Pressure-Collapsed Amorphous Mg(BH₄)₂: An Ultradense Complex Hydride Showing a Reversible Transition to the Porous Framework

Hydrogen-storage properties of complex hydrides depend of their form, such as a polymorphic form or an eutectic mixture. This Paper reports on an easy and reproducible way to synthesize a new stable form of magnesium borohydride by pressure-induced collapse of the porous γ-Mg­(BH₄)₂. This amorphous complex hydride was investigated by temperature-programmed synchrotron X-ray diffraction (SXRD), transmission electron microscopy (TEM), thermogravimetric analysis, differential scanning calorimetry analysis, and Raman spectroscopy, and the dynamics of the BH₄^– reorientation was studied by spin–lattice relaxation NMR spectroscopy. No long-range order is observed in the lattice region by Raman spectroscopy, while the internal vibration modes of the BH₄^– groups are the same as in the crystalline state. A hump at 4.9 Å in the SXRD pattern suggests the presence of nearly linear Mg–BH₄–Mg fragments constituting all the known crystalline polymorphs of Mg­(BH₄)₂, which are essentially frameworks built of tetrahedral Mg nodes and linear BH₄ linkers. TEM shows that the pressure-collapsed phase is amorphous down to the nanoscale, but surprisingly, SXRD reveals a transition at ∼90 °C from the dense amorphous state (density of 0.98 g/cm³) back to the porous γ phase having only 0.55 g/cm³ crystal density. The crystallization is slightly exothermic, with the enthalpy of −4.3 kJ/mol. The volumetric hydrogen density of the amorphous form is 145 g/L, one of the highest among hydrides. Remarkably, this form of Mg­(BH₄)₂ has different reactivity compared to the crystalline forms. The parameters of the reorientational motion of BH₄ groups in the amorphous Mg­(BH₄)₂ found from NMR measurements differ significantly from those in the known crystalline forms. The behavior of the nuclear spin–lattice relaxation rates can be described in terms of a Gaussian distribution of the activation energies centered on 234 ± 9 meV with the dispersion of 100 ± 10 meV