Thermodynamics of the α,β and γ polymorphs of AlH3

The thermodynamics of the α,β and γ polymorphs of AlH3 were determined using differential scanning calorimetry and ex situ X-ray diffraction. These results demonstrate that at around 100 °° C the decomposition of the β and γ polymorphs occurs by an initial phase transition to the α polymorph followed by decomposition of the α phase. The total heat evolved during the β→α transition is 1.5±0.41.5±0.4  kJ/mol AlH3 and 2.8±0.42.8±0.4  kJ/mol AlH3 during the γ→α transition. The transformation to the α phase is exothermic and is therefore likely to occur spontaneously at room temperature. A formation enthalpy of approximately −10−10  kJ/mol AlH3 was measured for α-AlH3, which is in good agreement with previous experimental and calculated results.