Synthesis and decomposition mechanisms of Mg2FeH6 studied by in-situ synchrotron X-ray diffraction and high-pressure DSC

Synthesis and decomposition mechanisms of ternary Mg2FeH6 were investigated using in-situ synchrotron radiation powder X-ray diffraction (SR-PXD) and high-pressure differential scanning calorimetry (HP-DSC). Two routes for synthesis of Mg2FeH6 were studied. The first utilizes a ball-milled homogeneous MgH2–Fe powder mixture and the second uses a mixture of Fe and Mg formed by decomposition of the ternary hydride, Mg2FeH6. In both cases the reaction mixture was sintered in a temperature range from RT to 500 °C under a hydrogen pressure of 100–120 bar. The reaction mechanisms were established using in-situ SR-PXD. The formation of Mg2FeH6 consists of two steps with MgH2 as an intermediate compound, and the presence of magnesium was not observed. In contrast, the decomposition of Mg2FeH6 was found to be a single-step reaction. Additionally, both reactions were investigated using HP-DSC under similar conditions as in the SR-PXD experiments in order to estimate reaction enthalpies and temperatures. Mg2FeH6 was found to form from MgH2 and Fe under hydrogen pressure regardless of whether the MgH2 was introduced in the mixture or formed prior to creation of the ternary hydride.