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Thermal decomposition of Mg(AlH4)2 studied by in situ synchrotron X-ray diffraction

“…MgAlH 5 has not been observed by X-ray diffraction, but thermovolumetric [4] and TGA-MS [5] analyses show features compatible with several steps in the first decomposition reaction. Two alternative reaction paths have also been suggested for the second reaction step.…”

“…In a later paper, Dymova et al [4] suggest that reaction (1) in fact is the sum of two temperature-separated reactions involving an intermediate phase, MgAlH 5 :…”

“…Two alternative reaction paths have also been suggested for the second reaction step. Dymova et al [2,4], observed direct reaction between magnesium hydride and aluminium to one (380-420 • C) or two (270-310 • C) Al-Mg alloy phases, whereas Fichtner et al [5,6] found that alloy formation only occurred subsequent to decomposition of magnesium hydride to the constituent elements at around 300 • C.…”

Complex Hydrides

“…The evidence for the formation of the magnesium aluminum hydride was deduced from DTA experiments by comparing the DTA data of the products with those of the educts. Solvate-free preparations of Mg(AlH 4 ) 2 and of MgAlH 5 by mechanochemical activation were reported by Dymova et al [165]. However, the intermediate of the decomposition, MgAlH 5 , was never observed by others.…”

“…The decomposition enthalpy of Mg(AlH 4 ) 2 is thus far below that required for reversible hydrogen storage at room temperature (30-40 kJ mol À1 H 2 ) [180]. Dymova et al proposed the existence of MgAlH 5 formed by two different pathways: (i) during the mechanochemical activation where Mg(AlH 4 ) 2 and MgH 2 react at elevated temperature or (ii) during the decomposition of Mg(AlH 4 ) 2 between 120 and 155 C [165]. However, the existence of this phase has never been shown by X-ray diffraction.…”