Theoretical investigations on the phase transition of pure and Li-doped AlH3

Abstract

In order to solve a contradiction between early theoretical prediction and experiments concerning the γ → α phase transition of aluminum hydride, models of Li-doped AlH₃ were constructed and investigated theoretically. Thermodynamic calculations show that the γ → α transition of pure AlH₃ absorbs energy, and the changes in Gibbs free energy are in range of 1.74–1.99 kJ mol⁻¹ at 298–380 K. These are opposite to the experimental fact that the γ- to α-phase transition takes place at 380 K. However, the changes in enthalpy and Gibbs free energy in the γ → α phase transition of Li-doped AlH₃ are negative. The doping of Li decreases the activation energy of the γ → α transition and introduces more metastable states between them. As the doping content increases, both the changes in enthalpy and Gibbs free energy (ΔH_γ→α and ΔG_γ→α) decrease. The experimental ΔH_γ→α value (−2.83 kJ mol⁻¹) is between those of doped AlH₃ with 1/23 and 1/11 Li-content (−0.87 and −5.62 kJ mol⁻¹ for Al₂₃LiH₇₀ and Al₁₁LiH₃₄, respectively). Heat capacity C_P(T) increases as the Li-doping content increases. The C_P(T) of Al₂₃LiH₇₀ is consistent with the experiments. Considering the thermodynamic evidence and the experimental conditions for AlH₃ preparation, the aluminum hydride synthesized by the reaction of LiAlH₄ + AlCl₃ is probably Li-doped with a Li content of 1/23. The changes in enthalpy and Gibbs free energy, as well as the activation energy for the γ → α phase transition can be increased if the Li-doped AlH₃ is purified.