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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

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

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