Hydride formation in Mg-based systems processed by reactive milling

Abstract

The possibilities to produce quaternary Mg-based transition-metal complex hydrides have been explored. Mg₂Mn_1−xFe_x (x = 0.5, 0) elemental powder mixtures were ball milled in a reactive D₂ atmosphere (about 5 MPa). The results were compared with the formation of Mg₂(FeD₆)_0.5(CoD₅)_0.5 from Mg–Fe–Co powders. The changes of D₂ pressure were monitored during milling and deuterium absorption was detected within the first 15 h of milling. Powder X-ray and neutron diffraction analysis for the as-milled Mg₂Mn_0.5Fe_0.5 powder showed a large fraction of unreacted elemental Mn, as well as the formation of a cubic K₂PtCl₆-type phase (a = 6.452 Å). The latter could be identified as Mg₂FeD₆. Infrared spectroscopy confirmed the presence of [FeD₆]⁴⁻ anions (stretching band at 1261 cm⁻¹) and ruled out the formation of [MnH₆]⁵⁻ complexes. As for the as-milled Mg₂Mn, powder diffraction analysis showed the formation of MgD₂ as the main reaction product. A small fraction of a cubic K₂PtCl₆-type phase (a = 6.526 Å) was also observed. This was identified as a Mg₂FeD₆-type compound. It was likely formed as a result of Fe impurities originating from the milling media and detected by EDX analysis. Infrared spectroscopy confirmed the presence of the [FeD₆]⁴⁻ complex (stretching band at 1259 cm⁻¹) and showed an additional band at 1313 cm⁻¹. The likelihood of this band arising from Mn–D bonds is discussed. Finally, deuterium desorption upon heating, was investigated for all systems by differential scanning calorimetry.