We report on a study of thermodynamic functions and phase stability of lithium alanates $\mathrm{Li}\mathrm{Al}{\mathrm{H}}_4$ and ${\mathrm{Li}}_3\mathrm{Al}{\mathrm{H}}_6$ using ab initio density-functional and quasiharmonic phonon calculations. The calculated thermodynamic functions are in good agreement with available experimental data and are used to study the decomposition reactions of these materials. The results show that the decomposition of $\mathrm{Li}\mathrm{Al}{\mathrm{H}}_4$ is irreversible under all temperature and pressure conditions considered, indicating that a direct synthesis of $\mathrm{Li}\mathrm{Al}{\mathrm{H}}_4$ from the solid reaction of $(\frac{1}{3}{\mathrm{Li}}_3\mathrm{Al}{\mathrm{H}}_6+\frac{2}{3}\mathrm{Al}+{\mathrm{H}}_2)$ is not possible. Meanwhile, the calculated results suggest that ${\mathrm{Li}}_3\mathrm{Al}{\mathrm{H}}_6$ can be used as a rechargeable hydrogen-storage medium under certain temperature and pressure conditions. We construct the temperature-pressure phase diagram and present a discussion in conjunction with an analysis of recent experimental results.

• Received 23 February 2007

DOI:https://doi.org/10.1103/PhysRevB.76.024112