Thermal Decomposition of Magnesium Borohydride: New Insights from Synchrotron X-Ray Scattering

Magnesium borohydride [Mg(BH₄)₂] has been extensively investigated as a promising material with applications in energy storage, having properties favorable for both hydrogen and electrochemical storage processes. Recent efforts regarding hydrogen storage to determine conditions optimal for dehydrogenation and hydrogenation have focused on identifying environmental factors that promote various polyborane intermediate pathways. In the present study, we demonstrate the impact of the synthesis route, residual impurities, sample processing, and starting crystalline phase on the structural transformations and decomposition pathways of Mg(BH₄)₂. Using synchrotron powder X-ray diffraction (PXRD) and small-angle X-ray scattering (SAXS), we provide evidence of the residual dimethyl sulfide solvent coordinated inside the pores of ?-Mg(BH₄)₂even after post-synthetic de-solvation. In situ temperature-resolved PXRD and temperature programed desorption (TPD) indicate that these impurity molecules are removed by heating to 100 °C in vacuo. Furthermore, when ?-Mg(BH₄)₂is heated slowly under vacuum, we observe a ?- to α-phase transformation in place of the ϵ-Mg(BH₄)₂"intermediate" structure, which provides a direct connection between the α- and ?-Mg(BH₄)₂decomposition pathways. At higher temperatures (?150 °C), Mg(BH₄)₂transitions to the well-known β-phase structure. Although XRD results suggest that the crystalline structures of β-Mg(BH₄)₂are identical regardless of the starting material, SAXS and transmission electron microscopy indicate that when the ?-phase is used as the starting structure, the resulting β-Mg(BH₄)₂material exhibits intergranular (i.e., skeletal) porosity, not observed when annealed from the α-phase. These variations of the microstructure may contribute to differences in dehydrogenation mechanisms observed in TPD data among the three currently existing "as-synthesized" Mg(BH₄)₂structural phases.