Theoretical modeling of hydrogen storage materials: Prediction of structure, chemical bond character, and high-pressure behavior

Density-functional theory (DFT) is a powerful tool to predict crystal structure, chemical bond character, and high-pressure behavior of materials. In this report, we show the application of DFT to study such properties for complex hydrides. The structural parameters for the experimentally known Li3AlH6 phase have been successfully reproduced within an accuracy of less than 1% and the crystal structure of KAlH4 has been predicted. From examination of the density of state, we find that these materials have insulating behavior with a band gap of ∼ 3.5 and 5.5 eV for Li3AlH6 and KAlH4, respectively. From analyses of charge density, charge transfer, electron localization function, crystal orbital Hamilton, and Mulliken population we find that the interaction between Li/K and [AlH4]/[AlH6] is essentially pure ionic, whereas within the [AlH4]/[AlH6] unit the interaction is partially ionic and partially covalent. Even though these materials are very soft the Al-H interaction is relatively strong compared with the other interactions. Subject to external pressure the equilibrium structure of Li3AlH6 is unstable. We predicted that this compound undergoes three successive structural phase transitions under pressure: α to β at 18.64 GPa, β to γ at 28.85 GPa, and γ to ɛ at 68.79 GPa. KAlH4 is stable and no pressure induced structural transitions were identified.