Neutron Powder Diffraction, x-ray absorption and Mössbauer spectroscopy on Mg2FeH6

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.