Preparation and thermal properties of aluminum hydride polymorphs

Aluminum hydride (AlH3) is one of the most promising hydrogen and energy storage materials, and it has a high gravimetric and volumetric density of hydrogen. In this work, three aluminum hydride polymorphs (α-AlH3, β-AlH3, and γ-AlH3) were prepared through the desolvation of AlH3-etherate using the organometallic synthesis method, and AlH3-etherate, which has the molecular formula of AlH3·0.22Et2O, was also obtained. The synthesis conditions were discussed in detail, and the structure and morphology of the samples were characterized using FTIR, SEM, and XRD. The thermal properties of AlH3 polymorphs were experimentally investigated under heating and isothermal processes. The results suggest that the α-polymorph is the most stable of the three polymorphs, and the decomposition of the less stable polymorphs, β-AlH3 and γ-AlH3, typically occurs via an exothermic transformation to the α-phase (≥100 °C) followed by the decomposition of α-AlH3 phase into Al and H2. However, a fraction of γ- and β-polymorphs decompose directly to Al and H2 at low temperatures (<100 °C). The direct decomposition of the γ- and β-phases is faster than that of the α-phase due to the lower total formation enthalpy. AlH3-etherate is first desolvated to γ-AlH3, which immediately transforms into the α-phase during the heating process.