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In this work, we investigate the representative case of the homometallic Co ludwigite Co$^{2+}_2$Co$^{3+}$O$_2$BO$_3$ ($Pbam$ space group) with four distinct Co crystallographic sites [$M1$-$M4$] surrounded by oxygen octahedra. The mixed-valent character of the Co ions up to at least $T=873$ K is verified through x-ray absorption near-edge structure (XANES) experiments. Single crystal x-ray diffraction (XRD) and neutron powder diffraction (NPD) confirm that the Co ions at the $M4$ site are much smaller than the others at low temperatures, consistent with a Co$^{3+}$ oxidation state at $M4$ and Co$^{2+}$ at the remaining sites. The size difference between the Co ions in the $M4$ and $M2$ sites is continuously reduced upon warming above $\approx 370$ K, indicating a gradual charge redistribution within the $M4$-$M2$-$M4$ (424) ladder in the average structure. An increasing structural disorder, is noted above $\approx 370$ K, The local Co-O distance distribution, revealed by Co $K$-edge Extended X-Ray Absorption Fine Structure (EXAFS) data and analyzed with an evolutionary algorithm method, is similar to that inferred from the XRD crystal structure below $\approx 370$ K. At higher temperatures, the local Co-O distance distribution remains similar to that found at low temperatures, at variance with the average crystal structure obtained with XRD. We conclude that the oxidation states Co$^{2+}$ and Co$^{3+}$ are instantaneously well defined in a local atomic level at all temperatures, however the thermal energy promotes local defects in the charge-ordered configuration of the 424 ladders upon warming. These defects coalesce into a phase-segregated state within a narrow temperature interval ($475< T < 495$ K). Finally, a transition at $\approx 500$ K revealed by differential scanning calorimetry (DSC) in the iron ludwigite Fe$_3$O$_2$BO$_3$ is discussed., 22 pages, 19 figures

The synthesis of the double perovskite Sr2FeWO6 through the direct combustion technique is reported. A complete characterization of the crystalline structure is performed from Synchrotron X-ray measurements and subsequent Rietveld analysis. The results evidence that this material crystallizes in a monoclinic structure, P21/n space group, with strong distortions in the Fe-O6 and W-O6 octahedra, which is consistent with Glazer's notation a-b+a-. A superficial morphological study through scanning electron microscopy images reveals that this synthesis technique allows obtaining nanometric grains with 160 nm mean size. The semiquantitative compositional analysis through spectroscopy of X-ray energy dispersion by electrons shows that the produced material does not contain other chemical elements besides those expected form the stoichiometric formula. Results of analyzes of spectroscopy of photoelectrons emitted by X-rays suggest that there is 66.9% of the material with oxidation states Fe2+ and W6+ and 33.1% with oxidation states Fe3+ and W5+. This bivalence of Fe and W cations is corroborated by means the M\"ossbauer spectroscopy technique. Finally, the magnetic response in AC susceptibility curves allows us to affirm that at low temperatures the material adopts an antiferromagnetic behavior with TN=32 K, with a tendency to the ferromagnetic state above T=150 K mostly due to the presence of Fe3+ ions in the high spin configuration 3d5, S = 5/2., Comment: 26 pages, 10 figures, 2 tables

Static magnetic order of quasi two-dimensional FeAs compounds Sr4A2O6-xFe2As2, with A = Sc and V, has been detected by 57Fe Moessbauer and muon spin relaxation ({\mu}SR) spectroscopies. The non-superconducting stoichiometric (x = 0) A = Sc system exhibits a static internal/hyperfine magnetic field both at the 57Fe and {\mu}+ sites, indicating antiferromagnetic order of Fe moments below TN = 35 K with ~ 0.1 Bohr magneton per Fe at T = 2 K. The superconducting and oxygen deficient (x = 0.4) A = V system exhibits a static internal field only at the {\mu}+ site below TN ~ 40 K, indicating static magnetic order of V moments co-existing with superconductivity without freezing of Fe moments. These results suggest that the 42622 FeAs systems belong to the same paradigm with the 1111 and 122 FeAs systems with respect to magnetic behavior of Fe moments., Comment: 4 pages 4 figures: for information, contact tomo@lorentz.phys.columbia.edu