verwey-type transition metal oxides in terms of the applicability of the Verwey charge ordering model. We show that a complete charge disproportionation (?) is not experimentally observed in any of these compounds, the maximum ? being less than 0.5?e?. Regarding the theoretical framework, we will outline some relevant implications for the description of the physics of 3d transition metal oxides of this critical re-examination of the experimental facts on magnetite. Electronic localization should then occur involving more than one transition metal atom, so the definition of ionic d?states loses its meaning in mixed valence transition metal oxides.

High resolution x-ray absorption near edge structure at the Mn K edge of manganites

We report a systematic study of manganese perovskites, RE1-xDxMnO3 (RE = rare earth; D = Ca, Sr), by means of x-ray absorption near edge structure (XANES) spectroscopy at the Mn K edge. High resolution has been achieved by recording the intensity of the Mn Kβ fluorescence spectrum. All the samples show a unique resonance at the Mn K edge. This common feature does not depend on small changes in the local structure around the Mn atom. The XANES spectra of mixed (0<x<1) composition compounds cannot be obtained by linear combination of REMnO3 and DMnO3 spectra. Therefore, Mn3+ and Mn4+ ions cannot be distinguished in intermediate RE1-xDxMnO3 compounds, either spatially or temporally. The possibility of a charge disproportionation is also considered and discussed in the paper.

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.

Analysis of the x-ray resonant scattering at the Mn K edge in half-doped mixed valence manganites

We reconsider the interpretation of x-ray resonant scattering experiments performed on the so-called charge-ordered manganites with Mn3+/Mn4+ = 1. The comparison between these experiments and our x-ray absorption data shows that no real Mn3+/Mn4+ charge ordering occurs in these compounds. However, these experiments demonstrate the presence of two different types of manganese atom with a different local geometrical structure. We propose a structural model which accounts for these resonant scattering experiments. Within this model, the charge-ordering phase transition can be explained as a structural phase transition driven by the softening of a phonon mode. This produces a periodic arrangement of local distortions, which is responsible for the observed resonances. Furthermore, electronic localization in these materials would occur on a length scale larger than the atomic one.

Stability of the tetragonal Mn 3 + O 6 distortions in the LaMn 1 − x Ga x O 3 series by x-ray absorption spectroscopy

The

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

{\mathrm{LaMn}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}{\mathrm{O}}_{3}

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

series has been studied by means of x-ray absorption spectroscopy at both Mn and Ga

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

K

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

edges. A homovalent substitution between

An x-ray spectroscopic study of A2FeMoO6 and Sr2Fe1−xCrxMoO6 double perovskites

{\mathrm{Mn}}^{3+}