Thermodynamics of the α, β and γ polymorphs of AlH3

The thermodynamics of the alpha, beta and gamma polymorphs of AlH3 were determined using differential scanning calorimetry and ex situ X-ray diffraction. These results demonstrate that at around 100 degrees C the decomposition of the beta and gamma polymorphs occurs by an initial phase transition to the a polymorph followed by decomposition of the a phase. The total heat evolved during the beta -> alpha transition is 1.5 +/- 0.4 kJ/mol AlH3 and 2.8 +/- 0.4 kJ/mol AlH3 during the gamma -> alpha transition. The transformation to the alpha phase is exothermic and is therefore likely to occur spontaneously at room temperature. A formation enthalpy of approximately -10 kJ/mol AlH3 was measured for alpha-AlH3, which is in good agreement with previous experimental and calculated results. (c) 2005 Elsevier B.V. All rights reserved.