Decomposition reactions for NaAlH ₄, Na ₃AlH ₆, and NaH: First-principles study

The electronic properties and lattice dynamics of the sodium alanate phases have been studied by the density functional calculations. The phases include NaAlH ₄ (space group, I4 ₁/a), Na ₃AlH ₆ (space group, P2 ₁/n), and NaH (space group, Fm-3m). The electronic properties are discussed on the basis of the electronic band structures, the atomic charges, the bond overlap population analysis, and the Born effective charges. The phonon dispersion relations and phonon density of states (DOS) of the phases are calculated by a direct force-constant method. Within the quasiharmonic approximation, the calculated thermodynamic functions including the heat capacity, the vibrational enthalpy, and the vibrational entropy are in good agreement with experimental values. Three decomposition reactions are studied based on the thermodynamic functions. The reactions are (1) NaAlH ₄ → 1/3Na ₃AlH ₆ + 2/3Al + H ₂, (2) 1/3Na ₃AlH ₆ → NaH + 1/3Al + 1/2H ₂, and (3) NaH → Na + 1/2H ₂. The reactions (1), (2), and (3) are predicted to take place at 285, 390, and 726 K, respectively, which are in good agreement with the experiment (353, 423, and 698 K, respectively). The individual contributions to the reactions including the enthalpy and entropy are investigated. We found that the enthalpy for the reaction is almost constant, and the net entropy contribution (TΔS) to the reaction is approximately equal to the entropy contribution of the H ₂ gas molecule (produced in that reaction).