Search

Four oxyfluorides of the title series (x = 0.00, 0.25, 0.50, 0.75) have been stabilized by topotactic treatment of perovskite precursors Sr1−xBaxFeO3−δ prepared by soft-chemistry procedures, yielding reactive materials that can easily incorporate a substantial amount of F atoms at moderate temperatures, thus avoiding the stabilization of competitive SrF2 and BaF2 parasitic phases. XRD and Neutron Powder Diffraction (NPD) measurements assess the phase purity and yield distinct features concerning the unit cell parameters’ variation, the Sr and Ba distribution, the stoichiometry of the anionic sublattice and the anisotropic displacement factors for O and F atoms. The four oxyfluorides are confirmed to be cubic in all of the compositional range, the unit cell parameters displaying Vergard’s law. All of the samples are magnetically ordered above room temperature; the magnetic structure is always G-type antiferromagnetic, as shown from NPD data. The ordered magnetic moments are substantially high, around 3.5 μB, even at room temperature (RT). Temperature-dependent Mössbauer data allow identifying Fe3+ in all of the samples, thus confirming the Sr1−xBaxFeO2F stoichiometry. The fit of the magnetic hyperfine field vs. temperature curve yields magnetic ordering TN temperatures between 740 K (x = 0.00) and 683 K (x = 0.75). These temperatures are substantially higher than those reported before for some of the samples, assessing for stronger Fe-Fe superexchange interactions for these specimens prepared by fluorination of citrate precursors in mild conditions.

We have studied the crystallographic structure, the oxidation states and the electronic structure of the ternary metal hydride Mg2FeH6 prepared by high-pressure and high-temperature reaction from the simple hydrides. This procedure provides a well-crystallized sample essential to perform Neutron Powder Diffraction (NPD) experiments. By NPD we accessed to subtle crystallographic details of Mg2FeH6, such as the hydrogen occupancy and the metal-hydrogen distances and angles; a full hydrogen composition with six hydrogen atoms per unit formula is determined. No magnetic structure was observed by NPD at 2 K. The oxidation states of the atoms inside the material and the electronic structure of Mg2FeH6 have been evaluated by x-ray absorption near-edge spectroscopy (XANES). Unexpectedly, the Fe K-edge spectrum shows a zero oxidation state of Fe atoms in agreement with Mössbauer data. Our results suggest a metallic nature of the hydride in contrast with the ionic character previously reported for this compound., We thank the financial support of the Spanish Ministry of Economy and Competitivity to the project MAT2013-41099-R, and for beamtime provision.