Synthesis and decomposition mechanisms of Mg 2FeH 6 studied by in-situ synchrotron X-ray diffraction and high-pressure DSC

Abstract

Synthesis and decomposition mechanisms of ternary Mg 2FeH 6 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 Mg 2FeH 6 were studied. The first utilizes a ball-milled homogeneous MgH 2–Fe powder mixture and the second uses a mixture of Fe and Mg formed by decomposition of the ternary hydride, Mg 2FeH 6. 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 Mg 2FeH 6 consists of two steps with MgH 2 as an intermediate compound, and the presence of magnesium was not observed. In contrast, the decomposition of Mg 2FeH 6 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. Mg 2FeH 6 was found to form from MgH 2 and Fe under hydrogen pressure regardless of whether the MgH 2 was introduced in the mixture or formed prior to creation of the ternary hydride.