Using first-principles metadynamics simulation to predict new phases and probe the phase transition of NaAlH_4

摘要

A first-principles-based metadynamics technique was used to study the solidsolid phase transition of NaAlH_4 under normal pressure and at temperatures close to the melting point in order to understand the mechanism of hydrogen release from NaAlH_4. Two orthorhombic phases with space symmetry groups of Cmcm and Pbcm were determined based on the structures isolated from the metadynamics trajectories starting from the α phase (I4_1/a) of NaAlH_4. The I4_1/a → Cmcm → Pbcm transitions can be correlated with the symmetry-lowering process (D _(2h) → C_(2v) → C_s) of the AlH _4~- complex ions. Thermodynamics analysis revealed that the Cmcm phase forms from the I4_1/a phase at around the melting point (548K). Our results are in qualitative agreement with a previous Raman scattering measurement, which showed a transition at ～503K. The Cmcm phase is expected to be a key intermediate in hydrogen desorption prior to Na _3AlH_6 formation.